Rate of Blood Flow Through the Muscles
During rest, blood flow through skeletal muscle averages 3 to 4 ml/min/100 g of muscle. During extreme exercise in the well-conditioned athlete, this can increase 15- to 25-fold, rising to 50 to 80 ml/min/100 g of muscle.
Blood Flow During Muscle Contractions. Figure 21–1 shows a record of blood flowchanges in a calf muscle of a human leg during strongrhythmical muscular exer-cise. Note that the flow increases and decreases with each muscle contraction. At the end of the contractions, the blood flow remains very high for a few seconds but then fades toward normal during the next few minutes.
The cause of the lower flow during the muscle contraction phase of exercise is compression of the blood vessels by the contracted muscle. During strong tetanic contraction, which causes sustained compression of the blood vessels, theblood flow can be almost stopped, but this also causes rapid weakening of the contraction.
Increased Blood Flow in Muscle Capillaries During Exercise. During rest, some musclecapillaries have little or no flowing blood. But during strenuous exercise, all the capillaries open. This opening of dormant capillaries diminishes the dis-tance that oxygen and other nutrients must diffuse from the capillaries to the contracting muscle fibers and sometimes contributes a twofold to threefold increased capillary surface area through which oxygen and nutrients can diffuse from the blood.
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