**Blood is pumped through the heart into two circulatory systems; the pulmonary system (the lungs) and the systemic system (the rest of the body). In the pulmonary system, deoxygenated (oxygen‐poor) blood enters the right atrium from the various parts of the body via three major vessel: the superior vena cava (from the upper body), the inferior vena cava (from the lower body), and the coronary sinus (from the wall of the heart).

The components of the heart and blood flow through the heart.

From the right atrium, blood flows to the right ventricle and is pumped to the lungs via the right and left pulmonary arteries (one goes to each lung).  At the lungs, deoxygenated blood releases carbon dioxide and takes on oxygen, becoming oxygenated (oxygen‐rich).  The blood then returns to the heart via the four pulmonary veins and is emptied into the left atrium, where it now goes into the systemic circulatory system. The left atrium pumps the blood into the left ventricle, where it is then pumped into the ascending aorta.  The aorta and its branches carry the oxygenated blood to the various body tissues and the heart muscle itself.  Because the blood needs to travel such a great distance throughout the body, the left ventricle is the most robust of the four chambers and can have two to four times the muscle mass of the right ventricle (that pumps blood to just the lungs).

To prevent backflow of blood into the heart chambers, each is equipped with a valve that can control the flow. Valves are made of thick connective tissue surrounded by endocardium, and they open and close in response to pressure changes in the heart as it contracts and relaxes. There are two types of valves: atrioventricular and semilunar. The valves found between the atria and ventricles are called atriventricular valves and consist of flaps that respond to pressure changes in the ventricle.  When the ventricles are relaxed between pumps, the valve flaps open (pointing into the ventricle) and blood can move from the atrium into the ventricle.  When the ventricle is contacting and full of blood, the valve flaps are forced shut and stop the blood from backwashing into the atrium.  These valves are slightly different in the right and left sides of the heart. Between the right atrium and ventricle the valve has three flaps and is thus referred to as a tricuspid valve, and between the left atrium and ventricle the valve has two flaps and is referred to as a bicuspid or mitral valve.  Semilunar valves are found on both the pulmonary artery and the aorta, and function to keep blood from flowing backward into the ventricles of the heart.  The semilunar valves get their name from the three half‐moon shaped cusps they have that allow the blood to flow in only one direction.**

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