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

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Falling O₂ concentration

Rising CO₂ and falling O₂ concentration

Rising CO₂ concentration

Falling CO₂ concentration

answer is D.

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Detailed Solution

When you hold your breath, the first gas change in blood that would lead to the urge to breathe is the increase in CO₂ levels. The body can tolerate a short-term decrease in oxygen levels, but as CO₂ builds up in the body, it creates an uncomfortable feeling and the urge to exhale. The increase in CO₂ causes an increase in H⁺. Increase in CO₂ and H⁺ stimulates the central and peripheral chemoreceptors, which signal the respiratory centre to increase breathing rate.
The role of oxygen in the regulation of respiratory rhythm is quite insignificant. The chemoreceptors are relatively insensitive to changes in pO₂. The peripheral chemoreceptors respond dramatically only when pO₂ decreases to less than 60 mm Hg.

(1) Falling CO₂ concentration would decrease the breathing rate.

(2) and (3) Falling O₂ would be the last to lead to the urge to breathe.

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