Most of the bodies blood is in the viens of the systemic system. The venus reserve can mobilize blood when there is vasoconstriction. When someone exercises the sympatic nervous system is activated and the viens are constricted which in turn increased the blood returned to the heart. This increased the end diastolic volume of the heart, the SV, cardiac output, and blood pressure. Blood flow is proportional to a change in pressure over resistance. The more pressure there is the greater the blood flow, and as resistance increases blood flow also increases. Resistance is proportional to 1/r^4 therefore doubling the radius of the vessel is equal to 16x more resistance.
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