What role do precapillary sphincters play in microvascular regulation?

• A. They regulate perfusion by opening/closing microvasculature in response to local cues and autonomic signals.
• B. They set the heartbeat rate.
• C. They regulate venous return.
• D. They secrete endothelin to cause systemic constriction.

Answer: (C)

Precapillary sphincters control how much blood actually enters the capillary networks by opening or constricting at the entry to the capillaries. They respond to local metabolic needs and neural signals, so active tissues receive more blood when needed and less when demand is low. This fine-tunes tissue perfusion and enables capillary recruitment to maximize exchange where it’s most needed. They aren’t responsible for setting heart rate or directly governing venous return. Some systemic constrictors like endothelin exist, but the main role of these sphincters is to regulate capillary entry and local perfusion in response to local cues and autonomic input.