A heartbeat is a two-part pumping action that takes about a second. The two parts of the heartbeat are called diastole and systole. The heart will beat 3.5 billion times throughout a normal life span. A healthy lifestyle that includes reducing risk factors for cardiovascular disease allows the heart to remain strong and reliable for a hundred years or more.
Blood returning from the body, heart muscle, and lungs collects in the upper chambers (the right and left atria) of the heart. The heart’s natural pacemaker (the SA node) sends out an electrical signal that causes the atria to contract. This atrial contraction pushes blood down through the tricuspid and mitral valves into the lower chambers of the heart (the right and left ventricles). This, longer, part of the pumping phase is called diastole.
By the time the ventricles are filled, the electrical signal that began in the SA node has traveled along a pathway of cells to the ventricles, causing them to contract. The tricuspid and mitral valves close to prevent back flow, and the pulmonary and aortic valves open. Blood flows from the right ventricle into the lungs to pick up oxygen; the oxygen-rich blood in the left ventricle flows to the heart and the rest of the body. This, shorter, second part of the pumping phase is called systole. After the blood flows out of them, the ventricles relax, and the pulmonary and aortic valves close. The lower pressure in the ventricles causes the tricuspid and mitral valves to open, and the cycle begins again. This series of contractions is repeated over and over again, increasing during times of exertion and decreasing during rest.
When the body is at rest, the heart normally beats 60 to 80 times per minute. Resting heart rate rises with increasing age, and usually decreases with improved physical fitness. In conjunction with the heart, the brain and sensory system monitor external conditions (e.g., temperature, stress, physical activity) and adjust the cardiovascular system to accommodate the needs of the body.
Texas Heart Institute www.texasheartinstitute.com/HIC/Anatomy/systole.cfm