When the precapillary sphincter located at the arteriole contracts, which of the following occur?

• A. Less blood enters into the capillaries
• B. Volume decreases and the Blood Capillary pressure will also decrease
• C. Less filtration occurs
• D. All of the above

Answer: (D)

Contracting the precapillary sphincter at the entrance to a capillary bed raises resistance to flow into that bed. This means less blood enters the capillaries, so the capillary bed’s volume drops and the capillary hydrostatic pressure falls. With the capillary hydrostatic pressure reduced, the driving force for filtration into the interstitial space also decreases. In short, fewer fluids move into the capillaries, the capillary pressure and volume decline, and filtration diminishes. All of these outcomes occur when the sphincter contracts, so the answer reflects that comprehensive effect.