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Circulatory Changes

The circulatory system also adapts to training. When you exercise, blood flow is redistributed-less blood goes to all major organs except the heart and brain, and more blood flows to the working muscles and skin. At rest, 20 percent of your blood flows to the muscles, compared to 88 percent at maximum exertion.

Arteries and veins have the capability to either constrict or dilate, rapidly redistributing blood flow to meet the demands of exercise. During exercise, the arteries dilate in the working muscles and blood flow increases through the smallest vessels (capillaries), which were previously closed. The increased flow of blood to the muscles increases the exchanges of oxygen, the release of heat and the removal of metabolic wastes: lactic acid and carbon dioxide.

The nervous system prepares the body for exercise by secreting hormones signaling dilation of the blood vessels in the heart and working muscles, and secretion of hormones in inactive tissue for constriction of blood vessels. With training, these systems act more efficiently and rapidly to redistribute blood.

Blood redistribution takes several minutes; starting or stopping exercise abruptly doesn't allow these changes to occur smoothly. An abrupt start can leave you breathless and strain unprepared muscles. An abrupt stop can leave you lightheaded when the blood pools in the working muscles, due to the sudden reduction of pumping action from the leg muscles to return blood to the heart.

The flow of blood through the capillaries is critical to maximum exertion. The number of capillaries in muscles increases with training, while the blood becomes thinner and therefore better able to flow through the capillaries.

The total number of red blood cells stays the same or slightly increases, but with adaptation to aerobic training, more water and dissolved proteins are added to the plasma volume to effectively thin the blood. The result is an increase in the total plasma volume and a decrease in the relative concentration of the red blood cells.

Dilution, this reduced concentration, is referred to as pseudo (false) anemia or athlete's anemia. It is not a true anemia since the total number of red blood cells is normal or slightly increased. This is a helpful adaptation to training that allows more blood to flow to tissues.

What are some of the muscular adaptations to exercise?

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Foreword: Billie Jean King

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