Which scenario would promote dilation of feeder arterioles and precapillary sphincters in systemic capillary beds?

• A. Local increase in pH
• B. Local increase in CO2
• C. Local release of norepinephrine
• D. Local increase in oxygen

Answer: (B)

Metabolic control of microcirculation drives dilation of small vessels when tissue activity is high. When cells generate more CO2, it builds up in the interstitial fluid and lowers local pH. This chemical milieu relaxes vascular smooth muscle directly and stimulates the endothelium to release nitric oxide, broadening the feeder arterioles and opening precapillary sphincters so more blood can enter the capillary beds. The result is increased capillary recruitment and flow to deliver oxygen and remove waste products.

A local rise in pH would reduce the signals that promote dilation and can favor constriction. Norepinephrine released locally tends to cause vasoconstriction via alpha-adrenergic receptors, not dilation. A local increase in oxygen signals that enough oxygen is present and typically reduces the need for increased flow, so it does not promote dilation.