Sympathetic Nervous System

Sympathetic nervous system activity also causes the release of epinephrine from the adrenal medulla. Epinephrine enters the blood stream, and is delivered to the heart where it binds with SA node receptors. Binding of epinephrine leads to further increase in heart rate. Cardiac output is the volume of blood pumped by the heart per minute (mL blood/min). Cardiac output is a function of heart rate and stroke volume. The heart rate is simply the number of heart beats per minute.

Blood vessels provide resistance to the flow of blood because of friction between moving blood and the wall of the vessel. The TPA refers to the sum total of vascular resistance to the flow of blood in the systemic circulation. Because of their small radii, arterioles provide the greatest resistance to blood flow in the arterial system. Adjustments in the radii of arterioles has a significant effect on TPA, which in turn has a significant effect on MAP. Resistance and pressure are directly proportional to each other. If resistance increases, then pressure increases. When the radii of arterioles decrease with vasoconstriction, TPA increases, which causes MAP to increase.