Introduction

Respiratory ventilation adjusts dynamically to maintain arterial blood gases within tight limits. Partial pressure of oxygen (Po2) in inspired air critically influences ventilation rate through chemoreceptor-mediated reflexes. Understanding how ventilation changes with different inspired oxygen levels is important for clinical and physiological insight. This article examines key statements about ventilatory responses to varying Po2 and identifies the combination of fully correct ones.

Analysis of the Statements

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

• True.
  When inspired Po2 falls below approximately 60 mm Hg, peripheral chemoreceptors in carotid bodies are strongly stimulated, resulting in a marked increase in ventilation to improve oxygen uptake.

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

• False (context-dependent).
  Normal resting ventilation in humans is about 6 L/min at sea level, where inspired Po2 is approximately 150 mm Hg (21% oxygen at atmospheric pressure). This statement is generally true, but ventilation can vary with metabolic demand and individual variations, so as a standalone statement, it’s not absolute.

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

• True with nuance.
  Above 60 mm Hg, ventilation remains relatively stable or only slightly increases. Significant ventilatory changes occur mainly under hypoxia below this threshold.

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

• False.
  Increased ventilation typically lowers alveolar Pco2 by blowing off more CO2. An increase in ventilation diminishes alveolar CO2, so this statement contradicts physiology.

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

• True.
  Hypoxia stimulates anaerobic metabolism in tissues, producing lactic acid, which increases H+ concentration, thereby lowering blood pH (increased acidity).

Summary of Correctness

Finding the Option With All Correct Statements

Reviewing the options for combined correctness:

• (1) A, B, and C — All true statements (correct)

• (2) B, C, and D — D is false, so incorrect

• (3) C, D, and E — D is false, so incorrect

• (4) A, B, and E — All true statements (correct)

Between options (1) and (4), (4) contains statements A, B, and E, which are unequivocally true. Statement C is true but less essential as an increase in ventilation above 60 mm Hg is minimal, so (4) is the better representation of well-established physiological changes.

Physiological Explanation

• Below 60 mm Hg PO2: Peripheral chemoreceptors strongly stimulate ventilation.

• At normal inspired PO2 (~150 mm Hg): Ventilation is stable around 6 L/min at rest.

• High ventilation decreases alveolar and arterial Pco2 by promoting CO2 elimination.

• Hypoxia leads to metabolic acidosis reflected by increased H+ concentration.

Summary Table

Conclusion

The option that best represents the combination of all correct statements about ventilation changes with varying inspired oxygen content is:

(4) A, B, and E