Which heart chamber has the thickest muscle wall? | Reference.com Which heart chamber has the thickest muscle wall? A: Quick Answer The heart chamber with the thickest muscle wall is the left ventricle. Ventricles are the lower two chambers, whereas the atria are the upper two chambers of the heart. The two ventricles have thicker muscular walls than the two atria. Full Answer The atria receive blood from the veins. When they contract, blood transfers from the atria to the ventricles, so the atria do not need to be as muscular as the ventricles.The ventricles of the heart have thicker muscular walls because they pump blood throughout the body. The pressure created in the ventricles causes blood to rush into the large arteries of the body such as the aorta and the pulmonary artery. The left ventricle is twice as muscular as the right because it has to do more work by pumping blood to all the organs of the body. In contrast, the right ventricle only pumps deoxygenated blood to the lungs, which are a short distance away.

Heart Anatomy & Blood Flow - Heart Anatomy - HealthCommunities.com Heart Anatomy & Blood Flow Print Anatomy of the Heart The heart is the hollow, muscular organ in the thoracic cavity (chest) that maintains the circulation of blood throughout the body. It is surrounded by a membrane called the pericardium. The pericardium consists of a layer of fibrous connective tissue and a layer of thin, serous (i.e., produces a secretion) tissue and is attached to the vena cava, the aorta, the diaphragm, and the sternum. The pericardial cavitythe potential space between the pericardium and the heartcontains the watery pericardial fluid. This fluid prevents friction between the pericardium and the heart. The heart wall consists of the epicardium (inner layer), the myocardium (middle layer comprised of cardiac muscle tissue), and the endocardium (lining of the myocardium that covers the heart valves). The heart has a right side and a left side. Each side has a relatively thin-walled chamber that receives blood returning to the heart (atrium) and a muscular chamber that pumps blood out of the heart (ventricle).

Blood Flow The flow of blood through the heart is controlled by the opening and closing of valves and the contraction and relaxation of the myocardium. Heart valves are controlled by pressure changes within each chamber and contraction and relaxation are controlled by the heart's conduction system. Blood that has traveled through the body returns to the heart and enters the right atrium. This blood flows through the tricuspid valve into the right ventricle. The right ventricle pumps the blood to the lungs, where it absorbs oxygen. Oxygen-rich blood returns from the lungs and enters the heart through the left atrium. Blood passes from the left atrium through the mitral valve and into the left ventricle. The left ventricle, the largest and most muscular of the four chambers, is the main pumping chamber of the heart. When the left ventricle contracts, blood is pumped through the aortic valve into the main artery of the body (aorta). The aorta supplies blood to smaller arteries that travel to the head, arms, abdomen, and legs. These arteries supply oxygen-rich blood to the organs and tissues of the body, which require oxygen to function. The coronary arteries supply oxygen-rich blood to the tissues of the heart.

Oxygen-poor blood travels from organs and tissues to the heart through veins. The vena cava is the major vein that returns blood to the right atrium of the heart. The vena cava superior returns blood from the head, neck, upper extremities, and chest. The vena cava inferior returns blood from the lower extremities, the pelvis, and the abdomen. The coronary sinus drains blood from the coronary arteries into the right atrium. Conduction System An electrical impulse travels through the heart and initiates contractions of the chambers. The heart's "spark plug" is an area of specialized heart tissue called the sinoatrial node (SA node), which is located in the right atrium. Each time the SA node "fires," an electrical impulse is generated that travels through the right and left atria, signaling these chambers to contract and pump blood into the ventricles. The impulse then travels into another area of specialized heart tissue called the atrioventricular node (AV node), which is located between the atria and the ventricles.

The electrical impulse is conducted through the AV node and wire-like pathways (Purkinje fibers) to the ventricles, signaling the ventricles to contract and pump blood into the lungs and throughout the body. The normal sequence of electrical activation of the chambers of the heart is called sinus rhythm. It occurs each time the heart beats, usually about 60 to 80 times every minute. In a normal heartbeat, the atria contract simultaneously while the ventricles relax. Then, the ventricles relax and the atria contract. The term systole refers to contraction and the term diastole refers to relaxation. A heartbeat consists of the systole and diastole of the atria and the systole and diastole of the ventricles. Publication Review By: the Editorial Staff at HealthCommunities.com Published: 28 Feb 2002 The Human Heart, Health Resource Center, CSULB Health Resource Center HRC Blog The Human Heart The heart is one of the major parts of our body. It is as small as your fist but does a major job in our body. The heart delivers blood throughout our bodies, by arteries, capillaries and veins.

