Our Heart

The human heart sits between the two lungs in the area called the mediastinum, also known as the middle of the chest. Two thirds of the heart are on the left side of the chest and one third on the right. A healthy heart is roughly one and a half times the size of a person's closed fist and weighs 250 to 350 grams. In shape it resembles a rounded cone, with the tip pointing down and forward. Because the tip of the heart lies so close to the chest wall, each heartbeat can be felt on the chest.

The heart acts as a muscular pump and keeps blood continuously moving through the body. As blood flows through the vessels it supplies every cell with oxygen and nutrients. Blood also carries away metabolic waste products like carbon dioxide.

The heart is divided into two halves by the septum, with each half performing very different functions. The right side of the heart draws oxygen-poor blood from the veins and pumps it into the pulmonary circulation, where it is enriched with oxygen. From the lungs, the blood then enters the left side of the heart, is pumped onward into the aorta-the main artery-and thus returns to the systemic circulation. Each half of the heart is further divided into two chambers: a smaller upper chamber (atrium), where the blood collects, and a larger, stronger lower chamber (ventricle), which draws blood from the atrium and pumps it back into the systemic or pulmonary circulation.

To enable the pumping action and ensure that blood flows in the right direction, the heart has four valves. Blood flows from the right atrium through the tricuspid valve, which has three leaflets, into the right ventricle. From there, it passes through the pulmonary valve, a semilunar valve, into the pulmonary artery and thus into the pulmonary circulation. From there, it returns to the heart via the left atrium. It then passes through the mitral valve, which has two leaflets, into the left ventricle and is pumped through the aortic valve, the second semilunar valve in the heart, into the aorta and thus into the systemic circulation. The coronary arteries also branch off directly where the aorta begins at the aortic valve. They wrap around the heart and supply it with blood. A heart attack occurs when one of the coronary arteries is blocked.

The two sides of the heart work in the same rhythm. The heart and the blood vessels together form the cardiovascular system.

The heart is a unique organ. It is the only muscle capable of stimulating itself independently, in a synchronized, regular way, and adjusted to the body's needs. For this there is the conduction system in the heart. The sinus node mentioned earlier is located in the right atrium. It is the body's natural pacemaker. Electrical impulses are generated there and passed on to the other stations in the heart. Specialized heart muscle cells pick up these impulses and conduct them across both atria. The next station is the AV node (atrioventricular node), which lies between the right atrium and right ventricle. It delays the impulse slightly so the atria can contract and push blood into the ventricles. From the AV node the impulse is passed to the bundle of His, from which the two bundle branches carry the impulse on to the Purkinje fibers. This ensures that all regions of the heart are reached.

A major feature of the conduction system is that the next station down the line can generate an impulse if the previous one fails. If the sinus node stops sending signals, the AV node can produce impulses itself-but at a slower rhythm. If the AV node also stops producing impulses, the bundle of His, the bundle branches, and the Purkinje fibers can still stimulate the heart so that it will beat, but at a very slow rate.

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