When you hold your breath, which of the following gas changes in blood would first lead to the urge to breathe?

Options

(a) rising CO2 concentration
(b) falling CO2 concentration
(c) rising CO2 and falling O2 concentration
(d) falling O2 concentration

Correct Answer:

rising CO2 concentration

Explanation:

Chemorecepters in the medulla get stimulated by increase in CO₂ concentration in the blood of arteries. Decrease in O₂ concentration has no significatant effect on chemoreceptors.