assignment of human heart

Biology Article

Human Heart

The heart is a muscular organ that is situated in the front of the chest. It pumps blood all through the body in a process called circulation. Apart from the heart, the blood vessels and blood as a unit constitute the cardiovascular system.

Table of Contents

Introduction
Functions of the Human Heart

Types of Circulation

Pericardium, structure of the heart wall, chambers of the heart, blood vessels, facts about human heart.

Important Questions about the Human Heart
Practise Labelling the Human Heart Diagram

Introduction to the Human Heart

The human heart is one of the most important organs responsible for sustaining life. It is a muscular organ with four chambers. The size of the heart is the size of about a clenched fist.

The human heart functions throughout a person’s lifespan and is one of the most robust and hardest working muscles in the human body.

Besides humans, most other animals also possess a heart that pumps blood throughout their bodies. Even invertebrates such as grasshoppers possess a heart like pumping organ, though they do not function the same way a human heart does.

Also Refer: Human Circulatory System

Position of Heart in Human Body

The human heart is located between the lungs in the thoracic cavity, slightly towards the left of the sternum (breastbone). It is derived from the embryonic mesodermal germ layer.

The Function of Heart

The function of the heart in any organism is to maintain a constant flow of blood throughout the body. This replenishes oxygen and circulates nutrients among the cells and tissues.

Following are the main functions of the heart:

One of the primary functions of the human heart is to pump blood throughout the body.
Blood delivers oxygen, hormones, glucose and other components to various parts of the body, including the human heart.
The heart also ensures that adequate blood pressure is maintained in the body

There are two types of circulation within the body, namely pulmonary circulation and systemic circulation.

Pulmonary circulation (blue) and Systemic circulation (red)

Pulmonary circulation is a portion of circulation responsible for carrying deoxygenated blood away from the heart , to the lungs and then bringing oxygenated blood back to the heart.
Systemic circulation is another portion of circulation where the oxygenated blood is pumped from the heart to every organ and tissue in the body, and deoxygenated blood comes back again to the heart.

Now, the heart itself is a muscle and therefore, it needs a constant supply of oxygenated blood. This is where another type of circulation comes into play, the coronary circulation.

Coronary circulation is an essential portion of the circulation, where oxygenated blood is supplied to the heart. This is important as the heart is responsible for supplying blood throughout the body.
Moreover, organs like the brain need a steady flow of fresh, oxygenated blood to ensure functionality.

In a nutshell, the circulatory system plays a vital role in supplying oxygen, and nutrients and removing carbon dioxide and other wastes from the body. Let us gain a deeper insight into the various anatomical structures of the heart:

Structure of the Human Heart

The human heart is about the size of a human fist and is divided into four chambers, namely two ventricles and two atria . The ventricles are the chambers that pump blood and the atrium are the chambers that receive blood. Among these both the right atrium and ventricle make up the “right heart,” and the left atrium and ventricle make up the “left heart.” The structure of the heart also houses the biggest artery in the body – the aorta .

The right and the left region of the heart are separated by a wall of muscle called the septum. The right ventricle pumps the blood to the lungs for re-oxygenation through the pulmonary arteries. The right semilunar valves close and prevent the blood from flowing back into the heart. Then, the oxygenated blood is received by the left atrium from the lungs via the pulmonary veins. Read on to explore more about the structure of the heart.

External Structure of Heart

One of the very first structures which can be observed when the external structure of the heart is viewed is the pericardium.

The human heart is situated to the left of the chest and is enclosed within a fluid-filled cavity described as the pericardial cavity. The walls and lining of the pericardial cavity are made up of a membrane known as the pericardium.

The pericardium is a fibre membrane found as an external covering around the heart. It protects the heart by producing a serous fluid, which serves to lubricate the heart and prevent friction between the surrounding organs. Apart from the lubrication, the pericardium also helps by holding the heart in its position and by maintaining a hollow space for the heart to expand itself when it is full. The pericardium has two exclusive layers—

Visceral Layer: It directly covers the outside of the heart.
Parietal Layer: It forms a sac around the outer region of the heart that contains the fluid in the pericardial cavity.

The heart wall is made up of 3 layers, namely:

Epicardium – Epicardium is the outermost layer of the heart. It is composed of a thin-layered membrane that serves to lubricate and protect the outer section.
Myocardium – This is a layer of muscle tissue and it constitutes the middle layer wall of the heart. It contributes to the thickness and is responsible for the pumping action.
Endocardium – It is the innermost layer that lines the inner heart chambers and covers the heart valves. Furthermore, it prevents the blood from sticking to the inner walls, thereby preventing potentially fatal blood clots.

Internal Structure of Heart

The internal structure of the heart is rather intricate with several chambers and valves that control the flow of blood.

Vertebrate hearts can be classified based on the number of chambers present. For instance, most fish have two chambers, and reptiles and amphibians have three chambers. Avian and mammalian hearts consists of four chambers. Humans are mammals; hence, we have four chambers, namely:

Left atrium
Right atrium
Left ventricle
Right ventricle

Atria are thin and have less muscular walls and are smaller than ventricles. These are the blood-receiving chambers that are fed by the large veins.

Ventricles are larger and more muscular chambers responsible for pumping and pushing blood out into circulation. These are connected to larger arteries that deliver blood for circulation.

The right ventricle and right atrium are comparatively smaller than the left chambers. The walls consist of fewer muscles compared to the left portion, and the size difference is based on their functions. The blood originating from the right side flows through the pulmonary circulation, while blood arising from the left chambers is pumped throughout the body.

In organisms with closed circulatory systems, the blood flows within vessels of varying sizes. All vertebrates, including humans, possess this type of circulation. The external structure of the heart has many blood vessels that form a network, with other major vessels emerging from within the structure. The blood vessels typically comprise the following:

Veins supply deoxygenated blood to the heart via inferior and superior vena cava, and it eventually drains into the right atrium.
Capillaries are tiny, tube-like vessels which form a network between the arteries to veins.
Arteries are muscular-walled tubes mainly involved in supplying oxygenated blood away from the heart to all other parts of the body. Aorta is the largest of the arteries and it branches off into various smaller arteries throughout the body.

Also Refer: Difference between Arteries and Veins

Valves are flaps of fibrous tissues located in the cardiac chambers between the veins. They ensure that the blood flows in a single direction (unidirectional). Flaps also prevent the blood from flowing backwards. Based on their function, valves are of two types:

Atrioventricular valves are between ventricles and atria. The valve between the right ventricle and right atrium is the tricuspid valve, and the one which is found between the left ventricle and left atrium is known as the mitral valve.
Semilunar valves are located between the left ventricle and the aorta. It is also found between the pulmonary artery and the right ventricle.

Frequently Asked Questions

1. what is pulmonary circulation explain..

Pulmonary circulation is a type of blood circulation responsible for carrying deoxygenated blood away from the heart, and to the lungs, where it is oxygenated. The system then brings oxygenated blood back to the heart to be pumped throughout the body.

2. Define systemic circulation.

In systemic circulation, the heart pumps the oxygenated blood through the arteries to every organ and tissue in the body, and then back again to the heart through a system of veins.

3. Elaborate on coronary circulation and its significance.

The heart is a muscle, and it needs a constant supply of oxygenated blood to survive and work effectively. This is where coronary circulation fulfils this function through a network of arteries and veins in the heart. The coronary arteries supply oxygenated blood to the heart, and the cardiac veins drain the blood once it has been deoxygenated by the tissues of the heart.

4. Briefly explain the structure of the human heart.

The human heart is divided into four chambers, namely two ventricles and two atria. The ventricles are the chambers that pump blood and atrium are the chambers that receive the blood. Among which, the right atrium and ventricle make up the “right portion of the heart”, and the left atrium and ventricle make up the “left portion of the heart.”

5. Name the chambers of the heart.

6. what is pericardium explain its function..

The pericardium is a fibrous membrane that envelops the heart. It also serves a protective function by producing a serous fluid, which lubricates the heart and prevents friction between the surrounding organs. Furthermore, the pericardium also holds the heart in its position and provides a hollow space for the heart to expand and contract.

7. Explain the three layers of the heart wall.

Epicardium – This is the outermost layer of the heart. It is composed of a thin layer of membrane that protects and lubricates the outer section.
Myocardium – This is a layer of muscle tissue that constitutes the middle layer wall of the heart. It is responsible for the heart’s “pumping” action.
Endocardium – The innermost layer that lines the inner heart chambers and covers the heart valves. Prevents blood from sticking, thereby avoiding the formation of fatal blood clots.

8. Explain the three major blood vessels of the human body.

The blood vessels comprise:

Veins – It supplies deoxygenated blood to the heart via inferior and superior vena cava, eventually draining into the right atrium.
Capillaries – They are minuscule, tube-like vessels which form a network between the arteries and veins.
Arteries – These are muscular-walled tubes responsible for supplying oxygenated blood away from the heart to all other parts of the body.

9. What is the function of the heart valves? Provide examples of various valves.

Valves are flaps of tissues that are present in cardiac chambers between the veins. They prevent the backflow of blood. Examples include the atrioventricular valves, tricuspid valves, mitral valves and the semilunar valves.

10. What is meant by myocardial infarction?

Myocardial infarction is a serious medical condition where the blood flow to the heart is reduced or entirely stopped. This causes oxygen deprivation in the heart muscles, and prolonged deprivation can cause tissues to die.

Label the Heart Diagram below:

Practice your understanding of the heart structure. Drag and drop the correct labels to the boxes with the matching, highlighted structures.

Instructions to use:

Hover the mouse over one of the empty boxes.
One part in the image gets highlighted.
Identify the highlighted part and drag and drop the correct label into the same box.
After finishing all the labelling, check your answers.
Semilunar valve
Pulmonary artery
Pulmonary vein

Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!

Select the correct answer and click on the “Finish” button Check your score and answers at the end of the quiz

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Heart Definition

The heart is a muscular organ that pumps blood throughout the body. It is located in the middle cavity of the chest, between the lungs. In most people, the heart is located on the left side of the chest, beneath the breastbone.

The heart is composed of smooth muscle. It has four chambers which contract in a specific order, allowing the human heart to pump blood from the body to the lungs and back again with high efficiency. The heart also contains “pacemaker” cells which fire nerve impulses at regular intervals, prompting the heart muscle to contract.

This animation shows the functioning of this extraordinarily complex pump in action. As you read this article, try scrolling back up and seeing if you can spot the chambers, valves, and blood vessels we’re discussing in action:

The heart is one of the most vital and delicate organs in the body. If it does not function properly, all other organs – including the brain – begin to die from lack of oxygen within just a few minutes. As of 2009, the most common cause of death in the world was heart disease.

Most heart disease occurs as a result of age or lifestyle. Cholesterol can build up in the arteries as a person gets older, and this is more likely for people who have diets high in saturated fat and cholesterol. Rarely, however, heart disease can also occur due to a virus or bacterium that infects the heart or its protective tissues.

Scientists have had some success replicating the heart’s pumping action with artificial pumps, but these pumps can be rejected by the body, and they break down over time.

The four-chambered heart found in mammals and birds is more efficient than the one, two, or three-chambered hearts found in some other animals. It is thought that warm-blooded animals need highly efficient circulation to satisfy their cells’ high demand for oxygen. This is especially true of humans, whose huge brains require a near-constant supply of oxygen to function!

Function of the Heart

The heart pumps blood through our immense and complicated circulatory systems at high pressure. It is a truly impressive feat of engineering, as it must circulate about five liters of blood through a full 1,000 miles worth of blood vessels each minute! We will talk more about how the heart accomplishes this remarkable task under the “Heart Structure” section below.

The pumping action of the heart allows the movement of many substances between organs in the body, including nutrients, waste products, and hormones and other chemical messengers. However, arguably the most important substance it circulates is oxygen.

Oxygen is required for animal cells to perform cellular respiration. Without oxygen, cells cannot break down food to produce ATP, the cellular currency of energy. Soon, none of their energy-dependent processes can function. Without its energy-dependent processes, a cell dies.

Neural tissues, including the brain, are particularly sensitive to oxygen deprivation. Neural tissues maintain a special cellular chemistry which must be constantly maintained through the consumption of lots and lots of energy. If ATP production stops, neural cells can begin to die within minutes.

For this reason, the body has taken many special measures to protect the heart. It is located below the strongest part of the ribcage and cushioned between the lungs. It is also surrounded by a protective membrane called the pericardium, which is filled with additional cushioning fluid.

Heart Structure

The heart’s unique design allows it to accomplish the incredible task of circulating blood through the human body. Here we will review its essential components, and how and why blood passes through them.

Layers of the Heart Wall

The heart has three layers of tissue, each of which serve a slightly different purpose. These are:

The Epicardium . The epicardium is also sometimes considered a part of the protective pericardial membrane around the heart. It helps to keep the heart lubricated and protected.
The Myocardium . The myocardium is the muscle of the heart. You can remember this because the root word “myo” comes from “muscle,” while “cardium” comes from “heart.” The myocardium is an incredibly strong muscle that makes up most of the heart. It is responsible for pumping blood throughout the body.
The Endocardium . The endocardium is a thin, protective layer on the inside of the heart. It is made of smooth, slippery endothelial cells, which prevent blood from sticking to the inside of the heart and forming deadly blood clots.

