A healthy 30-year-old woman is on a commercial airliner with a cabin barometric pressure of 565 mm Hg. Using the alveolar gas equation, what is the inspiratory partial pressure of alveolar oxygen in mm Hg?

• A. 27
• B. 60
• C. 109
• D. 515

Answer: (C)

To determine the inspiratory partial pressure of alveolar oxygen in a commercial airliner setting, we can use the alveolar gas equation, which provides a way to calculate the partial pressure of gases in the alveoli of the lungs.

At cabin pressure, the total barometric pressure is given as 565 mm Hg. The partial pressure of oxygen in the alveoli can be determined using the following equation:

[ P_AO_2 = (P_{barometric} - P_{H2O}) \times F_{O2} ]

In this equation:

• ( P_{barometric} ) is the barometric pressure (565 mm Hg in this case).

• ( P_{H2O} ) is the partial pressure of water vapor, which we can estimate. At body temperature (37°C), the partial pressure of water vapor is approximately 47 mm Hg.

• ( F_{O2} ) is the fractional concentration of oxygen in the inhaled air, which is roughly 0.21 (21% oxygen in typical air).

Applying these values:

1. Calculate the effective pressure of dry gas:

[ 565 , \text{mm Hg} - 47 , \text{mm