It is important for us to understand how it works and how to take care of our heart. The heart is a strong, muscular pump that continuously works around the clock. The heart can be seen as the power needed for life. Each day the average heart "beats," or expands and contracts, 100,000 times and pumps about 2,000 gallons of blood. In an average lifetime, the human heart beats more than 2.5 billion times. Anatomy of the Heart The heart is divided into four chambers. The upper two are the right and left atria ; the lower two, the right and left ventricles. The heart also has four valves that open and close to allow blood to flow in only one direction when the heart contracts (beats). The four heart valves are: the tricuspid valve, located between the right atrium and right ventricle; the pulmonary or pulmonic valve, between the right ventricle and the pulmonary artery; the mitral valve, between the left atrium and left ventricle; the aortic valve, between the left ventricle and the aorta. Each valve has a set of flaps (also called leaflets or cusps).

The right side of the heart receives dark, bluish blood from the superior and inferior vena cava. It pumps this blood to the lungs where waste gas (carbon dioxide) is removed and oxygen is picked up. The bright red oxygenated blood returns to the left side of the heart which pumps it out through the aorta to be distributed by smaller arteries to all parts of the body. The cardiovascular system, comprising both the heart and the blood vessels, circulates blood throughout the body. It carries essential supplies of food and fuel to every living cell and exchanges them for potentially harmful waste products. The adult circulation consists of thousands of miles of tubing containing about 10 pints (4.7l) of blood. The blood is kept flowing round the body by the pumping action of the heart. Blood is made up of a pale yellow liquid (plasma) containing dissolved nutrients and wastes, plus blood cells, hormones, proteins and other substances. Most numerous of these cells are the disk-shaped red blood cells. Their color comes from the presence of the substance hemoglobin, which combines with oxygen. When red corpuscles charged with oxygen approach body cells, the oxygen is delivered in exchange for the waste product carbon dioxide.

Other two-way transportation of materials takes place between the body cells and the plasma, and all unwanted substances are carried away in the blood for excretion by the kidneys, lungs, and liver. Plasma also contains white blood cells, which help to fight infection, and platelets, which are involved in blood clotting. Blood Flow In its passage through the body, blood is carried in tubes known as arteries and veins. Most arteries transport oxygen-rich (oxygenated) blood, whereas most veins transport carbon dioxide-rich (deoxygenated) blood. The largest artery is the aorta, which stems directly from the heart. The aorta and other large arteries have thick walls lined with muscle. Blood flow is assisted by the contraction of this muscle and the impetus given by the heartbeat. The "push" from the heart can be felt as the pulse wherever large arteries run near the body surface. As they penetrate the tissues, arteries split into narrow branches called arterioles, which in turn divide into capillaries. It is through the very thin capillary walls that the blood gives up its oxygen and nutrients and receives carbon dioxide and wastes. Deoxygenated blood in the capillaries flows into narrow veins (venules), and then into veins.

The two largest veins, the venae cavae, return this blood to the heart. Veins have thin walls compared with those of arteries, and blood moves through the veins much more slowly. Blood flow in the veins is assisted by the action of muscles in surrounding tissues, and backflow is prevented by one-way valves. The walls of arteries and veins comprise the same four tissues, but arteries have more muscle to withstand more blood pressure. Blood flow in veins is aided by nearby muscles; valves prevent backflow. Blood flow in the heart is controlled by valves. The tricuspid valve controls flow from right atrium to right ventricle; the mitral valve controls flow from left atrium to left ventricle. Valves also guard the entrances to the aorta and pulmonary artery in order to prevent backflow of blood after the heart contracts. The deoxygenated blood delivered to the heart along the veins is no use to body cells until it has been recharged with oxygen. To ensure reoxygenation, the circulation has a second "loop." In this part of the system, blood rich in carbon dioxide travels from the heart along the pulmonary artery to the lungs, where carbon dioxide is exchanged for oxygen breathed in.