Chambers of the Heart

The heart has four chambers, which are designed to pump blood from the body to the lungs and back again with extremely high efficiency. Here we’ll see what the four chambers are, and how they do their jobs:

The Right Atrium . The right and left atria are the smaller chambers of the heart, and they have thinner, less muscular walls. This is because they only receive blood from the veins – they don’t have to pump it back out through the whole circulatory system! The right atrium only has to receive blood from the body’s veins and pump it into the left ventricle, where the real pumping action starts.
The Right Ventricle . The ventricles are larger chambers with stronger, thicker walls. They are responsible for pumping blood to the organs at high pressures. There are two ventricles because there are two circuits blood needs to be pumped through – the pulmonary circuit, where blood receives oxygen from the lungs, and the body circuit, where oxygen-filled blood travels to the rest of the body. Maintaining these two separate circuits with two separate ventricles is much more efficient than simply pumping blood to the lungs and allowing it to flow to the rest of the body from there. With two ventricles, the heart can generate twice the force, and deliver oxygen to our cells much faster. The right ventricle is the one attacked to the pulmonary circuit. It pumps blood through the pulmonary artery and to the lungs, where the blood fills with oxygen, at high pressure. The blood then returns to…
The left atrium receives oxygenated blood from the pulmonary veins. It pumps this blood into the left ventricle, which…
The left ventricle pumps blood throughout the rest of the body.

After the blood has circulated through the body and the oxygen has been exchange for carbon dioxide waste from the body’s cells, the blood re-enters the right atrium and the process begins again.

In most people, this whole circulatory path only takes about a minute to complete!

Valves of the Heart

You may be wondering how the heart ensures that blood flows in the right direction between these chambers and blood vessels. You may also have heard of “heart valves” referred to in a medical context.

Heart valves are just that – biological valves that only allow blood to flow through the heart in one direction, ensuring that all the blood gets to where it needs to be.

Here is a list of the most important valves in the heart, and an explanation of why they are important:

The Tricuspid Valve . The tricuspid valve is what is called an “atrioventricular” valve. As you might guess by the name, it ensures that blood only flows from the atrium to the ventricle – not the other way around. These atrioventricular valves have to stand up to very high pressures to ensure that no blood gets through, as the ventricle contracts quite powerfully to squeeze blood out. The tricuspid valve is the valve that ensures that blood in the right ventricle goes into the pulmonary artery and reaches the lungs, instead of being pushed back into the right atrium.
The Pulmonary Valve . The pulmonary valve is what is called a semilunar valve. Semilunar valves are found in arteries leaving the heart. Their role is to prevent blood from flowing backwards from the arteries into the heart’s chambers. This is important because the ventricles “suck” blood in from the atria by expanding after they have expelled blood into the arteries. Without properly functioning semilunar valves, blood can flow back into the ventricle instead of going to the rest of the body. This drastically decreases the efficiency with which the heart can move oxygenated blood through the body. The pulmonary valve lies in between the pulmonary artery and the left ventricle, where it ensures that blood pumped into the pulmonary artery continues to the lungs instead of returning to the heart.
The Mitral Valve . The mitral valve is the other atrioventricular valve. This one lies between the left atrium and the left ventricle. It prevents blood from flowing back from the ventricle into the atrium, ensuring that that blood is pumped to the rest of the body instead! The mitral valve lies at the opening of the aorta, which is the largest blood vessel in the body. The aorta is the central artery from which all other arteries fill. It is thicker than a garden hose, extends all the way from our hearts down to our pelvis, where it splits in two to become the femoral artery of each leg.
The Aortic Valve . As you might have guessed, the aorta needs a semilunar valve just like the pulmonary artery does. The aortic valve prevents blood from flowing backwards from the aorta into the left ventricle as the left ventricle “sucks” in oxygenated blood from the left atrium.

Many people have minor irregularities with these valves, such as mitral valve prolapse, which make their hearts less efficient or more prone to experiencing problems. People with minor valve issues can often lead a normal, healthy life.

However, total failure of any of these valves can be catastrophic for the heart and for blood flow. That’s why people with conditions like mitral valve prolapse are often turned down by the military and other programs that involve conditions which can be very taxing for the heart.

The Sinoatrial Node

The sinoatrial node is another very important part of the heart. It is a group of cells in the wall of the right atrium of the heart – and it is what keeps the heart pumping!

The cells in the sinoatrial node produce small electrical impulses in a regular rhythm. These impulses are what drive the contractions of the four chambers of the heart.

Artificial pacemakers replicate the action of the sinoatrial node by making similar electrical impulses for people whose sinoatrial node isn’t functioning properly. However, healthy people have a natural pacemaker built right into the heart!

Moore, K. L., Agur, A. M., & Dalley, A. F. (2018). Clinically oriented anatomy. Philadelphia: Wolters Kluwer. Heart. (n.d.). Retrieved July 08, 2017, from http://www.innerbody.com/image/card01.html (n.d.). Retrieved July 08, 2017, from https://training.seer.cancer.gov/anatomy/cardiovascular/heart/structure.html Blood Vessels. (2017, May 19). Retrieved July 08, 2017, from https://www.fi.edu/heart/blood-vessels

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At the Heart of It All: Anatomy and Function of the Heart

The heart is a hollow, muscular organ that pumps oxygenated blood throughout the body and deoxygenated blood to the lungs. This key circulatory system structure is comprised of four chambers. One chamber on the right receives blood with waste (from the body) and another chamber pumps it out toward the lungs where the waste is exhaled. One chamber on the left receives oxygen-rich blood from the lungs and another pumps that nutrient-rich blood into the body. Two valves control blood flow within the heart’s chambers, and two valves control blood flow out of the heart.

1. The Heart Wall Is Composed of Three Layers

The muscular wall of the heart has three layers. The outermost layer is the epicardium (or visceral pericardium). The epicardium covers the heart, wraps around the roots of the great blood vessels, and adheres the heart wall to a protective sac. The middle layer is the myocardium . This strong muscle tissue powers the heart’s pumping action. The innermost layer, the endocardium , lines the interior structures of the heart.

2. The Atria Are the Heart’s Entryways for Blood

The left atrium and right atrium are the two upper chambers of the heart. The left atrium receives oxygenated blood from the lungs. The right atrium receives deoxygenated blood returning from other parts of the body. Valves connect the atria to the ventricles, the lower chambers. Each atrium empties into the corresponding ventricle below.

3. Each Heart Beat Is a Squeeze of Two Chambers Called Ventricles

The left and right ventricles pump blood out of the heart

The ventricles are the two lower chambers of the heart. Blood empties into each ventricle from the atrium above, and then shoots out to where it needs to go. The right ventricle receives deoxygenated blood from the right atrium, then pumps the blood along to the lungs to get oxygen. The left ventricle receives oxygenated blood from the left atrium, then sends it on to the aorta. The aorta branches into the systemic arterial network that supplies all of the body.

4. The Valves Are Like Doors to the Chambers of the Heart

Four valves regulate and support the flow of blood through and out of the heart. The blood can only flow one way—like a car that must always be kept in drive. Each valve is formed by a group of folds, or cusps, that open and close as the heart contracts and dilates. There are two atrioventricular (AV) valves, located between the atrium and the ventricle on either side of the heart: The tricuspid valve on the right has three cusps, the mitral valve on the left has two. The other two valves regulate blood flow out of the heart. The aortic valve manages blood flow from the left ventricle into the aorta. The pulmonary valve manages blood flow out of the right ventricle through the pulmonary trunk into the pulmonary arteries.

5. The Cardiac Cycle Includes All Blood Flow Events the Heart Accomplishes in One Complete Heartbeat

Click to play an animation of the heart through the cardiac cycle

The muscular wall of the heart powers contraction and dilation. Each contraction and relaxation is a heartbeat. Ventricular contractions, called systole , force blood out of the heart through the pulmonary and aortic valves. Diastole occurs when blood flows from the atria to fill the ventricles.

Download Heart Lab Manual

External Sources

“ How the Heart Works, ” an overview of heart function from the University of Michigan Health.

A description of the heart from the 1918 edition of Gray's Anatomy of the Human Body.

Visible Body Web Suite offers thousands of models to help understand and communicate how the human body looks and works.

Functions of the Blood

Blood Vessels: The Circulatory Network

Pulmonary and Systemic Circulation

Circulatory System Pathologies

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Author: Jana Vasković, MD • Reviewer: Alexandra Osika Last reviewed: November 03, 2023 Reading time: 12 minutes

The heart is a muscular organ that pumps blood around the body by circulating it through the circulatory/vascular system. It is found in the middle mediastinum , wrapped in a two-layered serous sac called the pericardium . The heart is shaped as a quadrangular pyramid, and orientated as if the pyramid has fallen onto one of its sides so that its base faces the posterior thoracic wall, and its apex is pointed toward the anterior thoracic wall. The great vessels that originate from the heart, radiate their branches to the head and neck , the thorax and abdomen and the upper and lower limbs .

The heart holds a special position in anatomical sciences. For instance, you can live without your spleen or with only one kidney , you can even regrow your liver–but you cannot live without a heart. This page will introduce you to the anatomy of the heart.

Key facts about the heart
Borders	Superior (atria, auricles), inferior (right and left ventricles), left (left auricle, left ventricle), right (right atrium) borders
Surfaces	Sternocostal (right ventricle), diaphragmatic (mostly right ventricle, portion of left ventricle), pulmonary (cardiac impression) surfaces
Chambers	Atria (left and right), ventricles (left and right)
Emerging/entering vessels	Pulmonary veins (-> left atrium), superior and inferior vena cavae (-> right atrium), aorta (left ventricle ->), pulmonary artery (right ventricle ->)
Valves	ricuspid, ulmonary, itral, ortic valves ry ulling y orta
Blood supply	(sinuatrial nodal branch, right marginal branch, atrioventricular nodal branch, posterior interventricular branch) (circumflex branch, anterior interventricular branch) (great, middle and , left marginal vein, left posterior ventricular veins)

Heart anatomy

Heart valves

Blood flow through the heart

Coronary circulation, great vessels of the heart, clinical notes, related articles.

The heart has five surfaces: base (posterior), diaphragmatic (inferior), sternocostal (anterior), and left and right pulmonary surfaces. It also has several margins: right, left, superior, and inferior:

The right margin is the small section of the right atrium that extends between the superior and inferior vena cava .
The left margin is formed by the left ventricle and left auricle.
The superior margin in the anterior view is formed by both atria and their auricles.
The Inferior margin is marked by the right ventricle.

Inside, the heart is divided into four heart chambers: two atria (right and left) and two ventricles (right and left).

The right atrium and ventricle receive deoxygenated blood from systemic veins and pump it to the lungs , while the left atrium and ventricle receive oxygenated blood from the lungs and pump it to the systemic vessels which distribute it throughout the body.

The left and right sides of the heart are separated by the interatrial and interventricular septa which are continuous with each other. Furthermore, the atria are separated from the ventricles by the atrioventricular septa . Blood flows from the atria into the ventricles through the atrioventricular orifices (right and left)–openings in the atrioventricular septa. These openings are periodically shut and open by the heart valves , depending on the phase of the heart cycle .

Although there are a lot of structures in the heart diagrams, you shall not worry, we’ve got them all covered for you in these articles and video tutorials. Be sure to check out our specially designed heart anatomy quiz which will help you to master the heart anatomy.

Heart valves separate atria from ventricles, and ventricles from great vessels. The valves incorporate two or three leaflets (cusps) around the atrioventricular orifices and the roots of great vessels.

The cusps are pushed open to allow blood flow in one direction, and then closed to seal the orifices and prevent the backflow of blood. Backward prolapse of the cusps is prevented by the chordae tendineae –also known as the heart strings–fibrous cords that connect the papillary muscles of the ventricular wall to the atrioventricular valves.

There are two sets of valves: atrioventricular and semilunar. The atrioventricular valves prevent backflow from the ventricles to the atria:

The right atrioventricular/tricuspid valve is between the right atrium and right ventricle. It has three cusps/leaflets: anterior/anterosuperior, septal, and posterior/inferior.
The left atrioventricular/bicuspid valve is also called the mitral valve since it only has two cusps and resembles a miter in shape. It is between the left atrium and left ventricle and has two cusps/leaflets: anterior/aortic and posterior/mural.

It is very easy to remember which is which if you use a mnemonic ! Just memorise LAB RAT and you're set!

L eft A trium: B icuspid

Right A trium: T ricuspid

Semilunar valves prevent backflow from the great vessels to the ventricles.

The pulmonary semilunar valve is between the right ventricle and the opening of the pulmonary trunk . It has three semilunar cusps/leaflets: anterior/non-adjacent, left/left adjacent, and right/right adjacent.
The aortic semilunar valve is between the left ventricle and the opening of the aorta. It has three semilunar cusps/leaflets: left/left coronary, right/right coronary, and posterior/non-coronary.

In clinical practice, the heart valves can be auscultated, usually by using a stethoscope. In order to be successful, one needs to master the surface projections of the heart .

We have created a quiz, so you can test your newly acquired knowledge on the heart valves.

Heart valves mnemonic

Need an easy way to remember the four heart valves? Memorise the phrase ' T ry P ulling M y A orta', which stands for:

T ricuspid P ulmonary M itral A ortic

You can then go on to solidify your knowledge about heart valve anatomy with this study unit:

The blood flow through the heart is quite logical. It happens with the heart cycle, which consists of the periodical contraction and relaxation of the atrial and ventricular myocardium ( heart muscle tissue ). Systole is the period of contraction of the ventricular walls, while the period of ventricular relaxation is known as diastole. Note that whenever the atria contract, the ventricles are relaxed and vice versa. Let’s put into words the heart blood flow diagram:

The right atrium receives deoxygenated blood from the superior and inferior venae cavae and coronary sinus
The right atrium contracts pushing blood through the right atrioventricular valve into the right ventricle. The right ventricle then contracts passing the blood into the pulmonary trunk via the pulmonary valve to reach the lungs
In the lungs, the blood gets oxygenated then moves back into the heart entering the left atrium through the pulmonary veins.
The left atrium contracts and pushes the blood into the left ventricle through the left atrioventricular valve.
The left ventricle pushes oxygenated blood through the aortic semilunar valve into the aorta, from which blood is distributed throughout the body.

The heart cycle is regulated completely subconsciously by an autonomic nerve plexus called the cardiac plexus .

Do you find the anatomy of the heart confusing? Learn actively all the features of this organ and cement them long term by testing yourself using Kenhub's diagrams, quizzes and worksheets of the heart !

