Which mechanism increases respiratory stimulation when PaO2 falls and inhibits it when PaO2 rises?

• A. Chemoreceptor stimulation
• B. Baroreceptor reflex
• C. Hypoxic drive
• D. Hering-Breuer reflex

Answer: (C)

Hypoxic drive is the mechanism by which ventilation is adjusted in response to arterial oxygen levels, mediated mainly by peripheral chemoreceptors in the carotid bodies. When PaO2 falls, these chemoreceptors increase their firing, signaling the brainstem to raise the respiratory rate and depth to boost oxygen uptake. When PaO2 rises, their activity decreases, dampening the drive to breathe.

This peripheral oxygen-sensing system works alongside central chemoreceptors that respond more to CO2 and pH, but the specific response to changes in PaO2—the increase in drive with hypoxemia and the inhibition with normoxia or hyperoxia—is due to hypoxic drive. Other reflexes, like baroreceptor reflexes or the Hering-Breuer reflex, don't directly regulate ventilation in response to PaO2.