The pulmonary artery is the only artery to carry deoxygenated blood. The newly oxygenated blood is carried back to the heart along the pulmonary vein, the only vein to transport oxygenated blood. The heart is a muscular organ about the size of a clenched fist. The structure and action of the heart are designed to serve the two loops of the circulation. Inside, the heart is divided vertically by a muscular wall. On each side of this wall is an upper chamber (atrium) and a thicker, lower chamber (ventricle). Blood moves through each side of the heart systematically. Deoxygenated blood is delivered into the right atrium. It then enters the right ventricle, from where it is pumped out into the pulmonary artery and to the lungs. Oxygenated blood returning in the pulmonary veins flows into the left atrium. This blood enters the left ventricle and is then pumped into the aorta for circulation. The flow of blood in each side of the heart is controlled by a series of valves. The pumping action of the heart is achieved by the contraction of the cardiac muscle, of which the heart is largely composed.

The rhythm of the heartbeat is regulated by bursts of electrical impulses sent out by a concentration of specialized heart tissue called the pacemaker. Under the influence of the pacemaker, the heart of an adult at rest beats at a rate of 60 to 80 beats a minute. The pacemaker also helps to ensure the correct sequence of activities during each heartbeat; first the two atria contract, followed rapidly by the ventricles. The powerful contraction of the ventricles pushes blood into the aorta and pulmonary artery. This period of contraction (systole) is followed by a period of relaxation (diastole), during which the heart refills. The complete sequence is accompanied by electrical activity of the muscle, which can be monitored as an electrocardiogram (EKG). An electrocardiogram (EKG) records the waves of nerve impulses of each heartbeat. The P wave starts just before the atria contract; the QRS wave indicates the contraction of the ventricles; and the T wave indicates the recovery period before the next contraction. Aorta Aorta is the largest artery in the body, supplying blood to the organs alongside its course.

It starts at the left ventricle of the heart, then arches upward and backward, giving off branch arteries to the heart muscles, head, and arms. The aorta then runs down the back of the chest, in front of the spine and esophagus (gullet), to reach the abdomen. Just above the pelvis, the aorta divides into the common iliac arteries to the legs Vena cava is one of the two veins (superior vena cava and inferior vena cava) that drain blood from all parts of the body (except the heart) into the right atrium of the heart. The superior vena cava returns blood from the head, neck, and arms to the heart. The inferior vena cava returns blood from the chest, abdomen, and legs to the heart. High Blood Pressure Researchers in recent years have made great strides in the understanding and treatment of high blood pressure (hypertension). High blood pressure is a major risk factor for heart attack, heart failure, stroke, and kidney failure. Heart disease and stroke are the first and third leading causes of death, respectively, in the United States, according to the National Heart, Lung, and Blood Institute.

Significant increases in hypertension awareness, treatment and control rates have occurred since the 1960s. Despite those advances, the declines in stroke and heart attack in the U.S. during the last 30 years have leveled off and congestive heart failure and kidney failure are rising. References and Resources The Human Heart The Human Heart The heart is one of the most important organs in the entire human body. It is really nothing more than a pump, composed of muscle which pumps blood throughout the body, beating approximately 72 times per minute of our lives. The heart pumps the blood, which carries all the vital materials which help our bodies function and removes the waste products that we do not need. For example, the brain requires oxygen and glucose, which, if not received continuously, will cause it to loose consciousness. Muscles need oxygen, glucose and amino acids, as well as the proper ratio of sodium, calcium and potassium salts in order to contract normally. The glands need sufficient supplies of raw materials from which to manufacture the specific secretions. If the heart ever ceases to pump blood the body begins to shut down and after a very short period of time will die. The heart is essentially a muscle(a little larger than the fist).