The heart must also be supplied with oxygenated blood. This is done by the two coronary arteries: left and right.

Heart muscles work constantly (thank goodness!), so the heart has a very high nutrient need. The coronary arteries arise from the aortic sinuses at the beginning of the ascending aorta , and then circle the heart–giving off several branches. In this way, oxygenated blood reaches every part of the heart.Venous blood from the heart is collected into the cardiac veins : middle, posterior, and small. They are all tributaries to coronary sinus –a large vessel that delivers deoxygenated blood from the myocardium to the right atrium.We’ve got you covered with coronary vasculature anatomy here.

Check out our clinical cases as well to see how bad it is when something is wrong with coronary circulation .

The great vessels of the heart are the: aorta , pulmonary artery , pulmonary vein , and superior and inferior vena cava . Why are they called the great vessels? Because they are large in size; the diameter of the ascending aorta is 2.1 centimeters, which is like the size of an American nickel (five-cent coin), and they all carry blood to and from the heart. Oh, not to mention that the aorta gives off branches which supply the entire body with oxygenated blood.

Major branches of the aorta include the brachiocephalic trunk , the left common carotid artery and the left subclavian artery. The superior vena cava receives blood from the upper half of the body via the left and right brachiocephalic veins , and the inferior vena cava from the lower half, through the common iliac veins.

Test yourself on the blood vessels of the heart with our quiz.

There are many disorders that can affect the heart and its adjacent structures. Below are a collection of different diseases, to name but a few.

Angina Pectoris is a pain in the chest that comes and goes and is due to the lack of oxygenation of the myocardium. Stable angina is the most common form and occurs because of the severe narrowing of the coronary arteries. Pain is felt upon exertion and is treated with nitroglycerin.

Infective endocarditis is a bacterial or fungal infection of the heart and can include but is not limited to the cardiac valves. These vegetative growths can be acute or chronic and are dangerous because of the potential embolization risk if they were to fragment.

The variations of congenital heart disease usually have an unknown etiology. Certain syndromes that occur due to chromosomal abnormalities usually have secondary heart defects which can include atrial septal defects , ventricular septal defects , a patent ductus arteriosus and even in some cases transposition of the great vessels . Down syndrome and turner syndrome are among the most common and well known chromosomal abnormalities.

Illustrations:

Heart (right lateral view) - Yousun Koh
Right atrium and ventricle of the heart - Yousun Koh
Left atrium and ventricle of the heart - Yousun Koh
Heart valves diagram - Yousun Koh
Coronary circulation - Yousun Koh
Heart valves in a cadaver - Prof. Carlos Suárez-Quian

Articles within this topic:

Anterior cardiac veins
Anterior interventricular sulcus
Aortic valve
Apex of the heart
Ascending aorta
Atria of the heart
Atrioventricular node
Blood supply of the heart
Circumflex artery
Clinical case: Chronic thoracic aneurysm
Clinical case: Long ectopic left main coronary artery
Conduction system of the heart
Conus arteriosus
Coronary sinus
Coronary sulcus
Crista terminalis
Diagrams, quizzes and worksheets of the heart
Disorders of coronary vessels
Embryology of the heart
Fetal circulation
Fibrous skeleton of the heart
Great cardiac vein
Heart auscultation and percussion
Inferior vena cava
Innervation of the heart
Interatrial septum
Interventricular septum
Layers of the heart
Left anterior descending artery (LAD artery)
Left coronary artery
Ligamentum arteriosum and ductus arteriosus
Middle cardiac vein
Pectinate muscles
Pericardial cavity
Pericardium
Posterior interventricular artery
Pulmonary circulation
Pulmonary valve
Pulmonary veins
Right coronary artery
Right marginal artery
Sinoatrial node
Small cardiac vein
Smallest cardiac veins (Thebesian veins)
Superior vena cava
Surface projections of the heart
Tachycardia
Trabeculae carneae
Tributaries of the inferior vena cava
Valvular heart disease
Ventricles of the heart

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Human heart: Anatomy, function & facts

The human heart is an organ that pumps blood throughout the body via the circulatory system.

Additional resources

The human heart is an organ that pumps blood throughout the body via the vessels of the circulatory system , supplying oxygen and nutrients to the tissues and removing carbon dioxide and other wastes.

"The tissues of the body need a constant supply of nutrition in order to be active," said Dr. Lawrence Phillips, a cardiologist at NYU Langone Medical Center in New York. "If [the heart] is not able to supply blood to the organs and tissues, they'll die."

The human heart is located in the center of the chest - slightly to the left of the sternum (breastbone). It sits between your lungs and is encased in a double-walled sac called the pericardium, according to the Texas Heart Institute . The pericardium serves to protect the heart and anchor it inside the chest. Pericardial fluid acts as a lubricant between the outer layer, the parietal pericardium, and the inner layer, the serous pericardium. The fluid lubricates the heart during contractions and movements of the lungs and diaphragm .

Related: Heart of the Matter: 7 Things to Know About Your Ticker

What does the human heart look like?

In humans, the heart is roughly the size of a large fist and weighs between about 10 and 12 ounces (280 and 340 grams) in men, and between 8 and 10 ounces (230 and 280 grams) in women, according to Henry Gray's " Anatomy of the Human Body ."

The physiology of the heart basically comes down to "structure, electricity and plumbing," Phillips told Live Science.

The human heart has four chambers: two upper chambers (the atria) and two lower ones (the ventricles), according to the National Institutes of Health . The right atrium and right ventricle together make up the "right heart," and the left atrium and left ventricle make up the "left heart." A wall of muscle called the septum separates the two sides of the heart.

Related: Spaceflight and long-distance swimming shrink the heart

The heart's outer wall consists of three layers. The outermost wall layer, or epicardium, forms the inner wall of the pericardium. The middle layer, or myocardium, contains the muscle that contracts the heart. The inner layer, or endocardium, lines the heart chambers, according to the British Heart Foundation .

Connecting the upper and lower chambers of the heart are the atrioventricular (AV) valves — made up of the tricuspid valve and the mitral valve. The pulmonary semi-lunar valve separates the right ventricle from the pulmonary artery, and the aortic valve separates the left ventricle from the aorta. The heartstrings, or chordae tendinae, anchor the valves to heart muscles.

Related; What is heart rate variability

How does the human heart work?

The heart circulates blood through two pathways: the pulmonary circuit and the systemic circuit.

In the pulmonary circuit, deoxygenated blood leaves the right ventricle of the heart via the pulmonary artery and travels to the lungs; then the oxygenated blood returns through the pulmonary vein to the left atrium of the heart, according to the journal Biomedical Sciences .

In the systemic circuit, oxygenated blood leaves the heart and travels through the left ventricle to the aorta, and from there enters the arteries and capillaries where it supplies the body's tissues with oxygen. Deoxygenated blood returns through veins to the venae cavae, re-entering the heart's right atrium.

Of course, the heart is also a muscle, so it needs a fresh supply of oxygen and nutrients, too, Phillips said.

"After the blood leaves the heart through the aortic valve, two sets of arteries bring oxygenated blood to feed the heart muscle," he said. The left main coronary artery, on one side of the aorta, branches into the left anterior descending artery and the left circumflex artery. The right coronary artery branches out on the right side of the aorta.

Blockage of any of these arteries can cause a heart attack , or damage to the heart muscle, Phillips said. A heart attack is distinct from cardiac arrest, which is a sudden loss of heart function that usually occurs as a result of electrical disturbances of the heart rhythm. A heart attack can lead to cardiac arrest, but the latter can also be caused by other problems, he said.

Related: Do other animals get heart attacks?

The heart contains electrical "pacemaker" cells, which cause it to contract — producing a heartbeat.

"Each cell has the ability to be the 'band leader' and [to] have everyone follow," Phillips said. In people with an irregular heartbeat, or atrial fibrillation, every cell tries to be the band leader, he said, which causes them to beat out of sync with one another.

A healthy heart contraction happens in five stages. In the first stage (early diastole), the heart is relaxed. Then the atrium contracts (atrial systole) to push blood into the ventricle. Next, the ventricles start contracting without changing volume. Then the ventricles continue contracting while empty. Finally, the ventricles stop contracting and relax. Then the cycle repeats. Valves prevent backflow, keeping the blood flowing in one direction through the heart.

By the end of the day, your heart will have beaten around 100,000 times (around 60 to 80 beats per minute). This will pump around 1.5 gallons (around 6.8 liters) of blood per minute through the 60,000 miles (around 97,000 kilometers) of blood vessels that are in the human body , according to the Cleveland Clinic .

Can humans get heartworm?

Heartworm is a disease that affects pets - predominantly dogs - resulting in heart failure and organ damage, according to the U.S. Food and Drug Administration (FDA). The disease is caused by a parasitic worm - called Dirofilaria immitis - which enters the pet's body from an infected mosquito bite. Humans, however, are not a natural host for the parasites - the heartworm larvae often die before reaching adulthood - therefore cases in people are very rare. A review published in 2005 in the journal Veterinary Parasitology found that between 1941 and 2005, there had been 81 reported cases of heartworm in humans.

The National Institutes of Health has information about heart and vascular diseases .
The American Heart Association has tips about how to keep your heart healthy .

This article was updated on Oct. 22, 2021 by Live Science staff writer Scott Dutfield.

Follow Tanya Lewis on Twitter . Follow us @livescience , Facebook & Google+ .

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All Parts of Heart Anatomy

In Basic Terms
How It Works
Understanding Heart Failure
Remembering Each Part

The heart is a vital, fist-sized muscular organ located slightly on the left side of the chest. It consists of four main chambers: two atria and two ventricles. Understanding its basic anatomy is crucial to understanding how it functions. This article provides a comprehensive look at the heart's structure with a detailed, labeled diagram and realistic photos, guiding you through each part and its role in the circulatory system.

Heart Anatomy in Basic Terms

The heart is a crucial organ that functions as the body's pump, ensuring blood circulation throughout the body. It consists of four main chambers:

Left and right atria (upper chambers)
Left and right ventricles (lower chambers)

These chambers work in a coordinated manner to receive oxygen-poor blood, pump it to the lungs for oxygenation (adding blood to oxygen), and then distribute oxygen-rich blood to the rest of the body. The heart's structure also includes valves that prevent backflow and ensure blood flows in the correct direction.

The heart has three layers of tissue:

Endocardium : The innermost layer, provides a smooth lining for chambers and valves
Myocardium : The middle layer, composed of muscle tissue that enables heart contractions
Epicardium : The outermost layer protects the heart and reduces friction with surrounding structures.

Understanding the heart's external and internal anatomy is essential for comprehending how this organ functions to maintain blood circulation throughout the body.

External Anatomy

The external structure of the heart includes several key components:

Pericardium

The pericardium is a double-walled sac that encloses the heart. It has two layers:

A tough outer layer ( fibrous pericardium ) that protects the heart and anchors it to surrounding structures.
An inner layer ( serous pericardium ) that includes the parietal layer lining the outer shell and the visceral layer (epicardium) directly on the heart's surface, acting as a cushion to prevent rubbing.

Coronary Arteries and Veins

Coronary arteries are blood vessels that supply the heart muscle (myocardium) with oxygen-rich blood. Coronary veins remove oxygen-poor blood.

Key coronary arteries include:

Left coronary artery : This artery supplies blood to the left side of the heart, including the left ventricle and left atrium. It also divides into the left anterior descending artery (to supply blood to the front of the left side of the heart) and the circumflex artery (to supply blood to the outer region and back of the heart.
Right coronary artery : This artery supplies blood to the right side of the heart, including the right ventricle, right atrium, and important nodes that control heart rhythm. It branches into smaller arteries like the right posterior descending artery and acute marginal artery. Along with the left anterior descending artery, it supplies blood to the heart's middle section (septum).

Coronary veins collect oxygen-poor blood from the myocardium and return it to the heart's right atrium, completing the circulation cycle.

Major Blood Vessels :

Major blood vessels of the heart include:

Aorta : The largest artery in the body, carrying oxygen-rich blood from the left ventricle to the body.
Pulmonary arteries : Vessels that carry oxygen-poor blood from the right ventricle to the lungs.
Pulmonary veins : Vessels that carry oxygen-rich blood from the lungs to the left atrium.
Superior vena cava and inferior vena cava : Carry oxygen-poor blood from the body to the right atrium.

Internal Anatomy

The internal structure of the heart is designed to facilitate its function as a powerful pump. Here are the key components:

The heart has four chambers, including:

Right atrium : Receives oxygen-poor blood from the body through the superior and inferior vena cava.
Right ventricle : Pumps the oxygen-poor blood to the lungs via the pulmonary artery.
Left atrium : Receives oxygen-rich blood from the lungs through the pulmonary veins.
Left ventricle : Pumps the oxygen-rich blood to the rest of the body through the aorta.

The heart's valves that prevent backflow and ensure that the blood continues to flow in the right direction include:

Tricuspid valve : Located between the right atrium and right ventricle, the tricuspid valve has three flaps (cusps) that open to allow blood to flow from the right atrium to the right ventricle and close to prevent blood from flowing backward.
Pulmonary valve : Positioned between the right ventricle and the pulmonary artery, this valve opens to let blood flow from the right ventricle into the pulmonary artery, which leads to the lungs. It closes to prevent blood from returning to the right ventricle.
Mitral valve : The mitral valve has two flaps (cusps) between the left atrium and left ventricle. It opens to allow oxygen-rich blood from the left atrium to flow into the left ventricle and closes to prevent backflow into the atrium.
Aortic valve : Found between the left ventricle and the aorta, the aortic valve opens to allow blood to flow from the left ventricle into the aorta. This main artery carries oxygen-rich blood to the rest of the body. It closes to prevent blood from flowing back into the left ventricle.

The septum is the muscular wall that divides the heart into the left and right sides, preventing the mixing of oxygen-rich and oxygen-poor blood.

