Question

The partial pressures (in mm Hg) of oxygen (O2 ) and carbon dioxide (CO2 ) at alveoli (the site of diffusion) are

• A. pO2=159 and pCO2=0.3
• B. pO2=104 and pCO2=40
• C. pO2=40 and pCO2=45
• D. pO2=95 and pCO2=40

Answer 1

The Correct Option is B

To solve this question, we need to understand the concept of partial pressures in the alveoli, which is crucial for gas exchange in the lungs. The alveoli are tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged between the air and the blood. The partial pressures of these gases are significant factors that drive diffusion.

1. Partial Pressure of Oxygen (pO2): In the alveoli, oxygen enters the blood, and its partial pressure is lower than in atmospheric air due to the mixing of incoming oxygen-rich air with residual air that has given up some oxygen to the blood. The typical partial pressure of oxygen in the alveoli is approximately 104 mm Hg.
2. Partial Pressure of Carbon Dioxide (pCO2): Carbon dioxide, a waste product of metabolism, diffuses from the blood into the alveoli to be expelled from the body. The partial pressure of carbon dioxide in the alveoli is around 40 mm Hg.

Given these physiological values, let's evaluate the options:

• Option 1: pO2 = 159 mm Hg and pCO2 = 0.3 mm Hg - These values are typically found in atmospheric air, not in the alveoli.
• Option 2: pO2 = 104 mm Hg and pCO2 = 40 mm Hg - These are the typical values for alveolar gas composition, matching the physiological understanding explained above.
• Option 3: pO2 = 40 mm Hg and pCO2 = 45 mm Hg - These values are indicative of conditions found in venous blood, not alveolar gas.
• Option 4: pO2 = 95 mm Hg and pCO2 = 40 mm Hg - While these values could occur in different parts of the respiratory system, they are not standard for alveolar gas.

Therefore, the correct answer is pO2=104 and pCO2=40, aligning with the standard physiological conditions in the alveoli.