Question

which of the following would lead to relaxation of the precapillary sphincter?
multiple choice
decreased nitric acid levels
decreased lactate production by cells in the capillary bed
decreased co₂ concentration in the capillary bed
decreased tissue metabolism
decreased o₂ levels in tissue cells served by the capillary

Explanation:

Precapillary sphincters control blood flow to capillary beds. When tissue cells (served by the capillary) have decreased \(O_2\) levels (indicating higher metabolic demand as cells need more \(O_2\) for respiration), it triggers relaxation of precapillary sphincters. This allows more blood (and thus \(O_2\)) to flow to the tissue. Decreased nitric acid levels would not promote relaxation (nitric oxide is a vasodilator, but nitric acid is not relevant here). Decreased lactate production (lactate is a by - product of anaerobic metabolism, lower production means less metabolic need), decreased \(CO_2\) (a vasodilator when in higher concentrations, lower \(CO_2\) means less need for blood flow), and decreased tissue metabolism (less need for nutrients and \(O_2\) delivery) would all lead to sphincter constriction, not relaxation.

Answer:

Decreased \(O_2\) levels in tissue cells served by the capillary.