Anatomical Variations

Anatomical variations of the heart can include differences in size, shape, position, and the number of chambers or valves. These variations can sometimes occur without causing significant health issues, while in other cases, they may contribute to specific cardiac conditions or affect heart function.

Some examples of anatomical variations of the heart include:

Atrial Septal Defect (ASD) : This is a congenital (present at birth) heart defect where there is an abnormal opening in the septum (wall) between the atria (upper chambers) of the heart. ASDs can vary in size and may lead to abnormal blood flow between the atria, which can cause permanent damage to the lung blood vessels.
Ventricular Septal Defect (VSD) : Similar to ASD, VSD is a congenital defect, but it occurs in the septum between the heart's ventricles (lower chambers). This defect allows blood to flow between the ventricles, potentially leading to symptoms like poor infant growth and rapid breathing.
Mitral Valve Prolapse (MVP) : In MVP, the mitral valve's flaps do not close properly, causing them to bulge (prolapse) back into the left atrium during the heart's contraction. MVP is a common condition and often doesn't cause significant problems. However, in some cases, it can lead to symptoms like palpitations, chest pain, or irregular heartbeats.

How Your Heart Anatomy Works

Exploring how blood moves through it and how it beats can help explain how the heart functions.

Oxygen-rich and oxygen-poor blood travels through different parts of the heart, ensuring that the body receives the oxygen and nutrients it needs to function properly.

Here is how blood flows through the heart:

Deoxygenated blood enters the right atrium : Deoxygenated blood from the body enters the right atrium through the superior and inferior vena cava.
Passage to the right ventricle : The right atrium contracts, pushing blood through the tricuspid valve into the right ventricle.
Pulmonary circulation : The right ventricle contracts, sending deoxygenated blood through the pulmonary valve and into the pulmonary artery, which then carries it to the lungs for oxygenation.
Oxygenated blood returns to the heart : Oxygenated blood from the lungs returns to the heart via the pulmonary veins, entering the left atrium.
Passage to the left ventricle : The left atrium contracts, pushing blood through the mitral valve into the left ventricle.
Systemic circulation : The left ventricle contracts, sending oxygen-rich blood through the aortic valve into the aorta, distributing it to the rest of the body.

Heart Beat, Rate, and Pulse

The heart's muscle contractions are triggered by electrical signals from a specialized system known as the cardiac conduction system . This network regulates the pace and pattern of heartbeats.

During each heartbeat, an electrical impulse travels from the upper part of the heart to the lower part, prompting the heart to contract and pump blood. This rhythmic process unfolds through several sequential steps, including:

The heart's electrical signal originates in pacemaker cells within the sinus node (SN), which is located in the right atrium.
This signal moves through the atria, making them contract and push blood into the ventricles.
Next, the signal reaches the atrioventricular (AV) node, another group of pacemaker cells between the atria and ventricles. Here, it slows down slightly, allowing the ventricles to fill with blood.
The AV node then sends a signal that spreads along the ventricle walls, causing them to contract and pump blood out of the heart.
After this contraction, the ventricles relax, and the cycle restarts as the SA node generates a new electrical signal.

Heart rate is measured in beats per minute (bpm) and reflects the number of times the heart contracts in a minute. A normal resting heart rate is between 60 and 100 beats per minute.

Pulse is an artery's palpable expansion and contraction as blood is ejected from the heart during each heartbeat. It is commonly measured at the wrist's radial artery or the neck's carotid artery.

Understanding Heart Failure With Anatomy

Heart failure can result from various conditions that weaken or damage the heart muscle, impairing its ability to pump blood effectively. This can lead to a backup of blood in the heart's chambers or the blood vessels leading to the heart.

Left-Sided Heart Failure

In left-sided heart failure , the left ventricle is unable to pump enough oxygen-rich blood to meet the body's needs. This can occur due to conditions such as:

Coronary artery disease (CAD)
High blood pressure (hypertension)
A heart attack

As a result, blood may return to the lungs, causing symptoms like shortness of breath, fatigue, and coughing.

Right-Sided Heart Failure

Right-sided heart failure occurs when the right ventricle is unable to pump blood to the lungs for oxygenation effectively. This can be caused by conditions such as:

Left-sided heart failure
Lung diseases, such as chronic obstructive pulmonary disease (COPD)
High blood pressure in the lungs, called pulmonary hypertension

Blood may then back up into the veins, leading to symptoms like swelling in the legs, abdomen, and other parts of the body.

Other Conditions Affecting Heart Function

Some medical conditions can significantly impact heart function and overall cardiovascular health. Proper diagnosis, treatment, and management are essential to mitigate their effects and improve heart function.

Arrhythmias : Abnormal heart rhythms, like fast, slow, or irregular beats, can disrupt heart function. The most common type is atrial fibrillation, which causes a fast and irregular heartbeat.
Heart valve diseases : Problems with heart valves can cause inefficient blood flow, leading to symptoms like fatigue, shortness of breath, and dizziness.
Cardiomyopathy : Diseases of the heart muscle can weaken the heart's pumping ability, causing heart failure and irregular heartbeats.
Pulmonary hypertension : High blood pressure in lung arteries can strain the heart, leading to congestive heart failure. This condition occurs when the pulmonary arteries in the lung become narrowed.

How to Remember Each Part of the Heart

Remembering heart anatomy can be overwhelming, especially for students who need to have them memorized! Here are a few ways to quickly recall the anatomy and function of the heart's chambers:

Heart Chambers : Use "RA, RV, LA, LV" to remember the order of the chambers (right atrium, right ventricle, left atrium, left ventricle).

Valves of the Heart : To remember the AV valves and their order, think of "Try Pulling My Aorta " (Tricuspid Valve, Pulmonary Valve, Mitral Valve, Aortic Valve).

The heart is the pump that moves blood around your body. It has four main parts: two upper chambers called atria and two lower chambers called ventricles. These parts work together to get oxygen-rich blood to your body and oxygen-poor blood back to your heart. Understanding how the heart works and its basic structure helps us see why it's so important for overall health.

MedlinePlus. Heart chambers .

American Heart Association. Roles of your four heart valves .

National Heart, Lung, and Blood Institute. How the heart works.

Frontiers in Physiology. Physiology of pericardial fluid .

John Hopkins Medicine. Anatomy and function of the coronary arteries .

University of Rochester Medical Center. About the heart and blood vessels .

American Heart Association. Your aorta: the pulse of life .

UpToDate. Chambers and valves of the heart .

Triposkiadis F, Xanthopoulos A, Boudoulas KD, Giamouzis G, Boudoulas H, Skoularigis J. The interventricular septum: structure, function, dysfunction, and diseases . J Clin Med . 2022;11(11):3227. doi:10.3390/jcm11113227

American Heart Association. Atrial septal defect (ASD) .

American Heart Association. Ventricular septal defect .

American Heart Association. Problem: Mitral valve prolapse .

National heart, Lung, and Blood Institute. How blood flows through the heart .

American Heart Association. How the heart beats .

MedlinePlus. Normal heart rhythm .

MedlinePlus. Radial plus .

InformedHealth.org. Heart failure: learn more – types of heart failure . Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG);2023.

MedlinePlus. Arrythmia .

MedlinePlus. Heart valve diseases .

MedlinePlus. Cardiomyopathy.

MedlinePlus. Primary pulmonary hypertension .

By Sarah Jividen, RN Jividen is a freelance healthcare journalist. She has over a decade of direct patient care experience working as a registered nurse specializing in neurotrauma, stroke, and the emergency room.

19.1 Heart Anatomy

Learning objectives.

By the end of this section, you will be able to:

Describe the location and position of the heart within the body cavity
Describe the internal and external anatomy of the heart
Identify the tissue layers of the heart
Relate the structure of the heart to its function as a pump
Compare systemic circulation to pulmonary circulation
Identify the veins and arteries of the coronary circulation system
Trace the pathway of oxygenated and deoxygenated blood thorough the chambers of the heart

The vital importance of the heart is obvious. If one assumes an average rate of contraction of 75 contractions per minute, a human heart would contract approximately 108,000 times in one day, more than 39 million times in one year, and nearly 3 billion times during a 75-year lifespan. Each of the major pumping chambers of the heart ejects approximately 70 mL blood per contraction in a resting adult. This would be equal to 5.25 liters of fluid per minute and approximately 14,000 liters per day. Over one year, that would equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly 60,000 miles of vessels. In order to understand how that happens, it is necessary to understand the anatomy and physiology of the heart.

Location of the Heart

The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. Figure 19.2 shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity . The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage, as seen in Figure 19.2 . The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the superior lobe of the left lung, called the cardiac notch .

Everyday Connection

The position of the heart in the torso between the vertebrae and sternum (see Figure 19.2 for the position of the heart within the thorax) allows for individuals to apply an emergency technique known as cardiopulmonary resuscitation (CPR) if the heart of a patient should stop. By applying pressure with the flat portion of one hand on the sternum in the area between the line at T4 and T9 ( Figure 19.3 ), it is possible to manually compress the blood within the heart enough to push some of the blood within it into the pulmonary and systemic circuits. This is particularly critical for the brain, as irreversible damage and death of neurons occur within minutes of loss of blood flow. Current standards call for compression of the chest at least 5 cm deep and at a rate of 100 compressions per minute, a rate equal to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. If you are unfamiliar with this song, a version is available on www.youtube.com. At this stage, the emphasis is on performing high-quality chest compressions, rather than providing artificial respiration. CPR is generally performed until the patient regains spontaneous contraction or is declared dead by an experienced healthcare professional.

When performed by untrained or overzealous individuals, CPR can result in broken ribs or a broken sternum, and can inflict additional severe damage on the patient. It is also possible, if the hands are placed too low on the sternum, to manually drive the xiphoid process into the liver, a consequence that may prove fatal for the patient. Proper training is essential. This proven life-sustaining technique is so valuable that virtually all medical personnel as well as concerned members of the public should be certified and routinely recertified in its application. CPR courses are offered at a variety of locations, including colleges, hospitals, the American Red Cross, and some commercial companies. They normally include practice of the compression technique on a mannequin.

Interactive Link

Visit the American Heart Association website to help locate a course near your home in the United States. There are also many other national and regional heart associations that offer the same service, depending upon the location.

Shape and Size of the Heart

The shape of the heart is similar to a pinecone, rather broad at the superior surface and tapering to the apex (see Figure 19.2 ). A typical heart is approximately the size of your fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness. Given the size difference between most members of the sexes, the weight of a female heart is approximately 250–300 grams (9 to 11 ounces), and the weight of a male heart is approximately 300–350 grams (11 to 12 ounces). The heart of a well-trained athlete, especially one specializing in aerobic sports, can be considerably larger than this. Cardiac muscle responds to exercise in a manner similar to that of skeletal muscle. That is, exercise results in the addition of protein myofilaments that increase the size of the individual cells without increasing their numbers, a concept called hypertrophy. Hearts of athletes can pump blood more effectively at lower rates than those of nonathletes. Enlarged hearts are not always a result of exercise; they can result from pathologies, such as hypertrophic cardiomyopathy . The cause of an abnormally enlarged heart muscle is unknown, but the condition is often undiagnosed and can cause sudden death in apparently otherwise healthy young people.

Chambers and Circulation through the Heart

The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle . Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.

There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.

The right ventricle pumps deoxygenated blood into the pulmonary trunk , which leads toward the lungs and bifurcates into the left and right pulmonary arteries . These vessels in turn branch many times before reaching the pulmonary capillaries , where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood. Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins —the only post-natal veins in the body that carry highly oxygenated blood. The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.

The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava , which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions ( Figure 19.4 ).

Membranes, Surface Features, and Layers

Our exploration of more in-depth heart structures begins by examining the membrane that surrounds the heart, the prominent surface features of the heart, and the layers that form the wall of the heart. Each of these components plays its own unique role in terms of function.

The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium or pericardial sac . It also surrounds the “roots” of the major vessels, or the areas of closest proximity to the heart. The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is made of tough, dense connective tissue that protects the heart and maintains its position in the thorax. The more delicate serous pericardium consists of two layers: the parietal pericardium, which is fused to the fibrous pericardium, and an inner visceral pericardium, or epicardium , which is fused to the heart and is part of the heart wall. The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium.

In most organs within the body, visceral serous membranes such as the epicardium are microscopic. However, in the case of the heart, it is not a microscopic layer but rather a macroscopic layer, consisting of a simple squamous epithelium called a mesothelium , reinforced with loose, irregular, or areolar connective tissue that attaches to the pericardium. This mesothelium secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts. Figure 19.5 illustrates the pericardial membrane and the layers of the heart.

Disorders of the...

Heart: cardiac tamponade.

If excess fluid builds within the pericardial space, it can lead to a condition called cardiac tamponade, or pericardial tamponade. With each contraction of the heart, more fluid—in most instances, blood—accumulates within the pericardial cavity. In order to fill with blood for the next contraction, the heart must relax. However, the excess fluid in the pericardial cavity puts pressure on the heart and prevents full relaxation, so the chambers within the heart contain slightly less blood as they begin each heart cycle. Over time, less and less blood is ejected from the heart. If the fluid builds up slowly, as in hypothyroidism, the pericardial cavity may be able to expand gradually to accommodate this extra volume. Some cases of fluid in excess of one liter within the pericardial cavity have been reported. Rapid accumulation of as little as 100 mL of fluid following trauma may trigger cardiac tamponade. Other common causes include myocardial rupture, pericarditis, cancer, or even cardiac surgery. Removal of this excess fluid requires insertion of drainage tubes into the pericardial cavity. Premature removal of these drainage tubes, for example, following cardiac surgery, or clot formation within these tubes are causes of this condition. Untreated, cardiac tamponade can lead to death.

