The following statement suggest the changes in respiratory ventilation and the mechanisms of these changes when a normal human subject is allowed to inhale air containing different oxygen content:

A. The ventilation is markedly increased when Po₂ of the inspired air is less than 60 mm Hg.

B. The ventilation is 6 L/min when the Po₂ of the inspired air is about 150 mm Hg.

C. The ventilation is slightly increased when Po₂ of the inspired air is more than 60 mm Hg.

D. The increased ventilation due to the lower Po₂ in the inspired air causes higher alveolar Pco₂.

E. The H⁺ concentration in the arterial blood is increased when Po₂ of the inspired air is gradually decreased.

Which one of the following options represents the combination of all correct statements?

Correct option is A

The question discusses the mechanisms that regulate respiratory ventilation and how it is influenced by the partial pressure of oxygen (PO₂) in inspired air.

Statement A: The ventilation is markedly increased when PO₂ of the inspired air is less than 60 mm Hg.  Correct

• Hypoxia (low PO₂) triggers marked increases in ventilation to compensate for low oxygen levels. This response is most significant when PO₂ is less than 60 mm Hg, as the body increases ventilation to restore normal oxygen levels.
• Hence, this statement is correct.

Statement B: The ventilation is 6 L/min when PO₂ of the inspired air is about 150 mm Hg.  Correct

• Under normal atmospheric conditions (with PO₂ around 150 mm Hg), the ventilation rate is typically around 6 L/min for an average adult.
• Hence, this statement is correct.

Statement C: The ventilation is slightly increased when PO₂ of the inspired air is more than 60 mm Hg.  Correct

• Ventilation is slightly increased at higher PO₂ levels, but a more significant increase is seen when PO₂ drops below 60 mm Hg (hypoxic conditions).
• In normal conditions where PO₂ is above 60 mm Hg, ventilation increases slightly or remains stable. This indicates that the body responds more strongly only when oxygen levels become significantly lower.
• Hence, this statement is correct.

Statement D: The increased ventilation due to the lower PO₂ in the inspired air causes higher alveolar PCO₂.  Incorrect

• Increased ventilation due to low PO₂ is meant to expel excess CO₂ and decrease alveolar PCO₂. The body increases ventilation to blow off more CO₂, which would lower PCO₂ levels in the alveoli, not increase them.
• Hence, this statement is incorrect.

Statement E: The H⁺ concentration in the arterial blood is increased when PO₂ of the inspired air is gradually decreased.  Incorrect

• H⁺ concentration (acidity) in the arterial blood increases as a result of increased CO₂ (hypercapnia), not just from decreased PO₂.
• While hypoxia can trigger increased CO₂ retention and thus an increase in H⁺ concentration, this effect is secondary. The primary driver of H⁺ increase is CO₂, not low PO₂.
• Hence, this statement is incorrect.

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