Like any other muscle in the human body, it contracts and expands. Unlike skeletal muscles, however, the heart works on the "All -or-Nothing Law". That is, each time the heart contracts it does so with all its force. In skeletal muscles, the principle of "gradation" is present. The pumping of the heart is called the Cardiac Cycle, which occurs about 72 times per minute. This means that each cycle lasts about eight-tenths of a second. During this cycle the entire heart actually rests for about four-tenths of a second. Make-up of the Heart. The walls of the heart are made up of three layers, while the cavity is divided into four parts. There are two upper chambers, called the right and left atria, and two lower chambers, called the right and left ventricles. The Right Atrium, as it is called, receives blood from the upper and lower body through the superior vena cava and the inferior vena cava, respectively, and from the heart muscle itself through the coronary sinus. The right atrium is the larger of the two atria, having very thin walls.

The right atrium opens into the right ventricle through the right atrioventicular valve(tricuspid), which only allows the blood to flow from the atria into the ventricle, but not in the reverse direction. The right ventricle pumps the blood to the lungs to be reoxygenated. The left atrium receives blood from the lungs via the four pulmonary veins. It is smaller than the right atrium, but has thicker walls. The valve between the left atrium and the left ventricle, the left atrioventicular valve(bicuspid), is smaller than the tricuspid. It opens into the left ventricle and again is a one way valve. The left ventricle pumps the blood throughout the body. It is the Aorta, the largest artery in the body, which originates from the left ventricle. The Heart works as a pump moving blood around in our bodies to nourish every cell. Used blood, that is blood that has already been to the cells and has given up its nutrients to them, is drawn from the body by the right half of the heart, and then sent to the lungs to be reoxygenated.

Blood that has been reoxygenated by the lungs is drawn into the left side of the heart and then pumped into the blood stream. It is the atria that draw the blood from the lungs and body, and the ventricles that pump it to the lungs and body. The output of each ventricle per beat is about 70 ml, or about 2 tablespoons. In a trained athlete this amount is about double. With the average heart rate of 72 beats per minute the heart will pump about 5 litres per ventricle, or about 10 litres total per minute. This is called the cardiac output. In a trained athlete the total cardiac output is about 20 litres. If we multiply the normal, non-athlete output by the average age of 70 years, we see that the cardiac output of the average human heart over a life time would be about 1 million litres, or about 250,000 gallons(US)! Anatomy of the Heart - Texas Heart Institute Heart Information Center Four valves regulate blood flow through your heart: The tricuspid valve regulates blood flow between the right atrium and right ventricle.

The pulmonary valve controls blood flow from the right ventricle into the pulmonary arteries, which carry blood to your lungs to pick up oxygen. The mitral valve lets oxygen-rich blood from your lungs pass from the left atrium into the left ventricle. The aortic valve opens the way for oxygen-rich blood to pass from the left ventricle into the aorta, your body's largest artery. See also on this site: The Heartbeat The Conduction System (illustration) Electrical impulses from your heart muscle (the myocardium) cause your heart to contract. This electrical signal begins in the sinoatrial (SA) node, located at the top of the right atrium. The SA node is sometimes called the heart's "natural pacemaker." An electrical impulse from this natural pacemaker travels through the muscle fibers of the atria and ventricles, causing them to contract. Although the SA node sends electrical impulses at a certain rate, your heart rate may still change depending on physical demands, stress, or hormonal factors. The Circulatory System (illustration) The heart and circulatory system make up your cardiovascular system. Your heart works as a pump that pushes blood to the organs, tissues, and cells of your body.

Blood delivers oxygen and nutrients to every cell and removes the carbon dioxide and waste products made by those cells. Blood is carried from your heart to the rest of your body through a complex network of arteries, arterioles, and capillaries. Blood is returned to your heart through venules and veins. If all the vessels of this network in your body were laid end-to-end, they would extend for about 60,000 miles (more than 96,500 kilometers), which is far enough to circle the earth more than twice! See also on other sites: MedlinePlus Chambers of the Heart Chambers of the heart Chambers of the heart The heart has four chambers: two atria and two ventricles. The right atrium receives oxygen-poor blood from the body and pumps it to the right ventricle. The right ventricle pumps the oxygen-poor blood to the lungs . The left atrium receives oxygen-rich blood from the lungs and pumps it to the left ventricle. The left ventricle pumps the oxygen-rich blood to the body. Credits Primary Medical ReviewerWilliam H.