Surface Features of the Heart

Inside the pericardium, the surface features of the heart are visible, including the four chambers. There is a superficial leaf-like extension of the atria near the superior surface of the heart, one on each side, called an auricle —a name that means “ear like”—because its shape resembles the external ear of a human ( Figure 19.6 ). Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. You may also hear them referred to as atrial appendages. Also prominent is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), along the superior surfaces of the heart. Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. The anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart. Figure 19.6 illustrates anterior and posterior views of the surface of the heart.

The wall of the heart is composed of three layers of unequal thickness. From superficial to deep, these are the epicardium, the myocardium, and the endocardium (see Figure 19.5 ). The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium discussed earlier.

The middle and thickest layer is the myocardium , made largely of cardiac muscle cells. It is built upon a framework of collagenous fibers, plus the blood vessels that supply the myocardium and the nerve fibers that help regulate the heart. It is the contraction of the myocardium that pumps blood through the heart and into the major arteries. The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart. They form a figure 8 pattern around the atria and around the bases of the great vessels. Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. More superficial layers of ventricular muscle wrap around both ventricles. This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would. Figure 19.7 illustrates the arrangement of muscle cells.

Although the ventricles on the right and left sides pump the same amount of blood per contraction, the muscle of the left ventricle is much thicker and better developed than that of the right ventricle. In order to overcome the high resistance required to pump blood into the long systemic circuit, the left ventricle must generate a great amount of pressure. The right ventricle does not need to generate as much pressure, since the pulmonary circuit is shorter and provides less resistance. Figure 19.8 illustrates the differences in muscular thickness needed for each of the ventricles.

The innermost layer of the heart wall, the endocardium , is joined to the myocardium with a thin layer of connective tissue. The endocardium lines the chambers where the blood circulates and covers the heart valves. It is made of simple squamous epithelium called endothelium , which is continuous with the endothelial lining of the blood vessels (see Figure 19.5 ).

Once regarded as a simple lining layer, recent evidence indicates that the endothelium of the endocardium and the coronary capillaries may play active roles in regulating the contraction of the muscle within the myocardium. The endothelium may also regulate the growth patterns of the cardiac muscle cells throughout life, and the endothelins it secretes create an environment in the surrounding tissue fluids that regulates ionic concentrations and states of contractility. Endothelins are potent vasoconstrictors and, in a normal individual, establish a homeostatic balance with other vasoconstrictors and vasodilators.

Internal Structure of the Heart

Recall that the heart’s contraction cycle follows a dual pattern of circulation—the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail.

Septa of the Heart

The word septum is derived from the Latin for “something that encloses;” in this case, a septum (plural = septa) refers to a wall or partition that divides the heart into chambers. The septa are physical extensions of the myocardium lined with endocardium. Located between the two atria is the interatrial septum . Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis , a remnant of an opening in the fetal heart known as the foramen ovale . The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern.

Between the two ventricles is a second septum known as the interventricular septum . Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract.

The septum between the atria and ventricles is known as the atrioventricular septum . It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve , a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular valves . The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves . The interventricular septum is visible in Figure 19.9 . In this figure, the atrioventricular septum has been removed to better show the bicuspid and tricuspid valves; the interatrial septum is not visible, since its location is covered by the aorta and pulmonary trunk. Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton , or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves. The cardiac skeleton also provides an important boundary in the heart electrical conduction system.

Heart: Heart Defects

One very common form of interatrial septum pathology is patent foramen ovale, which occurs when the septum primum does not close at birth, and the fossa ovalis is unable to fuse. The word patent is from the Latin root patens for “open.” It may be benign or asymptomatic, perhaps never being diagnosed, or in extreme cases, it may require surgical repair to close the opening permanently. As much as 20–25 percent of the general population may have a patent foramen ovale, but fortunately, most have the benign, asymptomatic version. Patent foramen ovale is normally detected by auscultation of a heart murmur (an abnormal heart sound) and confirmed by imaging with an echocardiogram. Despite its prevalence in the general population, the causes of patent ovale are unknown, and there are no known risk factors. In nonlife-threatening cases, it is better to monitor the condition than to risk heart surgery to repair and seal the opening.

Coarctation of the aorta is a congenital abnormal narrowing of the aorta that is normally located at the insertion of the ligamentum arteriosum, the remnant of the fetal shunt called the ductus arteriosus. If severe, this condition drastically restricts blood flow through the primary systemic artery, which is life threatening. In some individuals, the condition may be fairly benign and not detected until later in life. Detectable symptoms in an infant include difficulty breathing, poor appetite, trouble feeding, or failure to thrive. In older individuals, symptoms include dizziness, fainting, shortness of breath, chest pain, fatigue, headache, and nosebleeds. Treatment involves surgery to resect (remove) the affected region or angioplasty to open the abnormally narrow passageway. Studies have shown that the earlier the surgery is performed, the better the chance of survival.

A patent ductus arteriosus is a congenital condition in which the ductus arteriosus fails to close. The condition may range from severe to benign. Failure of the ductus arteriosus to close results in blood flowing from the higher pressure aorta into the lower pressure pulmonary trunk. This additional fluid moving toward the lungs increases pulmonary pressure and makes respiration difficult. Symptoms include shortness of breath (dyspnea), tachycardia, enlarged heart, a widened pulse pressure, and poor weight gain in infants. Treatments include surgical closure (ligation), manual closure using platinum coils or specialized mesh inserted via the femoral artery or vein, or nonsteroidal anti-inflammatory drugs to block the synthesis of prostaglandin E2, which maintains the vessel in an open position. If untreated, the condition can result in congestive heart failure.

Septal defects are not uncommon in individuals and may be congenital or caused by various disease processes. Tetralogy of Fallot is a congenital condition that may also occur from exposure to unknown environmental factors; it occurs when there is an opening in the interventricular septum caused by blockage of the pulmonary trunk, normally at the pulmonary semilunar valve. This allows blood that is relatively low in oxygen from the right ventricle to flow into the left ventricle and mix with the blood that is relatively high in oxygen. Symptoms include a distinct heart murmur, low blood oxygen percent saturation, dyspnea or difficulty in breathing, polycythemia, broadening (clubbing) of the fingers and toes, and in children, difficulty in feeding or failure to grow and develop. It is the most common cause of cyanosis following birth. The term “tetralogy” is derived from the four components of the condition, although only three may be present in an individual patient: pulmonary infundibular stenosis (rigidity of the pulmonary valve), overriding aorta (the aorta is shifted above both ventricles), ventricular septal defect (opening), and right ventricular hypertrophy (enlargement of the right ventricle). Other heart defects may also accompany this condition, which is typically confirmed by echocardiography imaging. Tetralogy of Fallot occurs in approximately 400 out of one million live births. Normal treatment involves extensive surgical repair, including the use of stents to redirect blood flow and replacement of valves and patches to repair the septal defect, but the condition has a relatively high mortality. Survival rates are currently 75 percent during the first year of life; 60 percent by 4 years of age; 30 percent by 10 years; and 5 percent by 40 years.

In the case of severe septal defects, including both tetralogy of Fallot and patent foramen ovale, failure of the heart to develop properly can lead to a condition commonly known as a “blue baby.” Regardless of normal skin pigmentation, individuals with this condition have an insufficient supply of oxygenated blood, which leads to cyanosis, a blue or purple coloration of the skin, especially when active.

Septal defects are commonly first detected through auscultation, listening to the chest using a stethoscope. In this case, instead of hearing normal heart sounds attributed to the flow of blood and closing of heart valves, unusual heart sounds may be detected. This is often followed by medical imaging to confirm or rule out a diagnosis. In many cases, treatment may not be needed. Some common congenital heart defects are illustrated in Figure 19.10 .

Right Atrium

The right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins, the superior and inferior venae cavae, and the large coronary vein called the coronary sinus that drains the heart myocardium empty into the right atrium. The superior vena cava drains blood from regions superior to the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas inferior to the diaphragm: the lower limbs and abdominopelvic region of the body. It, too, empties into the posterior portion of the atria, but inferior to the opening of the superior vena cava. Immediately superior and slightly medial to the opening of the inferior vena cava on the posterior surface of the atrium is the opening of the coronary sinus. This thin-walled vessel drains most of the coronary veins that return systemic blood from the heart. The majority of the internal heart structures discussed in this and subsequent sections are illustrated in Figure 19.9 .

While the bulk of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface demonstrates prominent ridges of muscle called the pectinate muscles . The right auricle also has pectinate muscles. The left atrium does not have pectinate muscles except in the auricle.

The atria receive venous blood on a nearly continuous basis, preventing venous flow from stopping while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase and actively pump blood into the ventricles just prior to ventricular contraction. The opening between the atrium and ventricle is guarded by the tricuspid valve.

Right Ventricle

The right ventricle receives blood from the right atrium through the tricuspid valve. Each flap of the valve is attached to strong strands of connective tissue, the chordae tendineae , literally “tendinous cords,” or sometimes more poetically referred to as “heart strings.” There are several chordae tendineae associated with each of the flaps. They are composed of approximately 80 percent collagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the flaps to a papillary muscle that extends from the inferior ventricular surface. There are three papillary muscles in the right ventricle, called the anterior, posterior, and septal muscles, which correspond to the three sections of the valves.

When the myocardium of the ventricle contracts, pressure within the ventricular chamber rises. Blood, like any fluid, flows from higher pressure to lower pressure areas, in this case, toward the pulmonary trunk and the atrium. To prevent any potential backflow, the papillary muscles also contract, generating tension on the chordae tendineae. This prevents the flaps of the valves from being forced into the atria and regurgitation of the blood back into the atria during ventricular contraction. Figure 19.11 shows papillary muscles and chordae tendineae attached to the tricuspid valve.

The walls of the ventricle are lined with trabeculae carneae , ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band (see Figure 19.9 ) reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the inferior portion of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle.

When the right ventricle contracts, it ejects blood into the pulmonary trunk, which branches into the left and right pulmonary arteries that carry it to each lung. The superior surface of the right ventricle begins to taper as it approaches the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve that prevents backflow from the pulmonary trunk.

Left Atrium

After exchange of gases in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. While the left atrium does not contain pectinate muscles, it does have an auricle that includes these pectinate ridges. Blood flows nearly continuously from the pulmonary veins back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. This atrial contraction accounts for approximately 20 percent of ventricular filling. The opening between the left atrium and ventricle is guarded by the mitral valve.

Left Ventricle

Recall that, although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right (see Figure 19.8 ). Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The mitral valve is connected to papillary muscles via chordae tendineae. There are two papillary muscles on the left—the anterior and posterior—as opposed to three on the right.

The left ventricle is the major pumping chamber for the systemic circuit; it ejects blood into the aorta through the aortic semilunar valve.

Heart Valve Structure and Function

A transverse section through the heart slightly above the level of the atrioventricular septum reveals all four heart valves along the same plane ( Figure 19.12 ). The valves ensure unidirectional blood flow through the heart. Between the right atrium and the right ventricle is the right atrioventricular valve , or tricuspid valve . It typically consists of three flaps, or leaflets, made of endocardium reinforced with additional connective tissue. The flaps are connected by chordae tendineae to the papillary muscles, which control the opening and closing of the valves.

Emerging from the right ventricle at the base of the pulmonary trunk is the pulmonary semilunar valve, or the pulmonary valve ; it is also known as the pulmonic valve or the right semilunar valve. The pulmonary valve is comprised of three small flaps of endothelium reinforced with connective tissue. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk. This flow of blood fills the pocket-like flaps of the pulmonary valve, causing the valve to close and producing an audible sound. Unlike the atrioventricular valves, there are no papillary muscles or chordae tendineae associated with the pulmonary valve.

Located at the opening between the left atrium and left ventricle is the mitral valve , also called the bicuspid valve or the left atrioventricular valve . Structurally, this valve consists of two cusps, compared to the three cusps of the tricuspid valve. In a clinical setting, the valve is referred to as the mitral valve, rather than the bicuspid valve. The two cusps of the mitral valve are attached by chordae tendineae to two papillary muscles that project from the wall of the ventricle.

At the base of the aorta is the aortic semilunar valve, or the aortic valve , which prevents backflow from the aorta. It normally is composed of three flaps. When the ventricle relaxes and blood attempts to flow back into the ventricle from the aorta, blood will fill the cusps of the valve, causing it to close and producing an audible sound.

In Figure 19.13 a , the two atrioventricular valves are open and the two semilunar valves are closed. This occurs when both atria and ventricles are relaxed and when the atria contract to pump blood into the ventricles. Figure 19.13 b shows a frontal view. Although only the left side of the heart is illustrated, the process is virtually identical on the right.

Figure 19.14 a shows the atrioventricular valves closed while the two semilunar valves are open. This occurs when the ventricles contract to eject blood into the pulmonary trunk and aorta. Closure of the two atrioventricular valves prevents blood from being forced back into the atria. This stage can be seen from a frontal view in Figure 19.14 b .

When the ventricles begin to contract, pressure within the ventricles rises and blood flows toward the area of lowest pressure, which is initially in the atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid) valves to close. These valves are tied down to the papillary muscles by chordae tendineae. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight (see Figure 19.13 b ). However, as the myocardium of the ventricle contracts, so do the papillary muscles. This creates tension on the chordae tendineae (see Figure 19.14 b ), helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.

The aortic and pulmonary semilunar valves lack the chordae tendineae and papillary muscles associated with the atrioventricular valves. Instead, they consist of pocket-like folds of endocardium reinforced with additional connective tissue. When the ventricles relax and the change in pressure forces the blood toward the ventricles, the blood presses against these cusps and seals the openings.

Visit this site to observe an echocardiogram of actual heart valves opening and closing. Although much of the heart has been “removed” from this gif loop so the chordae tendineae are not visible, why is their presence more critical for the atrioventricular valves (tricuspid and mitral) than the semilunar (aortic and pulmonary) valves?

Heart Valves

When heart valves do not function properly, they are often described as incompetent and result in valvular heart disease, which can range from benign to lethal. Some of these conditions are congenital, that is, the individual was born with the defect, whereas others may be attributed to disease processes or trauma. Some malfunctions are treated with medications, others require surgery, and still others may be mild enough that the condition is merely monitored since treatment might trigger more serious consequences.

