Answer:
The radii of the arterioles constrict by humoral control and by nervous control.
Explanation:
The vascular system regulation is performed by the smooth muscle of the arterioles, which constitute the maximum resistance point. The smooth muscle relaxation provokes vasodilation, increasing its diameter. On the contrary, smooth muscle contraction provokes a decrease in the arteriole´s diameter, reducing the blood flow.
This muscle is under the control of the autonomous nervous system and certain hormones.
Substances such as adrenaline, noradrelanine, angiotensine, vasopresin, and endotelines cause vasoconstriction.
Also, there are many vasoconstrictor fibers of the sympathetic nerves that increase resistance and modify blood flow.
