Which of the following responses occurs first with peripheral chemoreceptors when detecting low PO2 levels?

• A. Increase in heart rate
• B. Immediate increase in ventilation
• C. Delayed increase in ventilation
• D. Decrease in ventilation

Answer: (B)

The immediate increase in ventilation is the correct response observed when peripheral chemoreceptors detect low levels of partial oxygen pressure (PO2) in the blood. Peripheral chemoreceptors, primarily located in the carotid and aortic bodies, are sensitive to changes in blood gas composition. When oxygen levels fall, these chemoreceptors respond rapidly by sending signals to the respiratory center in the brainstem.

This stimulation of the respiratory center leads to an immediate increase in the rate and depth of breathing, enhancing ventilation. The increase in ventilation serves to bring more oxygen into the lungs and provide it to the bloodstream, addressing the hypoxia created by the low PO2 levels.

The rapid nature of this response is crucial, as it allows the body to quickly compensate for decreases in oxygen availability during conditions such as high altitude, respiratory diseases, or other situations that impair oxygen delivery. While other physiological responses, such as changes in heart rate, may follow, the priority of the immediate response is to increase ventilation to restore adequate oxygen levels in the body.