Valvular disorders are often caused by carditis, or inflammation of the heart. One common trigger for this inflammation is rheumatic fever, or scarlet fever, an autoimmune response to the presence of a bacterium, Streptococcus pyogenes , normally a disease of childhood.

While any of the heart valves may be involved in valve disorders, mitral regurgitation is the most common, detected in approximately 2 percent of the population, and the pulmonary semilunar valve is the least frequently involved. When a valve malfunctions, the flow of blood to a region will often be disrupted. The resulting inadequate flow of blood to this region will be described in general terms as an insufficiency. The specific type of insufficiency is named for the valve involved: aortic insufficiency, mitral insufficiency, tricuspid insufficiency, or pulmonary insufficiency.

If one of the cusps of the valve is forced backward by the force of the blood, the condition is referred to as a prolapsed valve. Prolapse may occur if the chordae tendineae are damaged or broken, causing the closure mechanism to fail. The failure of the valve to close properly disrupts the normal one-way flow of blood and results in regurgitation, when the blood flows backward from its normal path. Using a stethoscope, the disruption to the normal flow of blood produces a heart murmur.

Stenosis is a condition in which the heart valves become rigid and may calcify over time. The loss of flexibility of the valve interferes with normal function and may cause the heart to work harder to propel blood through the valve, which eventually weakens the heart. Aortic stenosis affects approximately 2 percent of the population over 65 years of age, and the percentage increases to approximately 4 percent in individuals over 85 years. Occasionally, one or more of the chordae tendineae will tear or the papillary muscle itself may die as a component of a myocardial infarction (heart attack). In this case, the patient’s condition will deteriorate dramatically and rapidly, and immediate surgical intervention may be required.

Auscultation, or listening to a patient’s heart sounds, is one of the most useful diagnostic tools, since it is proven, safe, and inexpensive. The term auscultation is derived from the Latin for “to listen,” and the technique has been used for diagnostic purposes as far back as the ancient Egyptians. Valve and septal disorders will trigger abnormal heart sounds. If a valvular disorder is detected or suspected, a test called an echocardiogram, or simply an “echo,” may be ordered. Echocardiograms are sonograms of the heart and can help in the diagnosis of valve disorders as well as a wide variety of heart pathologies.

Visit this site for audio examples of heart sounds.

Career Connection

Cardiologist.

Cardiologists are medical doctors that specialize in the diagnosis and treatment of diseases of the heart. After completing 4 years of medical school, cardiologists complete a three-year residency in internal medicine followed by an additional three or more years in cardiology. Following this 10-year period of medical training and clinical experience, they qualify for a rigorous two-day examination administered by the Board of Internal Medicine that tests their academic training and clinical abilities, including diagnostics and treatment. After successful completion of this examination, a physician becomes a board-certified cardiologist. Some board-certified cardiologists may be invited to become a Fellow of the American College of Cardiology (FACC). This professional recognition is awarded to outstanding physicians based upon merit, including outstanding credentials, achievements, and community contributions to cardiovascular medicine.

Visit this site to learn more about cardiologists.

Cardiovascular Technologist/Technician

Cardiovascular technologists/technicians are trained professionals who perform a variety of imaging techniques, such as sonograms or echocardiograms, used by physicians to diagnose and treat diseases of the heart. Nearly all of these positions require an associate degree. Growth within the field is fast, projected at 29 percent from 2010 to 2020.

There is a considerable overlap and complementary skills between cardiac technicians and vascular technicians, and so the term cardiovascular technician is often used. Special certifications within the field require documenting appropriate experience and completing additional and often expensive certification examinations. These subspecialties include Certified Rhythm Analysis Technician (CRAT), Certified Cardiographic Technician (CCT), Registered Congenital Cardiac Sonographer (RCCS), Registered Cardiac Electrophysiology Specialist (RCES), Registered Cardiovascular Invasive Specialist (RCIS), Registered Cardiac Sonographer (RCS), Registered Vascular Specialist (RVS), and Registered Phlebology Sonographer (RPhS).

Visit this site for more information on cardiovascular technologists/technicians.

Coronary Circulation

You will recall that the heart is a remarkable pump composed largely of cardiac muscle cells that are incredibly active throughout life. Like all other cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients, and a way to remove wastes, so it needs a dedicated, complex, and extensive coronary circulation. And because of the critical and nearly ceaseless activity of the heart throughout life, this need for a blood supply is even greater than for a typical cell. However, coronary circulation is not continuous; rather, it cycles, reaching a peak when the heart muscle is relaxed and nearly ceasing while it is contracting.

Coronary Arteries

Coronary arteries supply blood to the myocardium and other components of the heart. The first portion of the aorta after it arises from the left ventricle gives rise to the coronary arteries. There are three dilations in the wall of the aorta just superior to the aortic semilunar valve. Two of these, the left posterior aortic sinus and anterior aortic sinus, give rise to the left and right coronary arteries, respectively. The third sinus, the right posterior aortic sinus, typically does not give rise to a vessel. Coronary vessel branches that remain on the surface of the artery and follow the sulci are called epicardial coronary arteries .

The left coronary artery distributes blood to the left side of the heart, the left atrium and ventricle, and the interventricular septum. The circumflex artery arises from the left coronary artery and follows the coronary sulcus to the left. Eventually, it will fuse with the small branches of the right coronary artery. The larger anterior interventricular artery , also known as the left anterior descending artery (LAD), is the second major branch arising from the left coronary artery. It follows the anterior interventricular sulcus around the pulmonary trunk. Along the way it gives rise to numerous smaller branches that interconnect with the branches of the posterior interventricular artery, forming anastomoses. An anastomosis is an area where vessels unite to form interconnections that normally allow blood to circulate to a region even if there may be partial blockage in another branch. The anastomoses in the heart are very small. Therefore, this ability is somewhat restricted in the heart so a coronary artery blockage often results in death of the cells (myocardial infarction) supplied by the particular vessel.

The right coronary artery proceeds along the coronary sulcus and distributes blood to the right atrium, portions of both ventricles, and the heart conduction system. Normally, one or more marginal arteries arise from the right coronary artery inferior to the right atrium. The marginal arteries supply blood to the superficial portions of the right ventricle. On the posterior surface of the heart, the right coronary artery gives rise to the posterior interventricular artery , also known as the posterior descending artery. It runs along the posterior portion of the interventricular sulcus toward the apex of the heart, giving rise to branches that supply the interventricular septum and portions of both ventricles. Figure 19.15 presents views of the coronary circulation from both the anterior and posterior views.

Diseases of the...

Heart: myocardial infarction.

Myocardial infarction (MI) is the formal term for what is commonly referred to as a heart attack. It normally results from a lack of blood flow (ischemia) and oxygen (hypoxia) to a region of the heart, resulting in death of the cardiac muscle cells. An MI often occurs when a coronary artery is blocked by the buildup of atherosclerotic plaque consisting of lipids, cholesterol and fatty acids, and white blood cells, primarily macrophages. It can also occur when a portion of an unstable atherosclerotic plaque travels through the coronary arterial system and lodges in one of the smaller vessels. The resulting blockage restricts the flow of blood and oxygen to the myocardium and causes death of the tissue. MIs may be triggered by excessive exercise, in which the partially occluded artery is no longer able to pump sufficient quantities of blood, or severe stress, which may induce spasm of the smooth muscle in the walls of the vessel.

In the case of acute MI, there is often sudden pain beneath the sternum (retrosternal pain) called angina pectoris, often radiating down the left arm in males but not in female patients. Until this anomaly between the sexes was discovered, many female patients suffering MIs were misdiagnosed and sent home. In addition, patients typically present with difficulty breathing and shortness of breath (dyspnea), irregular heartbeat (palpations), nausea and vomiting, sweating (diaphoresis), anxiety, and fainting (syncope), although not all of these symptoms may be present. Many of the symptoms are shared with other medical conditions, including anxiety attacks and simple indigestion, so differential diagnosis is critical. It is estimated that between 22 and 64 percent of MIs present without any symptoms.

An MI can be confirmed by examining the patient’s ECG, which frequently reveals alterations in the ST and Q components. Some classification schemes of MI are referred to as ST-elevated MI (STEMI) and non-elevated MI (non-STEMI). In addition, echocardiography or cardiac magnetic resonance imaging may be employed. Common blood tests indicating an MI include elevated levels of creatine kinase MB (an enzyme that catalyzes the conversion of creatine to phosphocreatine, consuming ATP) and cardiac troponin (the regulatory protein for muscle contraction), both of which are released by damaged cardiac muscle cells.

Immediate treatments for MI are essential and include administering supplemental oxygen, aspirin that helps to break up clots, and nitroglycerine administered sublingually (under the tongue) to facilitate its absorption. Despite its unquestioned success in treatments and use since the 1880s, the mechanism of nitroglycerine is still incompletely understood but is believed to involve the release of nitric oxide, a known vasodilator, and endothelium-derived releasing factor, which also relaxes the smooth muscle in the tunica media of coronary vessels. Longer-term treatments include injections of thrombolytic agents such as streptokinase that dissolve the clot, the anticoagulant heparin, balloon angioplasty and stents to open blocked vessels, and bypass surgery to allow blood to pass around the site of blockage. If the damage is extensive, coronary replacement with a donor heart or coronary assist device, a sophisticated mechanical device that supplements the pumping activity of the heart, may be employed. Despite the attention, development of artificial hearts to augment the severely limited supply of heart donors has proven less than satisfactory but will likely improve in the future.

MIs may trigger cardiac arrest, but the two are not synonymous. Important risk factors for MI include cardiovascular disease, age, smoking, high blood levels of the low-density lipoprotein (LDL, often referred to as “bad” cholesterol), low levels of high-density lipoprotein (HDL, or “good” cholesterol), hypertension, diabetes mellitus, obesity, lack of physical exercise, chronic kidney disease, excessive alcohol consumption, and use of illegal drugs.

Coronary Veins

Coronary veins drain the heart and generally parallel the large surface arteries (see Figure 19.15 ). The great cardiac vein can be seen initially on the surface of the heart following the interventricular sulcus, but it eventually flows along the coronary sulcus into the coronary sinus on the posterior surface. The great cardiac vein initially parallels the anterior interventricular artery and drains the areas supplied by this vessel. It receives several major branches, including the posterior cardiac vein, the middle cardiac vein, and the small cardiac vein. The posterior cardiac vein parallels and drains the areas supplied by the marginal artery branch of the circumflex artery. The middle cardiac vein parallels and drains the areas supplied by the posterior interventricular artery. The small cardiac vein parallels the right coronary artery and drains the blood from the posterior surfaces of the right atrium and ventricle. The coronary sinus is a large, thin-walled vein on the posterior surface of the heart lying within the atrioventricular sulcus and emptying directly into the right atrium. The anterior cardiac veins parallel the small cardiac arteries and drain the anterior surface of the right ventricle. Unlike these other cardiac veins, it bypasses the coronary sinus and drains directly into the right atrium.

Heart: Coronary Artery Disease

Coronary artery disease is the leading cause of death worldwide. It occurs when the buildup of plaque—a fatty material including cholesterol, connective tissue, white blood cells, and some smooth muscle cells—within the walls of the arteries obstructs the flow of blood and decreases the flexibility or compliance of the vessels. This condition is called atherosclerosis, a hardening of the arteries that involves the accumulation of plaque. As the coronary blood vessels become occluded, the flow of blood to the tissues will be restricted, a condition called ischemia that causes the cells to receive insufficient amounts of oxygen, called hypoxia. Figure 19.16 shows the blockage of coronary arteries highlighted by the injection of dye. Some individuals with coronary artery disease report pain radiating from the chest called angina pectoris, but others remain asymptomatic. If untreated, coronary artery disease can lead to MI or a heart attack.

The disease progresses slowly and often begins in children and can be seen as fatty “streaks” in the vessels. It then gradually progresses throughout life. Well-documented risk factors include smoking, family history, hypertension, obesity, diabetes, high alcohol consumption, lack of exercise, stress, and hyperlipidemia or high circulating levels of lipids in the blood. Treatments may include medication, changes to diet and exercise, angioplasty with a balloon catheter, insertion of a stent, or coronary bypass procedure.

Angioplasty is a procedure in which the occlusion is mechanically widened with a balloon. A specialized catheter with an expandable tip is inserted into a superficial vessel, normally in the leg, and then directed to the site of the occlusion. At this point, the balloon is inflated to compress the plaque material and to open the vessel to increase blood flow. Then, the balloon is deflated and retracted. A stent consisting of a specialized mesh is typically inserted at the site of occlusion to reinforce the weakened and damaged walls. Stent insertions have been routine in cardiology for more than 40 years.

Coronary bypass surgery may also be performed. This surgical procedure grafts a replacement vessel obtained from another, less vital portion of the body to bypass the occluded area. This procedure is clearly effective in treating patients experiencing a MI, but overall does not increase longevity. Nor does it seem advisable in patients with stable although diminished cardiac capacity since frequently loss of mental acuity occurs following the procedure. Long-term changes to behavior, emphasizing diet and exercise plus a medicine regime tailored to lower blood pressure, lower cholesterol and lipids, and reduce clotting are equally as effective.

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How the Heart Works
What the Heart Looks Like
How Blood Flows through the Heart
How the Heart Beats

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How the Heart Works The Heart

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The heart is an organ about the size of your fist that pumps blood through your body. It is made up of multiple layers of tissue.

Your heart is at the center of your circulatory system. This system is a network of blood vessels, such as arteries, veins, and capillaries, that carries blood to and from all areas of your body. Your blood carries the oxygen and nutrients that your organs need to work properly. Blood also carries carbon dioxide to your lungs so you can breathe it out. Inside your heart, valves keep blood flowing in the right direction.

