The heart is the pump that propels and circulates blood through the vessels of the cardiovascular system. Anatomically and functionally, its left and right halves can be described as the left heart and right heart. Likewise, its upper and lower halves can be described as the upper atrium and lower ventricle. Thus, it is best described by its four anatomical chambers, and although each has a function indispensable to the completion of a full cardiac cycle otherwise called a heartbeat, the left heart gives life-sustaining energy to all of the tissues of the human body.
Oxygen is absorbed by the capillary blood vessels that blanket the two lungs. These blood vessels merge into pulmonary veins that transport the oxygenated blood to the left heart. As the heart momentarily relaxes, one-way valves at the end of the pulmonary veins open to fill the heart’s left atrium with blood. When the atrium contracts, the valves close, forcing the chamber’s full contents into the left ventricle beneath.
The muscular ventricle of the left heart does all the work. Precisely one-tenth of a second after atrial contraction, the ventricle contracts, forcing the mitral valve separating the two chambers to close shut and the aortic valve to open. The contents of the ventricle is ejected out through the aorta, the largest blood vessel in the human body. When the pressure between the ventricle and the aorta equalizes, the valve between the two closes and the heart relaxes, to repeat this cardiac cycle about once every second at a normally relaxed bodily state.
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It would take an extraordinary amount of force to pump viscous liquid to the many tiny capillaries of the body’s remote extremities, without which their tissue cells would starve and die. The heart instead attains the force necessary through smaller, cumulative pulses, relying on the natural hydrodynamics of pressure difference to create a peristaltic wave. Blood’s pressure and flow decrease in the aorta when its valve closes, but higher pressure preceding and even higher ventricular pressure soon to follow advances the liquid.
The right atrium receives blood — via the superior and inferior vena cava blood vessels — that has been deoxygenated by all of the body’s tissues. Similarly to the left heart, the blood is forced down into the ventricle and is then ejected out through the pulmonary arteries to the lungs, where carbon dioxide and excess dihydrogen oxide is extracted from the blood and exhaled. Whereas the right heart is therefore responsible for removing the waste products of human metabolism, the left heart is responsible for supplying metabolism’s catalytic energy sources — oxygen, hydrogen and carbon-based nutrients.