The heart is constructed of four chambers: the right atrium and the right ventricle, and the left atrium and the left ventricle. These four chambers function as two side-by-side pumps, each of which sends blood through a completely different system of circulation. The right side of the heart pumps blood through the less forceful pulmonary circulation of the lungs, where oxygen-depleted blood is replenished in lung tissue with oxygen from the air we breathe. The left side of the heart pumps blood into the rest of your blood vessels, allowing  nourishing, oxygen-rich blood to leave the heart and travel throughout the rest of the body.
The atria receive blood from the body on the right side and from the lungs on the left side. The two ventricles are the pumping chambers that expel the blood. The two pumps operate in synchronized fashion, the two atria and then the two ventricles contracting and relaxing simultaneously. The two sides of the heart are separated by a thick mus- cular wall called the septum, which prevents blood from passing directly from one side of the heart to the other. To understand the mechanics of pulmonary (lung) circulation, you can trace the path of about half a cup of blood—the amount pumped in a given heartbeat through the right side of the heart. Blood enters the right atrium of the heart through two large veins: the superior vena cava, which collects blood from your head and upper body, and the inferior vena cava, which collects blood from your legs and abdomen. At this point, the red blood cells in the blood return- ing to the right atrium have delivered oxygen and nutrients to other body tissues. The depleted blood has a low oxygen content. The right atrium contracts and sends the blood through a one-way valve into the right ventri- cle, which in turn contracts and pushes the blood out through the pulmonary artery into the lungs. As the blood circulates through the lungs, it unloads carbon dioxide, a waste prod- uct of cellular function that it has carried from body tissues. The red blood cells then pick up fresh oxygen, and the blood is enriched, or oxygenated.

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Similarly, on the upper left side of the heart, circulation to the rest of your body starts with the left atrium. Bright red, oxygen-rich blood enters the chamber via the pulmonary vein, from the lungs. The walls of the left atrium contract and push the blood through a one-way valve into the left ventricle. Then the left atrium relaxes while the powerful left ventricle contracts with the considerable force required to propel the blood into the aorta—the major artery at the top of the heart that directs the blood throughout the body. The left ventricle is the main pump and the strongest muscle tissue in the heart.