Your heart’s electrical system controls the rate and rhythm of your heartbeat. A healthy heart supplies your body with the right amount of blood at the rate needed to work well. If disease or injury weakens your heart, your body’s organs will not receive enough blood to work normally. A problem with the electrical system — or the nervous or endocrine systems, which control your heart rate and blood pressure — can also make it harder for the heart to pump blood.

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Your heart does a lot of work to keep the body going. Learn about the organ’s amazing power and the functions of its many parts.

February 27, 2019 7:00 AM

Author | Michigan Medicine

This story was updated on January 31, 2020.

Each day, the average human heart beats about 100,000 times, pumping 2,000 gallons of blood through the body.

That's a lot of work for an organ no bigger than a large fist and weighing 8 to 12 ounces.

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In fact, the heart does more physical work than any other muscle over a lifetime.

Located between the lungs in the middle of the chest, the heart pumps blood through the network of arteries and veins known as the cardiovascular system. It pushes blood to the body's organs, tissues and cells.

Blood delivers oxygen and nutrients to every cell and removes the carbon dioxide and other waste products made by those cells.

Blood is carried from the heart to the rest of the body through a complex network of arteries, arterioles and capillaries. Blood is returned to the heart through venules and veins.

Parts of the human heart

The heart is made up of four chambers: two upper chambers known as the left atrium and right atrium and two lower chambers called the left and right ventricles.

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It is also made up of four valves: the tricuspid, pulmonary, mitral and aortic valves.

The right atrium receives non-oxygenated blood from the body's largest veins — superior vena cava and inferior vena cava — and pumps it through the tricuspid valve to the right ventricle.

The right ventricle pumps the blood through the pulmonary valve to the lungs, where it becomes oxygenated.

The left atrium receives oxygenated blood from the lungs and pumps it through the mitral valve to the left ventricle.

The left ventricle pumps oxygen-rich blood through the aortic valve to the aorta and the rest of the body.

The coronary arteries run along the surface of the heart and provide oxygen-rich blood to the heart muscle.

A web of nerve tissue also runs through the heart, conducting the complex signals that govern contraction and relaxation. A sac known as the pericardium surrounds the heart.

The outer layer of the pericardium surrounds the roots of the heart's major blood vessels, and the inner layer is attached to the heart muscle.

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Introduction

Shape and location

Pericardium, chambers of the heart, external surface of the heart.

Origin and development
Valves of the heart
Wall of the heart
Blood supply to the heart
Regulation of heartbeat
Electrocardiogram
Nervous control of the heart
The aorta and its principal branches
Superior vena cava and its tributaries
Inferior vena cava and its tributaries
Portal system
Venous pulmonary system
The capillaries
Human fetal circulation
Right-heart catheterization
Left-heart catheterization
Angiocardiography and arteriography
Noninvasive techniques

What are heart sounds?

human cardiovascular system

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Thompson Rivers University Pressbooks - Human Biology - Introduction to the Cardiovascular System
Healthline - Circulatory
Biology LibreTexts - Introduction to the Cardiovascular System
University of Hawai'i Pressbooks - Human Nutrition - The Cardiovascular System
Cleveland Clinic - Cardiovascular System
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cardiovascular system - Student Encyclopedia (Ages 11 and up)
Table Of Contents

human cardiovascular system , organ system that conveys blood through vessels to and from all parts of the body, carrying nutrients and oxygen to tissues and removing carbon dioxide and other wastes. It is a closed tubular system in which the blood is propelled by a muscular heart . Two circuits, the pulmonary and the systemic, consist of arterial , capillary , and venous components.

Understanding the human cardiovascular system

The primary function of the heart is to serve as a muscular pump propelling blood into and through vessels to and from all parts of the body. The arteries, which receive this blood at high pressure and velocity and conduct it throughout the body, have thick walls that are composed of elastic fibrous tissue and muscle cells. The arterial tree—the branching system of arteries—terminates in short, narrow, muscular vessels called arterioles , from which blood enters simple endothelial tubes (i.e., tubes formed of endothelial, or lining, cells) known as capillaries. These thin, microscopic capillaries are permeable to vital cellular nutrients and waste products that they receive and distribute. From the capillaries, the blood, now depleted of oxygen and burdened with waste products, moving more slowly and under low pressure , enters small vessels called venules that converge to form veins, ultimately guiding the blood on its way back to the heart.

This article describes the structure and function of the heart and blood vessels, and the technologies that are used to evaluate and monitor the health of these fundamental components of the human cardiovascular system. For a discussion of diseases affecting the heart and blood vessels, see the article cardiovascular disease . For a full treatment of the composition and physiologic function of blood, see blood , and for more information on diseases of the blood, see blood disease . To learn more about the human circulatory system , see systemic circulation and pulmonary circulation , and for more about cardiovascular and circulatory function in other living organisms, see circulation .

Description

The human heart and cardiovascular system explained

The adult human heart is normally slightly larger than a clenched fist, with average dimensions of about 13 × 9 × 6 cm (5 × 3.5 × 2.5 inches) and weight approximately 10.5 ounces (300 grams). It is cone-shaped, with the broad base directed upward and to the right and the apex pointing downward and to the left. It is located in the chest ( thoracic ) cavity behind the breastbone ( sternum ), in front of the windpipe ( trachea ), the esophagus , and the descending aorta , between the lungs , and above the diaphragm (the muscular partition between the chest and abdominal cavities). About two-thirds of the heart lies to the left of the midline.

The heart is suspended in its own membranous sac, the pericardium. The strong outer portion of the sac, or fibrous pericardium, is firmly attached to the diaphragm below, the mediastinal pleura on the side, and the sternum in front. It gradually blends with the coverings of the superior vena cava and the pulmonary (lung) arteries and veins leading to and from the heart. (The space between the lungs, the mediastinum , is bordered by the mediastinal pleura, a continuation of the membrane lining the chest. The superior vena cava is the principal channel for venous blood from the chest, arms, neck, and head.)

Smooth, serous (moisture-exuding) membrane lines the fibrous pericardium, then bends back and covers the heart. The portion of membrane lining the fibrous pericardium is known as the parietal serous layer (parietal pericardium), that covering the heart as the visceral serous layer (visceral pericardium or epicardium ).

The two layers of serous membrane are normally separated by only 10 to 15 ml (0.6 to 0.9 cubic inch) of pericardial fluid, which is secreted by the serous membranes. The slight space created by the separation is called the pericardial cavity . The pericardial fluid lubricates the two membranes with every beat of the heart as their surfaces glide over each other. Fluid is filtered into the pericardial space through both the visceral and parietal pericardia.

The heart is divided by septa, or partitions, into right and left halves, and each half is subdivided into two chambers. The upper chambers, the atria , are separated by a partition known as the interatrial septum; the lower chambers, the ventricles , are separated by the interventricular septum. The atria receive blood from various parts of the body and pass it into the ventricles. The ventricles, in turn, pump blood to the lungs and to the remainder of the body.

The right atrium , or right superior portion of the heart, is a thin-walled chamber receiving blood from all tissues except the lungs. Three veins empty into the right atrium, the superior and inferior venae cavae, bringing blood from the upper and lower portions of the body, respectively, and the coronary sinus, draining blood from the heart itself. Blood flows from the right atrium to the right ventricle. The right ventricle, the right inferior portion of the heart, is the chamber from which the pulmonary artery carries blood to the lungs.

The left atrium, the left superior portion of the heart, is slightly smaller than the right atrium and has a thicker wall. The left atrium receives the four pulmonary veins , which bring oxygenated blood from the lungs. Blood flows from the left atrium into the left ventricle. The left ventricle, the left inferior portion of the heart, has walls three times as thick as those of the right ventricle. Blood is forced from this chamber through the aorta to all parts of the body except the lungs.

Shallow grooves called the interventricular sulci , containing blood vessels, mark the separation between ventricles on the front and back surfaces of the heart. There are two grooves on the external surface of the heart. One, the atrioventricular groove, is along the line where the right atrium and the right ventricle meet; it contains a branch of the right coronary artery (the coronary arteries deliver blood to the heart muscle). The other, the anterior interventricular sulcus, runs along the line between the right and left ventricles and contains a branch of the left coronary artery.

On the posterior side of the heart surface, a groove called the posterior longitudinal sulcus marks the division between the right and left ventricles; it contains another branch of a coronary artery. A fourth groove, between the left atrium and ventricle, holds the coronary sinus, a channel for venous blood.

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Human Heart

Anatomy and functions of the human heart.

The human heart is the organ that pumps blood throughout the body via the vessels of the circulatory system, supplying oxygen and nutrients to the tissues and removing carbon dioxide and other wastes. Pumping the blood through the arteries, capillaries, and veins is the major function of the heart. It maintains proper circulation of blood. The human heart functions throughout a person’s lifespan and is responsible for the survival of living beings.

Human Heart Anatomy

The heart is almost the size of a large fist and weighs between about 280 to 340 grams in men and 230 to 280 grams in women.

Location of the Heart

The heart is situated between the lungs in the thoracic cavity. The name of this area is mediastinum. The cone-shaped heart base is at the top, behind the sternum, and the great vessels are entering or leaving here. The heart's apex (tip) points down and is just above the midline diaphragm to the left. That's why we might think of the heart as being on the left side because here you can hear or feel the strongest beat.

The heart is confined to the pericardial membranes. The fibrous pericardium is the outermost layer. It is a loose-fitting sac of strong fibrous connective tissue extending inferiorly over the diaphragm and superiorly over the bases of the large vessels entering and leaving the heart. Serous pericardium is a folded membrane; parietal and visceral layers are given by the fold.

The parietal pericardium is the lining of the fibrous pericardium. The visceral pericardium often called the epicardium, is located on the surface of the heart muscle. Serous fluid is present between the parietal and visceral pericardial membranes, preventing friction as the heartbeats.

Types of Circulation

The heart circulates blood mainly in two ways: pulmonary circulation and systemic circulation.

The Pulmonary Circulation: In the pulmonary circuit, deoxygenated blood leaves the right ventricle of the heart via the pulmonary artery and travels to the lungs, and then the oxygenated blood returns through the pulmonary vein to the left atrium of the heart.

The Systemic Circulation: In the systemic circuit, oxygenated blood leaves the heart and travels through the left ventricle to the aorta, and from there enters the arteries and capillaries where it supplies the body's tissues with oxygen.

There is also another type of circulation called coronary circulation in which oxygenated blood is supplied to the heart.

Layers of the Heart Wall

The three layers of the heart's wall are the epicardium (external layer), the myocardium (middle layer), and the endocardium (inner layer).

Epicardium: It is the outermost layer of the heart.

Myocardium: It is the middle layer of the heart and conducts the pumping action.

Endocardium: It is the innermost layer of the heart and thus covers heart valves.

Chambers of the Heart

The walls of the heart's four chambers are made of the myocardium called cardiac muscle. The chambers are lined with the endocardium, a simple squamous epithelium that also covers the heart valves and continues as its lining (endothelium) into the vessels. The endocardium's important physical feature is not its thinness, but its smoothness. This very smooth tissue prevents blood clotting as blood contact with a rough surface would initiate clotting.

The heart's upper chambers are the right and left atria, with relatively thin walls separated by a common myocardial wall called the interatrial septum. The lower chambers are the right and left ventricles with thicker walls and the interventricular septum separates them. As you can see, the atria receive blood from either the body or the lungs and the ventricles pump blood into the lungs or the body.

Right Atrium

The two big caval veins return blood to the right atrium from the body. The upper vena cava carries upper body blood, and the lower vena cava carries lower body blood. Blood flows into the right ventricle from the right atrium through the right atrioventricular (AV) valve or the tricuspid valve. The tricuspid valve consists of three endocardium flaps (or cusps) strengthened by connective tissue.

All valves in the circulatory system have the general purpose of preventing blood backflow. The purpose of the tricuspid valve is to prevent the flow of blood from the right ventricle to the right atrium when contracting the right ventricle. As the ventricle contracts, blood is forced to close the valve behind three-valve flaps.

Left Atrium

In the left atrium, blood comes from the lungs through four pulmonary veins. This blood flows through the left atrioventricular (AV) valve into the left ventricle, also known as the mitral valve or bicuspid valve. When the left ventricle contracts, the mitral valve prevents blood from the left ventricle to the left atrium.

Another function of the atria is to produce a hormone that is involved in maintaining blood pressure. When increased blood volume or blood pressure stretches atria's walls, the cells produce atrial natriuretic peptide (ANP), also known as the atrial natriuretic hormone (ANH).

ANP decreases kidney reabsorption of sodium ions to excrete more sodium ions in urine, which in turn increases water removal. Water loss reduces the volume of blood and blood pressure. You might have noticed that ANP is an antagonist to the aldosterone hormone, which increases blood pressure.

Right Ventricle

The tricuspid valve closes when the right ventricle contracts and the blood is pumped through the pulmonary artery (or trunk) to the lungs. The pulmonary semilunar valve is at the junction of this large artery and the right ventricle.

When the right ventricle contracts and pumps blood into the pulmonary artery, its three flaps are forced open. Blood tends to come back when the right ventricle relaxes, but this fills the valve flaps and closes the pulmonary valve to prevent blood from flowing back into the right ventricle.

Columns of the myocardium called papillary muscles are projecting into the lower part of the right ventricle. Fibrous connective tissue strands, the chordae tendineae, range from the papillary muscles to the tricuspid valve flaps. When the right ventricle contracts, the papillary muscles also contract and pull on the tendineae chordae to prevent the tricuspid valve from reversing.

If you've ever had a strong wind in your umbrella, you can see what would happen if the chordae tendineae and papillary muscles didn't anchor the flaps of the tricuspid valve.

Left Ventricle

The left ventricle walls are thicker than the right ventricle walls, allowing the left ventricle to contract more vigorously. The left ventricle, through the aorta, the body's largest artery, pumps blood to the body. At the junction between the aorta and the left ventricle is the aortic semilunar valve. The left ventricle's contraction force, which also closes the mitral valve, opens this valve.

