Heart and Circulation
The circulatory or cardiovascular system consists of the heart and blood vessels, and is responsible for supplying all of the body's tissues and organs with blood. The circulatory system also carries various waste products to the liver and kidneys for processing and elimination from the body. At the centre of the system, almost literally, is the heart, a powerful muscular pump that beats more than 100 ,000 times per day to send oxygen and nutrients around the body. HeartThe heart has distinct halves, each with an upper chamber (atrium) and lower chamber (ventricle). Each of these four chambers is connected to one or more blood vessels. The two upper chambers, or atria, receive blood returning under low pressure from the veins. The two lower chambers, the ventricles, pump blood from the atria out through the arteries, under high pressure, around the body. Compared to the ventricles, the atria are less muscular and have thinner walls. In a typical beat of the heart, blood passes from the two atria through one way valves into the corresponding ventricle. The ventricles then contract powerfully, forcing blood out through narrow openings, guarded by more valves, into the arteries. The left ventricle empties into the aorta, the body's main artery, about the diameter of a garden hose, transporting blood rich in oxygen to organs and tissues. The right ventricle pumps blood low in oxygen through the pulmonary arteries back to the lungs, where oxygen levels are renewed. In a lifetime, it is estimated that the average heart beats more than 2.5 billion times, and each of these beats sends a pressure wave through the arteries, causing their walls to expand. Each beat involves a sequence of contractions of the four chambers of the heart, controlled by electrical impulses that spread throughout the heart muscle. The heart rate and the volume of blood pumped with each heartbeat are not under conscious control, but function automatically. |
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