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Heart Function - How the Heart Works

By Mr Ghaz, January 19, 2011

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Heart Function - How the Heart Works

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The heart is large muscular organ in the middle of the chest. Although it is often thought of as being in the left-hand side of the body it actually straddles the mid-line with more of it on the left than the right. It weighs about 12 oz (340 gm) in men and a little less in women.

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The right-hand border of the heart lies more or less behind the right-hand border of the breastbone. On the left side of the breastbone the heart projects out as a sort of rounded triangle with its point lying just below the left nipple. This point can be felt pulsating with each heartbeat. It is called the apex beat.

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The job of the heart is to pump blood around two separate circulations. First it pumps blood out into the arteries via the aorta, the central artery of the body.

This blood circulates through the organs and tissues delivering food and oxygen to them. The blood then returns to the heart in the veins, having had all the oxygen absorbed from it.

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The heart then pumps the blood on its second circuit, this time to the lungs to replace the oxygen. It is then returned to the heart with its oxygen renewed.

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There are four main chambers in the heart operating the pumping arrangement. Each chamber is a muscular bag with walls which contract to push blood onward. The thickness of the muscle wall depends on the amount of work the chamber has to do. The left ventricle has the thickest walls as it does the largest share of the pumping.

The chambers are arranged in pairs, each having a thin-walled atrium which receives blood from the veins. Each atrium pumps the blood through a valve into a thicker-walled ventricle which pumps the blood into a main artery.

The two atria lie behind and above the two ventricles. Both the atria and both the ventricles lie side by side. The portions of their walls which separate them are called the interatrial and interventricular septums.

How It Works

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Blood returns to the heart from the lungs in the pulmonary veins with its oxygen store renewed. It goes into the left atrium which contracts and pushes the blood through a valve called the mitral valve into the left ventricle.

The left ventricle then contracts and as it does so the mitral valve shuts so the blood can only go out through the open aortic valve into the aorta. It then goes on into the tissues where it given up its oxygen.

The blood returns to the heart from the body in a large vein, the inferior vena cava, and from the head in the superior vena cava. It goes into the right atrium. This contracts and the blood passes through the tricuspid valve into the right ventricle.

A right ventricular contraction sends it out into the pulmonary artery, through the pulmonary valve, and through the lungs, where it has its oxygen renewed. It then returns to the heart in the pulmonary veins ready to start all over again. This process is repeated 50-60 times every minute.

The Valves

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Like so many pumps, the heart depends on a series of valves to work properly. On the right-hand side are the pulmonary and tricuspid valves; on the left-hand side are the aortic and mitral valves. The four valves open and close automatically to the chambers, so that it can flow in only one direction.

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The pulmonary and aortic valves are similar in structure. They have three leaf-like cusps, or leaflets, and are made of tough but thin fibrous tissue. The mitral and tricuspid valves are more complicated, though they are similar in structure. The mitral valve has two leaflets; the tricuspids valve three.

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Each of these valves sits in a ring between the atrium and the ventricle. The bases of the leaflets are attached to the ring, while the free edges touch each other and close the passage between the ventricle and atrium when the valve is closed. These free edges are also attached to a series of fine strings-called the chordae tendineae-which pass down into the ventricle and stop the valve from springing back into the atrium when under pressure.

Timing System

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With each heartbeat the two atria contract together and charge up the ventricles with blood. Then the ventricles both contract.

This ordered series of contractions depends upon a sophisticated electrical timing system.

The basic control comes from the sinuatrial node which is in the right atrium. Impulses pass from it through both atria and make them contract. There is another node, the atrioventricular, at the junction of the atria and ventricles.

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This delays the impulse to contract and then passes it down through a bundle of conducting fibers in the intraventriuclar septum called the Bundle of His. After passing through the bundle, the impulse spreads out into the ventricles causing them to contract after the two atria.

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Position of valves viewed from the front: The valves ensure that blood travels in only one direction through the heart by preventing backflow. They consist of two or three 'leaflets' which close off when blood has passed through. The mitral and aortic valves govern the flow of oxygenated blood on the left side of the heart, the tricupid and pulmonary valves control the passage of deoxygenated blood on the right.

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Valves-viewed from above