We also learned about three basic theories of circulation and came up with physiological justifications for each. The first, the fact that blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue needs, can be explained by either low oxygen for an increase in vasodilating products. These do not have to be mutually exclusive. the second was that cardiac output is controlled mainlly b the sum of all the local tissue flows. We explained that cardiac output is dependent on the volume of blood in the venous system. Third was that arterial pressure is controlled independently of either local blood flwo control or cardiac output control. We explained that heart rate multiplied by stroke volume equals cardiac output and that stroke volume is related to venous return but venous return itself is controlled by total peripheral resistance.
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