Circulatory response to exercise in health.

Engagement in muscular exercise involves complex local and nervous adjustments of the circulation. In the active muscles, including cardiac muscle, the resistance vessels relax in response to local chemical changes to provide an increase in blood flow adequate for their metabolic requirements. There is increased release of norepinephrine from the sympathetic nerve endings as a result of increased sympathetic outflow; the resultant alpha-receptor activation leads to constriction of both systemic resistance and capacitance vessels outside the active muscles, and the beta-receptor activation leads to an increase in heart rate, shortening of the refractory period, and enhancement of myocardial contractility. As a consequence, the filling pressure of the heart and arterial blood pressure are maintained, and the increase in left ventricular output is directed primarily to the active muscles. During upright exercise, the action of the leg muscle pump contributes to the maintenance of the cardiac filling pressure. As exercise continues and body temperature rises, the skin flow increases to dissipate heat from the body. Static exercise causes a greater increase in arterial blood pressure than dynamic exercise. This is due to the combination of an increase in cardiac output and in total systemic vascular resistance as a consequence of increase sympathetic outflow and mechanical compression of the vessels in the active muscles. The hemodynamic changes result from activation of ergoreceptors in the contracted muscles and from central command. The increase in pressure helps to oppose the mechanical compression. The arterial baroreceptors are reset so that they operate normally around the higher blood pressure.