Blahd, Jr., MD, FACEP - Emergency Medicine Adam Husney, MD - Family Medicine Specialist Medical ReviewerDavid Messenger, MD H. Michael O'Connor, MD - Emergency Medicine Martin J. Gabica, MD - Family Medicine Current as ofOctober 1, 2015 WebMD Medical Reference from Healthwise This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.© 1995-2015 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated. Top Picks SEER Training:Structure of the Heart Structure of the Heart Updates Structure of the Heart The human heart is a four-chambered muscular organ, shaped and sized roughly like a man's closed fist with two-thirds of the mass to the left of midline. The heart is enclosed in a pericardial sac that is lined with the parietal layers of a serous membrane. The visceral layer of the serous membrane forms the epicardium. Layers of the Heart Wall Three layers of tissue form the heart wall.

The outer layer of the heart wall is the epicardium, the middle layer is the myocardium, and the inner layer is the endocardium. Chambers of the Heart The internal cavity of the heart is divided into four chambers: Right atrium Left atrium Left ventricle The two atria are thin-walled chambers that receive blood from the veins. The two ventricles are thick-walled chambers that forcefully pump blood out of the heart. Differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which reflects the amount of force each chamber is required to generate. The right atrium receives deoxygenated blood from systemic veins; the left atrium receives oxygenated blood from the pulmonary veins. Valves of the Heart Pumps need a set of valves to keep the fluid flowing in one direction and the heart is no exception. The heart has two types of valves that keep the blood flowing in the correct direction. The valves between the atria and ventricles are called atrioventricular valves (also called cuspid valves), while those at the bases of the large vessels leaving the ventricles are called semilunar valves.

The right atrioventricular valve is the tricuspid valve. The left atrioventricular valve is the bicuspid, or mitral, valve. The valve between the right ventricle and pulmonary trunk is the pulmonary semilunar valve. The valve between the left ventricle and the aorta is the aortic semilunar valve. When the ventricles contract, atrioventricular valves close to prevent blood from flowing back into the atria. When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles. Pathway of Blood through the Heart While it is convenient to describe the flow of blood through the right side of the heart and then through the left side, it is important to realize that both atria and ventricles contract at the same time. The heart works as two pumps, one on the right and one on the left, working simultaneously. Blood flows from the right atrium to the right ventricle, and then is pumped to the lungs to receive oxygen. From the lungs, the blood flows to the left atrium, then to the left ventricle. From there it is pumped to the systemic circulation.

Blood Supply to the Myocardium The myocardium of the heart wall is a working muscle that needs a continuous supply of oxygen and nutrients to function efficiently. For this reason, cardiac muscle has an extensive network of blood vessels to bring oxygen to the contracting cells and to remove waste products. The right and left coronary arteries, branches of the ascending aorta, supply blood to the walls of the myocardium. After blood passes through the capillaries in the myocardium, it enters a system of cardiac (coronary) veins. Most of the cardiac veins drain into the coronary sinus, which opens into the right atrium. Anatomy of the Heart - NHLBI, NIH Anatomy of the Heart Share this page from the NHLBI on Blogger. Share this page from the NHLBI on Buzz. Share this page from the NHLBI on Delicious. Share this page from the NHLBI on Digg. Share this page from the NHLBI on Facebook. Add this link to the NHLBI to my browser. Email this page with Gmail. Bookmark this page with Google. Share this page from the NHLBI on LinkedIn. Share this page from the NHLBI on Messenger. Share this page from the NHLBI on MySpace. Share this page from the NHLBI on Reddit.

Share this page from the NHLBI on Stumble. Share this page from the NHLBI on Tumblr. Share this page from the NHLBI on Twitter. Anatomy of the Heart Your heart is located under your ribcage in the center of your chest between your right and left lungs. Its muscular walls beat, or contract, pumping blood to all parts of your body. The size of your heart can vary depending on your age, size, and the condition of your heart. A normal, healthy, adult heart usually is the size of an average clenched adult fist. Some diseases can cause the heart to enlarge. The Exterior of the Heart Below is a picture of the outside of a normal, healthy, human heart. Heart Exterior Figure A shows the location of the heart in the body. Figure B shows the front surface of the heart, including the coronary arteries and major blood vessels. In figure B, the heart is the muscle in the lower half of the picture. The heart has four chambers. The heart's upper chambers, the right and left atria (AY-tree-uh), are shown in purple. The heart's lower chambers, the right and left ventricles (VEN-trih-kuls), are shown in red.

