Functional anatomy of the heart

The heart is located in the center of the thoracic cavity. It sits directly above the muscles of the diaphragm, which separates the thorax from the abdomen, and lies beneath the sternum between the two lungs. The heart is enclosed and anchored in place by a double-walled fibrous sac referred to as the pericardium. The membranes of the pericardium produce a small amount of pericardial fluid that minimizes friction produced by the movement of the heart when it beats. To function mechanically as a pump, the heart must have  

The atria (sing, atrium) are chambers that receive blood returning to the heart through the veins. The blood then moves to the ventricles, or delivery chambers, of the heart. The powerful contractions of the ventricles generate a force 

In summary, the heart is a single organ consisting of two pumps the right heart delivers blood to the lungs and the left heart delivers blood to the rest of the body. Both pumps work simultaneously. The atria fill with blood and then contract at the same time and the ventricles fill with blood and then contract at the same time. Contraction of the atria occurs prior to contraction of the ventricles in order to ensure proper filling of the ventricles with blood. 

Two sets of valves in the heart maintain the one-way flow of blood as it passes through the heart chambers  

The semilunar valves separate the ventricles from their associated arteries. The pulmonary valve is found between the right ventricle and the pulmonary artery and the aortic valve is found between the left ventricle and the aorta. These valves prevent backward flow of blood from the pulmonary artery or the aorta into their preceding ventricles when the ventricles relax. The semilunar valves also have three cusps. There are no valves between the venae cavae or the pulmonary veins and the atria into which they deliver blood. The closure of the valves causes the lub-dub associated with the heart beat. Tire first heart sound, or the lub, occurs when the ventricles contract and the AV valves close. The second heart sound, or the dub, occurs when the ventricles relax and the semilunar valves close. 

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