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How does the heart work?

The heart is the main organ of your circulatory (or cardiovascular) system and is a pump that is mainly made up of muscle. The purpose of the heart is to move the blood through your blood vessels (arteries and veins). We think of the heart as a single pump, but it is really two connected pumps. The right side of the heart takes blood that is returning from the body (head, arms, intestines, legs, etc. – basically everything except the lungs) and pumps it to the lungs where it will be loaded with oxygen from the air. From the lungs, the blood then flows back to the left side of the heart. The left side of the heart's main job is to send the blood, now with fresh oxygen, on its way to the rest of the body.

The two pumps, or sides of the heart (right and left), are each made up of two chambers separated by valves. The blood from the body first returns to the right atrium, which functions mainly as a holding chamber, and then flows through the tricuspid valve into the right ventricle. From there, the right ventricle pumps the blood through the pulmonary valve into the vessels of the lung (the pulmonary artery and its branches). After passing by the air spaces of the lung, each red blood cell should be carrying a complete load of oxygen (medically, we say they are "fully saturated with oxygen"). The blood then continues through the pulmonary veins and into the left atrium. The left atrium is a holding chamber from which blood drains through the mitral valve into the left ventricle. The left ventricle is a strong pump with a thick layer of muscle – normally much thicker than the right ventricle -- that drives the blood, now fully loaded with oxygen, out through the aortic valve into the aorta and on to the rest of the body.

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In our discussions, we are going to concentrate on the part of the circulatory system that involves the pumping of the blood to and from the lungs. This is called the pulmonary (as opposed to the systemic or "main body") circulation. This path (right side of the heart > pulmonary arteries > lungs (capillaries next to alveoli) > pulmonary veins > left side of the heart) is normally a low pressure system without much resistance to the movement of the blood. Because the pressure and resistance to blood flow is very low, the normally thin muscle of the right ventricle has no trouble pumping blood through the lungs.

Note: The above description assumes normal heart and lungs. In pulmonary hypertension, your lungs and heart may have problems that interfere with parts of this picture (for example, getting oxygen from the lungs into the blood) and we discuss these issues in other sections.