Some of the main blood vessels (arteries and veins) that make up your circulatory system are directly connected to the heart. The Right Side of Your Heart In figure B above, the superior and inferior vena cavae are shown in blue to the left of the heart muscle as you look at the picture. These veins are the largest veins in your body. After your body's organs and tissues have used the oxygen in your blood, the vena cavae carry the oxygen-poor blood back to the right atrium of your heart. The superior vena cava carries oxygen-poor blood from the upper parts of your body, including your head, chest, arms, and neck. The inferior vena cava carries oxygen-poor blood from the lower parts of your body. The oxygen-poor blood from the vena cavae flows into your heart's right atrium and then to the right ventricle. From the right ventricle, the blood is pumped through the pulmonary (PULL-mun-ary) arteries (shown in blue in the center of figure B) to your lungs. Once in the lungs, the blood travels through many small, thin blood vessels called capillaries.

There, the blood picks up more oxygen and transfers carbon dioxide to the lungs—a process called gas exchange. To learn more about gas exchange, go to the Health Topics How the Lungs Work article. The oxygen-rich blood passes from your lungs back to your heart through the pulmonary veins (shown in red to the left of the right atrium in figure B). The Left Side of Your Heart Oxygen-rich blood from your lungs passes through the pulmonary veins (shown in red to the right of the left atrium in figure B above). The blood enters the left atrium and is pumped into the left ventricle. From the left ventricle, the oxygen-rich blood is pumped to the rest of your body through the aorta. The aorta is the main artery that carries oxygen-rich blood to your body. Like all of your organs, your heart needs oxygen-rich blood. As blood is pumped out of your heart's left ventricle, some of it flows into the coronary arteries (shown in red in figure B). Your coronary arteries are located on your heart's surface at the beginning of the aorta. They carry oxygen-rich blood to all parts of your heart.

The Interior of the Heart Below is a picture of the inside of a normal, healthy, human heart. Heart Interior Figure A shows the location of the heart in the body. Figure B shows a cross-section of a healthy heart and its inside structures. The blue arrow shows the direction in which oxygen-poor blood flows through the heart to the lungs. The red arrow shows the direction in which oxygen-rich blood flows from the lungs into the heart and then out to the body. Heart Chambers Figure B shows the inside of your heart and how it's divided into four chambers. The two upper chambers of your heart are called the atria. They receive and collect blood. The two lower chambers of your heart are called ventricles. The ventricles pump blood out of your heart to other parts of your body. The Septum An internal wall of tissue divides the right and left sides of your heart. This wall is called the septum. The area of the septum that divides the atria is called the atrial or interatrial septum. The area of the septum that divides the ventricles is called the ventricular or interventricular septum. Heart Valves Figure B shows your heart's four valves.

Shown counterclockwise in the picture, the valves include the aortic (ay-OR-tik) valve, the tricuspid (tri-CUSS-pid) valve, the pulmonary valve, and the mitral (MI-trul) valve. Blood Flow The arrows in figure B show the direction that blood flows through your heart. The light blue arrow shows that blood enters the right atrium of your heart from the superior and inferior vena cavae. From the right atrium, blood is pumped into the right ventricle. From the right ventricle, blood is pumped to your lungs through the pulmonary arteries. The light red arrow shows oxygen-rich blood coming from your lungs through the pulmonary veins into your heart's left atrium. From the left atrium, the blood is pumped into the left ventricle. The left ventricle pumps the blood to the rest of your body through the aorta. For the heart to work well, your blood must flow in only one direction. Your heart's valves make this possible. Both of your heart's ventricles have an "in" (inlet) valve from the atria and an "out" (outlet) valve leading to your arteries.

