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The human heart is a four chambered muscle that squeezes blood from one chamber to another in order to accept deoxygenated blood from the body, pump blood through the lungs for oxygenation, and then pump oxygenated blood out to the body. This is accomplished by internal electrical stimulation of contractile fibers within the cardiac muscle. A heart beat is traditionally presented as "thump-thump". This is systole-diastole, with systole made up of atria-ventricle contraction and diastole made up of atrial-ventricle relaxation in overlapping events.

The upper chambers are known as the atria and the lower chambers are the ventricles. The left and right side of the heart do not communicate directly with one another after birth and are separated by a thick interventricular or interatrial septum. The atrium and ventricle on each side is separated by a valve to prevent the backflow of blood, as are the exit points to the heart.

Blood enters the heart via the inferior and superior vena cava, the endpoints of the peripheral venous system carrying deoxygenated blood. The vena cave empty into the right atrium. When the right ventricle is relaxed, blood empties from the atrium through the tricuspid valve (or right atrioventricular valve) into the right ventricle. The atria contracts enough to finish the emptying immediately before the ventricle contracts. This is accomplished by the heart's natural pacemaker, which resides in the right atrium - the sinoatrial (SA) node. This specialized group of cardiac cells has the fastest rate of depolarization in the heart tissue, leading what is known as the heart beat. The electrical impulse generated travels to the atrioventricular (AV) node, which is in the interatrial septum. This is atrial systole.

In the interventricular septum is the bundle of His, known as the AV bundle, which receives a signal from the AV node and transmits it to the left and right bundle branches in the base of heart. The impulses travel along Purkinje fibers to other parts of the heart to produce ventricular systole. By this point the atria have begun diastole, which is the dispelling of the electrical impulse resulting in relaxation. When the right ventricle contracts, it pushes blood through the pulmonary semilunar valve into the pulmonary artery. The pulmonary artery carries deoxygenated blood to the lungs for oxygenation.

The blood then flows back into the heart from the lungs through the pulmonary vein into the left atrium. When the left ventricle relaxes, the blood drains into the left ventricle from the left atrium through the mitral valve (or bicuspid valve). The left ventricle has the thickest wall of any other chamber in the heart and contracts more forcibly. The blood is forced through the aortic semilunar valve into the aorta where it is dispensed to the various arteries throughout the body.

At any given time, either the atria or ventricles are filled with blood. The force of the blood flow closes the valves to prevent backflow and continue a one-way flow through the heart to prevent blood from pooling, which results in congestive heart failure and edema. It also prevents deoxygenated blood from reentering the circulation, which is what occurs when the foramen ovale does not properly close after birth.

The flow through the fetal human heart is slightly different from an adult heart. An opening (foramen ovale) in the interatrial septum bypasses the pulmonary circulation because the fetus does not yet breath with its lungs. Instead, blood is oxygenated by the maternal circulation. This opening is closed by a flap by the force of the first rush of blood upon breathing after birth.

Learn more about this author, Alicia M Prater PhD.
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