Which statement about precapillary sphincters is NOT true?

• A. They decrease the osmotic pressure in the thoroughfare channel.
• B. They regulate flow into capillary beds.
• C. They respond to local metabolic signals.
• D. They help distribute blood flow to tissues

Answer: (A)

Precapillary sphincters are rings of smooth muscle at the entrance to capillary beds that dial in blood flow by contracting or relaxing in response to local tissue needs. They open wider when tissues are metabolically active, driven by signals like increased CO2, lower pH, adenosine, and nitric oxide, which dilate the sphincters to boost capillary perfusion. By adjusting whether blood enters the capillaries or bypasses them via the metarteriole–thoroughfare channel, they help distribute blood flow to tissues according to demand. Osmotic pressure, set mainly by plasma protein concentration, is not controlled by the contraction state of these sphincters, so the statement about decreasing osmotic pressure is not true. The other roles—regulating flow into capillary beds, responding to local metabolic signals, and aiding distribution of blood flow to tissues—are accurate.