Healthy valves open and close in exact coordination with the pumping action of your heart's atria and ventricles. Each valve has a set of flaps called leaflets or cusps that seal or open the valve. This allows blood to pass through the chambers and into your arteries without backing up or flowing backward. Rate This Content: The Heart's Chambers and Valves The Heart's Chambers and Valves Search the site By Richard N. Fogoros, MD Updated January 09, 2017 The function of the heart is to pump the blood that bathes every organ of the body. The blood carries the oxygen and nutrients vital to the tissues and removes waste products. If the pumping action of the heart is disrupted, the body's organs begin to fail very quickly. So life itself is dependent on the efficient, continuous operation of the heart. The heart is a muscular organ roughly the size of your fist. As the heart muscle contracts or squeezes, it propels the blood out into the vascular system. The heart's chambers and valves are arranged to direct the flow of the blood as the heart beats. Heart’s Chambers and Valves The heart has four chambers.

The two ventricles (right and left) are muscular chambers that propel the blood out of the heart. The right ventricle pumps blood to the lungs, and the left ventricle pumps blood to all other organs. The two atria (right and left) hold the blood returning to the heart, and at just the right moment empty into the right and left ventricles. The four heart valves (tricuspid, pulmonary, mitral and aortic) keep the blood moving in the right direction through the heart. It is helpful to visualize the heart functioning as two separate pumps, working in series; the right heart pump, and the left heart pump. The Right Heart Pump The right heart pump consists of the right atrium, tricuspid valve, right ventricle, pulmonic valve, and pulmonary artery. Its job is to make sure “used” blood gets reloaded with oxygen. Oxygen-poor blood returning to the heart from the body’s tissues enters the right atrium. When the atria contract, the tricuspid valve opens and allows the blood to be pumped from the right atrium to the right ventricle.

Then, when the right ventricle contracts, the tricuspid valve closes (to prevent blood from washing backwards to the right atrium), and the pulmonic valve opens — so blood is ejected from the right ventricle and out to the pulmonary artery and the lungs, where it is replenished with oxygen. Human Heart: Anatomy, Function & Facts Human Heart: Anatomy, Function & Facts By Tanya Lewis, Staff Writer | March 22, 2016 01:07pm ET MORE Credit: Dreamstime The human heart is an organ that pumps blood throughout the body via 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." Human heart anatomy In humans, the heart is roughly the size of a large fist and weighs between about 10 to 12 ounces (280 to 340 grams) in men and 8 to 10 ounces (230 to 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 is about the size of a fist. Credit: tlorna Shutterstock 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. A double-walled sac called the pericardium encases the heart, which serves to protect the heart and anchor it inside the chest.

Between the outer layer, the parietal pericardium, and the inner layer, the serous pericardium, runs pericardial fluid, which lubricates the heart during contractions and movements of the lungs and diaphragm. The heart's outer wall consists of three layers. The outermost wall layer, or epicardium, is the inner wall of the pericardium. The middle layer, or myocardium, contains the muscle that contracts. The inner layer, or endocardium, is the lining that contacts the blood. The tricuspid valve and the mitral valve make up the atrioventricular (AV) valves, which connect the atria and the ventricles. 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. The sinoatrial node produces the electrical pulses that drive heart contractions. Human heart function 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 returns as oxygenated blood to the left atrium of the heart via the pulmonary vein. In the systemic circuit, oxygenated blood leaves the body via 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 via 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. The cardiovascular system circulates blood from the heart to the lungs and around the body via blood vessels. Credit: The BioDigital HumanTM developed by NYU School of Medicine and BioDigital Systems LLC "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 muscle of the heart, 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. 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. Facts about the human heart A human heart is roughly the size of a large fist. The heart weighs between about 10 to 12 ounces (280 to 340 grams) in men and 8 to 10 ounces (230 to 280 grams) in women. The heart beats about 100,000 times per day (about 3 billion beats in a lifetime). An adult heart beats about 60 to 80 times per minute. Newborns' hearts beat faster than adult hearts, about 70 to 190 beats per minute. The heart pumps about 6 quarts (5.7 liters) of blood throughout the body. The heart is located in the center of the chest, usually pointing slightly left. Editor’s Note: If you’d like more information on this topic, we recommend the following book:

Question: Which is the largest and most muscular of the four chambers of the human heart?
Answer: Left ventricle