Final answer:
At an elevation of 18,000 feet with a barometric pressure of 380 mmHg, and maintaining the same alveolar ventilation as at sea level, the estimated alveolar partial pressure of oxygen (alveolar PO2) would be approximately 40 mmHg.
Step-by-step explanation:
The question is asking for the estimated alveolar partial pressure of oxygen (alveolar PO2) at an elevation of 18,000 feet with a barometric pressure of 380 mmHg, assuming a constant alveolar ventilation rate as at sea level. At sea level, the atmospheric pressure is 760 mm Hg and the partial pressure of oxygen is 21% of this, which is calculated as:
Po₂ = (760 mm Hg) (0.21) = 160 mm Hg.
However, at higher altitudes such as 18,000 feet, the barometric pressure is lower, and thus the partial pressure of oxygen is also reduced. Although the proportion of oxygen in the atmosphere remains the same (21%), its partial pressure decreases proportionally with the barometric pressure. At 18,000 feet, the atmospheric pressure is 380 mmHg. Thus, the alveolar PO2 would be:
Po₂ = (380 mm Hg) (0.21) - Pco₂ (approximated as 40 mm Hg for alveolar CO2)
= 79.8 mm Hg - 40 mm Hg
≈ 40 mm Hg.
This result shows that at high altitudes with lower atmospheric pressure, the partial pressure of oxygen in the alveoli drops significantly, making it more difficult for the body to obtain necessary oxygen, which would be compounded further by any extra factors that reduce alveolar oxygen levels such as a respiratory dead space increase or a ventilation/perfusion mismatch.