Where is a precapillary sphincter located?

• A. At the beginning of a true capillary
• B. At the venous end of a capillary
• C. In the venule
• D. In the artery before the capillary bed

Answer: (A)

Precapillary sphincters are rings of smooth muscle that sit at the entrance to a capillary bed, right where the metarteriole transitions into true capillaries. Their job is to regulate whether blood actually enters the capillaries. Because their location is at the start of the true capillary network, they determine how much blood flows into the capillary bed or is diverted through the nearby thoroughfare channel. So identifying them as being at the beginning of a true capillary best captures both their position and function. They aren’t situated at the venous end of a capillary, in the venule, or upstream in an artery before the capillary bed.