DISTRIBUTION OF BLOOD FLOW
An individual’s blood volume remains relatively con-stant within the normal range appropriate to the size of the person. Active tissues, however, require more blood, that is, more oxygen, than do less active tissues. As active tissues and organs receive a greater propor-tion of the total blood flow, less active organs must receive less, or blood pressure will decrease markedly.
As mentioned previously, precapillary sphincters dilate in active tissues and constrict in less active ones. The arterioles also constrict to reduce blood flow to less active organs. This ensures that metabolically active organs will receive enough oxygen to function properly and that blood pressure for the body as a whole will be maintained within normal limits.
An example will be helpful here; let us use the body at rest and the body during exercise. Consult Fig. 13–11 as you read the following. Resting cardiac output is approximately 5000 mL per minute. Exercise cardiac output is three times that, about 15,000 mL per minute. Keep in mind that the volume of blood is the same in both cases, but that during exercise the blood is being circulated more rapidly.
Compare the amounts of blood flowing to various organs and tissues during exercise and at rest. During exercise, the heart receives about three times as much blood as it does when the body is at rest. The very active skeletal muscles receive about ten times as much blood. The skin, as an organ of heat loss, receives about four times as much blood. Other organs, how-ever, can function adequately with less blood. Blood flow is reduced to the digestive tract, to the kidneys, and to other parts of the body such as bones.
When the exercise ceases, cardiac output will gradually return to the resting level, as will blood flow to the various organs. These changes in the distribu- tion of blood ensure sufficient oxygen for active tissues and an appropriate blood pressure for the body as a whole.
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