Which of the following would NOT result in the dilation of the feeder arterioles and opening of the precapillary sphincters in systemic capillary beds?

• A. A local increase in pH
• B. Increased CO2 concentration
• C. Local release of nitric oxide
• D. Decreased oxygen concentration

Answer: (A)

The main idea is how blood flow to a tissue is locally controlled to match its metabolic needs. When a tissue is active, it produces signals that cause the small feed arteries to widen and the precapillary sphincters to open, increasing capillary perfusion. Those signals include higher CO2, lower oxygen, and the release of nitric oxide, all of which promote smooth muscle relaxation and dilation. A local rise in pH means the tissue is more alkaline, which reduces the stimulus for dilation and can even favor constriction, so it would not lead to dilation. The other factors described—increased CO2, nitric oxide release, and decreased O2—are all classic triggers of vasodilation to boost blood flow to meet metabolic demand.