How the heart works: a primer.The normal heart is a strong, muscular pump a little larger than a fist. It pumps blood continuously through the circulatory system circulatory system, group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the . Each day the average heart "beats" (expands and contracts) 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human heart beats more than 2.5 billion times. The circulatory system is the network of elastic tubes that carries blood throughout the body. It includes the heart, lungs, arteries, arterioles Arterioles Small blood vessels that carry arterial (oxygenated) blood. Mentioned in: Retinal Artery Occlusion arterioles, n (small arteries), and capillaries (very tiny blood vessels Blood vessels Tubular channels for blood transport, of which there are three principal types: arteries, capillaries, and veins. Only the larger arteries and veins in the body bear distinct names. ). These blood vessels carry oxygen- and nutrient-rich blood to all parts of the body. The circulatory system also includes venules venules (vēnˑ·yōōlz), n.pl small blood vessels that merge with the veins and return blood from other tissues to the heart. (small veins) and veins. These are the blood vessels that carry oxygen- and nutrient-depleted blood back to the heart and lungs. If all these vessels were laid end-to-end, they'd extend about 60,000 miles. The circulating blood brings oxygen and nutrients to all the body's organs and tissues, including the heart itself. It also picks up waste products from the body's cells. These waste products are removed as they're filtered through the kidneys, liver and lungs. How does the heart pump blood? A heart's four chambers must beat in an organized manner. This is governed by an electrical impulse. A chamber of the heart contracts when an electrical impulse moves across it. Such a signal starts in a small bundle of highly specialized cells in the right atrium--the sinoatrial node sinoatrial node n. Abbr. SAN A small mass of specialized cardiac muscle fibers located in the posterior wall of the right atrium of the heart that acts as a pacemaker of the cardiac conduction system by generating at regular intervals the (SA node SA node see sinoatrial. SA node Sinoatrial node, see there ), also called the sinus node. A discharge from this natural "pacemaker" causes the heart to beat. This pacemaker generates electrical impulses at a given raate, but emotional reactions and hormonal factors can affect its rate of discharge. This lets the heart rate respond to varying demands. Information provided by the National Institutes of Health. |
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