Final answer:
The statement is true; precapillary sphincters, a type of sphincter muscle, regulate blood flow to capillary beds based on the body's needs, affecting only 5-10% of capillaries at any moment through mechanisms of vasodilation and vasoconstriction.
Step-by-step explanation:
The statement that sphincter muscles regulate the opening and closing of arterioles leading to capillary beds, directing the flow of blood to different areas of the body depending on the body's needs, is true. Precapillary sphincters, which are circular rings of smooth muscle, control the flow of blood from a metarteriole to the capillaries it supplies. These muscle cells can tighten (constrict) or relax (dilate) in response to the body's requirements, which is influenced by nerve and hormone signals. For example, after eating, there is increased blood flow to the digestive system, whereas during exercise, the blood is more directed to the skeletal muscles.
Vasodilation and vasoconstriction are critical mechanisms in this regulation. Vasodilation leads to a widening of blood vessels, while vasoconstriction narrows them, and these processes are governed by the autonomic nervous system. Notably, only about 5-10% of our capillary beds have blood flowing through them at any given time, ensuring that only regions in need of more oxygen or with excess waste products receive additional blood flow. If all sphincters were to open simultaneously, the capillaries would hold all the blood in the body, which demonstrates their importance in maintaining overall circulatory health.