Which statement best describes the effect of precapillary sphincter contraction?

Which statement best describes the effect of precapillary sphincter contraction?

• A. It reduces blood entering into the capillaries
• B. It increases blood entering into the capillaries
• C. It has no effect on filtration
• D. It causes dilation of venules

Answer: (A)

Precapillary sphincters are smooth muscle rings at the entrance to capillary beds that regulate how much arterial blood reaches the capillaries. When they contract, the entrance narrows, so less blood can enter the capillaries. This reduces capillary perfusion in that bed (and can indirectly affect filtration there). Relaxation loosens the sphincter, allowing more blood to flow into the capillaries. They do not cause dilation of venules, and contracting does affect the amount of blood entering the capillaries.

Precapillary sphincters are smooth muscle rings at the entrance to capillary beds that regulate how much arterial blood reaches the capillaries. When they contract, the entrance narrows, so less blood can enter the capillaries. This reduces capillary perfusion in that bed (and can indirectly affect filtration there). Relaxation loosens the sphincter, allowing more blood to flow into the capillaries. They do not cause dilation of venules, and contracting does affect the amount of blood entering the capillaries.