When the left ventricle relaxes, the aortic valve closes to prevent blood from the aorta to the left ventricle. When the mitral valve closes, it prevents backflow of blood to the left atrium; the flaps of the mitral valve are also anchored by chordae tendineae and papillary muscles.

This is a fibrous connective tissue that anchors the valve flaps ' outer edges and prevents stretching of the valve openings. It also separates the atria and ventricles from the myocardium and prevents the contraction of the atria from reaching the ventricles except through the normal conduction path.

The right side of the heart receives deoxygenated blood from the body and pumps it into the lungs for oxygen collection and carbon dioxide release. The heart's left side receives oxygenated blood from the lungs, pumping it into the body. Both pumps work simultaneously, i.e. both atria and ventricles contract together.

Cardiac Conduction Pathway

The heart cycle is a sequence of mechanical events regulated by the myocardium's electrical activity. Cardiac muscle cells are capable of contracting spontaneously; there is no need for nerve impulses to cause contraction. The heart produces its own beat and the electrical impulses throughout the myocardium following a very specific route.

The heart's natural pacemaker is the sinoatrial (SA) node, a specialized group of heart muscle cells located in the right atrium wall just below the upper vena cava opening. The SA node is considered to be specialized because it has the fastest contraction rate, it depolarizes faster than any other part of the myocardium (60 to 80 times per minute).

The rapid entry of Na + ions and the reversal of charges on either side of the cell membrane is called depolarization. The SA node cells are more permeable to Na+ ions than any other muscle cells in the cardiac. They depolarize faster, then contract and initiate each heartbeat.

In the lower interatrial septum, impulses for contraction travel from the SA node to the atrioventricular (AV) node. The transmission of impulses from the SA node to the AV node results in atrial systole. Therefore, the only way for impulses from the atria to the ventricles is known as “bundle of his” or AV bundles.

In the upper interventricular septum, the AV bundle receives impulses from the AV node and communicates them to the right and left branches of the bundle. From the bundle branches, impulses travel along the fibers of Purkinje to the rest of the ventricular myocardium. The electrical activity of the atria and ventricles is easily depicted by an electrocardiogram (ECG).

If the SA node does not work properly, the heartbeat will be initiated by the AV node, but at a slower rate (50 to 60 beats per minute). The ventricle beat can also be generated by the AV bundle, but at a much slower rate (15 to 40 beats per minute). This can happen in certain types of heart disease that block the transmission of impulses from the atria to the ventricles.

A healthy adult has a 60 to 80 beats per minute resting heart rate (pulse), which is the SA node depolarization rate. A rate of less than 60 (with the exception of athletes) is called bradycardia while tachycardia is the state in which an extended or consistent rate of more than 100 beats per minute is observed.

This is a concise introduction to the anatomy and functioning of the human heart. This organ works non-stop since birth and circulates blood to every nook and corner of the human body.

FAQs on Human Heart

1. What are the chambers of the human heart?

There are four chambers of the human heart: left atrium, right atrium, left ventricle, and right ventricle. Atria are the parts that are thin and contain less muscular walls. They are smaller than ventricles. Ventricles are comparatively larger and more muscular chambers than atria and they are responsible for pumping blood out to the circulation.

2. Define in brief the structure of the Human Heart?

The heart is divided into four chambers, namely two ventricles and two atria. The ventricles are the types of chambers that pump blood and the atrium are the chambers that receive the blood. The Human heart consists of a wall and the wall is made up of three layers: epicardium, myocardium, and endocardium.

3. Explain the three layers of the heart wall.

The human heart wall is made up of three layers:

Epicardium – Epicardium is the outermost layer of the human heart. Epicardium is composed of a thin layer of membrane that protects and lubricates the outer section.

Myocardium – Myocardium is a layer of muscle tissue that constitutes the middle layer wall of the heart. The myocardium is also responsible for the heart’s pumping action.

Endocardium – Endocardium innermost layer that lines the inner heart chambers and covers the heart valves. Endocardium also prevents blood from sticking, thereby avoiding the formation of fatal blood clots.

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August 21, 2024

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What the unique shape of the human heart tells us about our evolution

by Aimee Drane, The Conversation

Mammals, from the mighty blue whale to the tiny shrew, inhabit nearly every corner of our planet. Their remarkable adaptability to different environments has long fascinated scientists, with each species developing unique traits to survive and thrive.

Despite such vast biological diversity, it was believed until recently that the structure and function of the heart across mammals was the same. But research from my colleagues and I reveals that the human heart is an outlier, distinctly different from those of our closest relatives, the great apes, including chimpanzees, bonobos, orangutans and gorillas.

So why are humans the odd ones out?

Humans diverged from chimpanzees (Pan Troglodytes), our last common ancestor, between five and six million years ago. In contrast, people evolved to stand upright to engage in greater amounts of activity, such as persistence hunting. And we have developed considerably larger brains.

These changes to humans' bodies were associated with a much greater metabolic demand, requiring more blood to be pumped to our muscles and brain. Our research suggests that the human heart has adapted to support our upright stance, movement and larger brain.

Over the last 10 years, we have been conducting assessments of the cardiovascular system of great apes across the globe. We have been fortunate enough to work with dedicated veterinarians and care staff in the UK, Europe, Africa and Asia. An important aspect of these assessments has been the use of cardiac ultrasound through which we can assess the structure and function of the heart, how large it is and how the muscle contracts, twists and rotates.

Our previous research has suggested that the structure of the human heart may be different to that of the chimpanzee. Using cardiac ultrasound, we discovered that the left ventricle —the heart's main pumping chamber—in chimpanzees contains bundles of muscles arranged in a mesh known as "trabeculations."

For our current study, we wanted to examine if trabeculations exist in the other great apes, which we found that they do. In contrast, humans have a smooth wall of the left ventricle. This difference is especially pronounced at the bottom of the left ventricle, where the human heart's smoothness is nearly four times greater than that of our great ape relatives.

Our research didn't just reveal structural differences in the human left ventricle compared to that of the great apes; we also uncovered an important difference in function. By using a specialized technique called " speckle-tracking echocardiography, " which tracks heart muscle movement during contraction and relaxation, we examined how the muscle thickens, twists, rotates and lengthens.

The results were striking. Humans, who have the least trabeculation, exhibited much greater twist and rotation at the apex (the tip of the heart) during contraction. In contrast, non-human great apes, with their heavily trabeculated hearts, showed much less movement.

We believe the human heart evolved away from the trabeculated structure seen in the other great apes to enhance its ability to twist and contract more efficiently. This increased twisting motion, along with the smooth ventricular walls, probably allows the human heart to pump a larger volume of blood with each beat. This meets the heightened demands of our physical activity and larger brains.

Our research challenges the assumption that heart structure is uniform across mammals. Instead, subtle but crucial differences in heart anatomy and function have emerged in response to unique environmental challenges.

Cardiac disease

While our current research has shed light on the evolution of the human heart, our work analyzing the hearts of endangered great apes continues. Sadly, cardiac disease is the leading cause of death in captive great apes.

Unlike humans, great apes do not appear to develop coronary artery disease. Rather, their heart muscle undergoes a fibrotic, or thickening, process which causes poor contraction and a susceptibility to arrythmia, which is a problem with the rhythm of the heartbeat. The cause of this disease is unknown. So, we at the International Primate Heart Project have been conducting assessments of great ape cardiovascular physiology across the globe to further understand the disease.

Before our involvement, little was known about the normal cardiovascular physiology of great apes. Through collaboration with veterinary practitioners, our research has generated vital data, significantly improving our understanding of human heart evolution, as well as the understanding, diagnosis and management of heart disease in great apes.

Provided by The Conversation

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IMAGES

Activity 1
Heart Anatomy Labeled Diagram
Heart Diagram and Blood Flow WS FA22
human-heart-diagram
10Th standard science lesson 14 human heart work
The human heart

COMMENTS

Human Heart
The human heart is one of the most important organs responsible for sustaining life. It is a muscular organ with four chambers. The size of the heart is the size of about a clenched fist. The human heart functions throughout a person's lifespan and is one of the most robust and hardest working muscles in the human body.
Heart (Human Anatomy): Overview, Function & Structure
The heart is a muscular organ that pumps blood throughout the body. It is located in the middle cavity of the chest, between the lungs. In most people, the heart is located on the left side of the chest, beneath the breastbone. The heart is composed of smooth muscle. It has four chambers which contract in a specific order, allowing the human ...
Heart
heart, organ that serves as a pump to circulate the blood.It may be a straight tube, as in spiders and annelid worms, or a somewhat more elaborate structure with one or more receiving chambers (atria) and a main pumping chamber (ventricle), as in mollusks. In fishes the heart is a folded tube, with three or four enlarged areas that correspond to the chambers in the mammalian heart.
The Heart
The muscular wall of the heart has three layers. The outermost layer is the epicardium (or visceral pericardium). The epicardium covers the heart, wraps around the roots of the great blood vessels, and adheres the heart wall to a protective sac. The middle layer is the myocardium. This strong muscle tissue powers the heart's pumping action.
Heart anatomy: Structure, valves, coronary vessels
Heart anatomy. The heart has five surfaces: base (posterior), diaphragmatic (inferior), sternocostal (anterior), and left and right pulmonary surfaces. It also has several margins: right, left, superior, and inferior: The right margin is the small section of the right atrium that extends between the superior and inferior vena cava .
Human heart: Anatomy, function & facts
Jump to: Anatomy. Function. Heartworm. Additional resources. The human heart is an organ that pumps blood throughout the body via the vessels of the circulatory system, supplying oxygen and ...
Heart Anatomy: Labeled Diagram and Quick Reference
Heart Anatomy in Basic Terms. The heart is a crucial organ that functions as the body's pump, ensuring blood circulation throughout the body. It consists of four main chambers: Left and right atria (upper chambers) Left and right ventricles (lower chambers) These chambers work in a coordinated manner to receive oxygen-poor blood, pump it to the ...
Anatomy of the human heart
Anatomy of the human heart. The heart is a muscular organ situated in the mediastinum. It consists of four chambers, four valves, two main arteries (the coronary arteries), and the conduction system. The left and right sides of the heart have different functions: the right side receives de-oxygenated blood through the superior and inferior ...
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17.2: Heart Anatomy. The vital importance of the heart is obvious. If one assumes an average rate of contraction of 75 contractions per minute, a human heart would contract approximately 108,000 times in one day, more than 39 million times in one year, and nearly 3 billion times during a 75-year lifespan.
19.1 Heart Anatomy
CPR. The position of the heart in the torso between the vertebrae and sternum (see Figure 19.2 for the position of the heart within the thorax) allows for individuals to apply an emergency technique known as cardiopulmonary resuscitation (CPR) if the heart of a patient should stop. By applying pressure with the flat portion of one hand on the sternum in the area between the line at T4 and T9 ...
How the Heart Works
The Heart. The heart is an organ about the size of your fist that pumps blood through your body. It is made up of multiple layers of tissue. Your heart is at the center of your circulatory system. This system is a network of blood vessels, such as arteries, veins, and capillaries, that carries blood to and from all areas of your body.
Anatomy of a Human Heart
Located between the lungs in the middle of the chest, the heart pumps blood through the network of arteries and veins known as the cardiovascular system. It pushes blood to the body's organs, tissues and cells. Blood delivers oxygen and nutrients to every cell and removes the carbon dioxide and other waste products made by those cells.
Heart Anatomy: Labeled Diagram, Structures, Blood Flow ...
Antibiotic Classes and Drug Names: The Pharmacology Trick. Function and anatomy of the heart made easy using labeled diagrams of cardiac structures and blood flow through the atria, ventricles, valves, aorta, pulmonary arteries veins, superior inferior vena cava, and chambers. Includes an exercise, review worksheet, quiz, and model drawing of ...
Human cardiovascular system
human cardiovascular system, organ system that conveys blood through vessels to and from all parts of the body, carrying nutrients and oxygen to tissues and removing carbon dioxide and other wastes. It is a closed tubular system in which the blood is propelled by a muscular heart. Two circuits, the pulmonary and the systemic, consist of ...
human heart structure and function
HUMAN HEART Heart is a muscular organ responsible for pumping blood through blood vessels by repeated contractions. The term "cardiac" means "related to the ...
Human Heart
The human heart is the organ that pumps blood throughout the body via the vessels of the circulatory system, supplying oxygen and nutrients to the tissues and removing carbon dioxide and other wastes. Pumping the blood through the arteries, capillaries, and veins is the major function of the heart. It maintains proper circulation of blood.
What the unique shape of the human heart tells us about our evolution
Our research suggests that the human heart has adapted to support our upright stance, movement and larger brain. Over the last 10 years, we have been conducting assessments of the cardiovascular ...

Human Heart

Types of Circulation

Introduction to the Human Heart

Position of Heart in Human Body

The Function of Heart

Structure of the Human Heart

External Structure of Heart

Internal Structure of Heart

Frequently Asked Questions

2. Define systemic circulation.

3. Elaborate on coronary circulation and its significance.

4. Briefly explain the structure of the human heart.

5. Name the chambers of the heart.

7. Explain the three layers of the heart wall.

8. Explain the three major blood vessels of the human body.

9. What is the function of the heart valves? Provide examples of various valves.

10. What is meant by myocardial infarction?

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Heart Definition

Function of the Heart

Heart Structure

Layers of the Heart Wall

Chambers of the Heart

Valves of the Heart

The Sinoatrial Node

Cite This Article

At the Heart of It All: Anatomy and Function of the Heart

1. The Heart Wall Is Composed of Three Layers

2. The Atria Are the Heart’s Entryways for Blood

3. Each Heart Beat Is a Squeeze of Two Chambers Called Ventricles

4. The Valves Are Like Doors to the Chambers of the Heart

5. The Cardiac Cycle Includes All Blood Flow Events the Heart Accomplishes in One Complete Heartbeat

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