Which Statement About Precapillary Sphincters is CORRECT

Question

Which statement about precapillary sphincters is CORRECT?
A) They contract when oxygen levels in the tissue are low.
B) They can close off the capillaries by contracting.
C) They are openings that allow large molecules and blood cells to leave the capillary.
D) When the precapillary sphincters are open, blood flows only through the thoroughfare channels.
E) They are voluntary.

Accepted Answer

Precapillary sphincters are rings of smooth muscle located at the entrance of capillary beds. By contracting, they can control blood flow into the capillaries, effectively closing them off to regulate the distribution of blood to tissues based on the body's needs. This mechanism does not involve the voluntary control or the passage of large molecules and blood cells out of the capillaries, nor does it imply that blood flows only through thoroughfare channels when they are open.