PiO2 at sea level with FiO2 = 0.21 is approximately:

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PiO2 at sea level with FiO2 = 0.21 is approximately:

PiO2 represents the partial pressure of oxygen in the inspired, humidified gas. To find it, use the equation PiO2 = FiO2 × (Pb − PH2O). At sea level, the barometric pressure Pb is about 760 mmHg, and the water vapor pressure PH2O at body temperature is about 47 mmHg, since air is humidified to 37°C in the airways. So PiO2 ≈ 0.21 × (760 − 47) ≈ 0.21 × 713 ≈ 150 mmHg. This is why the approximate value is around 150 mmHg. If you ignored the water vapor correction, you’d get about 0.21 × 760 ≈ 160 mmHg, which is slightly higher.

PiO2 represents the partial pressure of oxygen in the inspired, humidified gas. To find it, use the equation PiO2 = FiO2 × (Pb − PH2O). At sea level, the barometric pressure Pb is about 760 mmHg, and the water vapor pressure PH2O at body temperature is about 47 mmHg, since air is humidified to 37°C in the airways. So PiO2 ≈ 0.21 × (760 − 47) ≈ 0.21 × 713 ≈ 150 mmHg. This is why the approximate value is around 150 mmHg. If you ignored the water vapor correction, you’d get about 0.21 × 760 ≈ 160 mmHg, which is slightly higher.

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