After accounting for all subtractions in the respiratory system, what is the approximate PO2 at the alveoli?

• A. 47 mmHg
• B. 159 mmHg
• C. 100 mmHg
• D. 75 mmHg

Answer: (C)

The partial pressure of oxygen (PO2) in the alveoli is approximately 100 mmHg under normal conditions, which reflects the balance of various physiological factors involved in gas exchange. Atmospheric pressure at sea level is about 760 mmHg, and the amount of oxygen in the atmosphere is roughly 21%, which translates to an approximate partial pressure of oxygen starting at about 160 mmHg (0.21 x 760 mmHg).

However, during inhalation, the air is humidified in the respiratory tract, which introduces water vapor that exerts its own pressure (approximately 47 mmHg at body temperature). This means that when we calculate the effective partial pressure of oxygen in the alveoli, we need to account for the reduction caused by the presence of this water vapor.

The calculation looks like this:

• Start with atmospheric PO2: 160 mmHg

• Subtract the water vapor pressure: 160 mmHg - 47 mmHg = 113 mmHg.

At the alveolar level, there are other factors, including the mixing of fresh air with the air already in the lungs and the completion of gas exchange, which results in a final average PO2 closer to 100 mmHg. This value reflects the equilibrium state