Why is alveolar PO2 (approximately 104 mmHg) lower than atmospheric PO2 (160 mmHg) in human lungs?
Answer and explanation
Correct answer: B. Inhaled air is humidified with water vapor and mixes continuously with residual alveolar CO2
- Step 1: Inhaled air is saturated with water vapor (partial pressure 47 mmHg at 37 deg C), diluting gas concentrations. Step 2. Continuous alveolar gas exchange adds CO2 (~40 mmHg) and removes O2, lowering alveolar PO2 to ~104 mmHg. Answer: Inhaled air is humidified with water vapor and mixes continuously with residual alveolar CO2. Why not the others: Surfactant lowers surface tension, does not destroy O2; capillaries absorb O2, not repel; trachea conducts air without gas absorption.
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