Partial pressure of inspired oxygen (PIO2) decreases with altitude, which is important because it reduces tissue oxygenation. Which statement is correct?

• A. Partial pressure of inspired oxygen decreases with altitude, reducing oxygen available for tissue oxygenation
• B. Partial pressure of inspired oxygen increases with altitude, improving tissue oxygenation
• C. PIO2 remains constant with altitude
• D. PIO2 is not related to oxygen delivery

Answer: (A)

As altitude increases, barometric pressure falls, so the partial pressure of inspired oxygen declines. PIO2 is determined roughly by FiO2 times the driving pressure for oxygen entering the lungs, (Pb − PH2O). With FiO2 about 0.21 and PH2O about 47 mmHg, a drop in Pb directly lowers PIO2. For example, sea level PIO2 is about (760−47)×0.21 ≈ 149 mmHg, while at higher altitude it drops substantially. This lowers alveolar PO2, decreases arterial PO2, and reduces diffusion of oxygen into the blood, ultimately diminishing tissue oxygen delivery. Therefore, the statement that PIO2 decreases with altitude, reducing oxygen available for tissue oxygenation, is correct. The other ideas contradict how oxygen pressure and transport work: PIO2 does not increase with altitude, it does not remain constant, and it is directly related to oxygen delivery.