What happens to total atmospheric pressure and partial pressure of oxygen as altitude increases?

What happens to total atmospheric pressure and partial pressure of oxygen as altitude increases?

• A. Total atmospheric pressure decreases; partial pressure of oxygen decreases
• B. Total atmospheric pressure increases; partial pressure of oxygen decreases
• C. Total atmospheric pressure decreases; partial pressure of oxygen increases
• D. Total atmospheric pressure remains constant; partial pressure of oxygen decreases

Answer: (A)

As altitude increases, the total atmospheric pressure falls. Oxygen makes up about 21% of the air, so its partial pressure is the fraction times the total pressure (P_O2 ≈ 0.21 × P_total). When the total pressure drops, the partial pressure of oxygen drops as well. This means both total atmospheric pressure and the partial pressure of oxygen decrease with altitude, which is why less oxygen is available for diffusion into the blood at higher elevations. Example: at sea level, P_total ≈ 760 mmHg and P_O2 ≈ 0.21 × 760 ≈ 160 mmHg. At a higher altitude, P_total might be around 380 mmHg, giving P_O2 ≈ 0.21 × 380 ≈ 80 mmHg.

As altitude increases, the total atmospheric pressure falls. Oxygen makes up about 21% of the air, so its partial pressure is the fraction times the total pressure (P_O2 ≈ 0.21 × P_total). When the total pressure drops, the partial pressure of oxygen drops as well. This means both total atmospheric pressure and the partial pressure of oxygen decrease with altitude, which is why less oxygen is available for diffusion into the blood at higher elevations.

Example: at sea level, P_total ≈ 760 mmHg and P_O2 ≈ 0.21 × 760 ≈ 160 mmHg. At a higher altitude, P_total might be around 380 mmHg, giving P_O2 ≈ 0.21 × 380 ≈ 80 mmHg.