The human heart contains many intricate elements. The primary structures of the heart include the right and left atria, the right and left ventricles, the aorta, the pulmonary arteries, and both the atrioventricular and semilunar valves. These structures operate via electrical signals from the brain that allow the heart to distribute blood throughout the body.
Deoxygenated blood returning from the body enters the right atrium and passes through the atrioventricular tricuspid valve into the right ventricle. The blood is then pumped through the pulmonary semilunar valve and the pulmonary arteries to the lungs, where it receives oxygen and releases carbon dioxide. The oxygen-rich blood is then moved from the left atrium to the left ventricle through the atrioventricular bicuspid valve. Finally, the blood is pumped past the aortic semilunar valve and through the aorta to the rest of the body. The signature "lub-dup" heartbeat sound is caused by the sequential closing of the atrioventricular and semilunar valves, and not the contraction of the ventricles as is commonly believed. The resting adult heart beats between 60 and 100 times per minute, depending on the fitness level of the individual. Greater levels of physical fitness correspond to slower heart rates, as fit individuals are more efficient at processing the oxygen in the blood the heart pumps.