the partial pressure of oxygen in the alveolar air is

Options

A. 104 mmHg
B. 120 mmHg
C. 40 mmHg
D. 90 mmHg

Answer

### Step-by-Step Solution:

1. **Understanding Partial Pressure**:
- Partial pressure is the pressure exerted by an individual gas in a mixture of gases. In the case of air, it consists primarily of nitrogen, oxygen, carbon dioxide, and other gases.

2. **Identifying the Components of Air**:
- Air is a mixture, and the gases within it exert their own partial pressures. The partial pressure of oxygen (denoted as pO2) is the focus here.

3. **Considering the Alveoli**:
- The alveoli are tiny air sacs in the lungs where gas exchange occurs. Oxygen-rich air enters the alveoli, while carbon dioxide is expelled.

4. **Comparing Partial Pressures**:
- The partial pressure of oxygen in the atmosphere (pO2 in atmosphere) is approximately 159 mm Hg. However, in the alveoli, the partial pressure of oxygen is lower due to the presence of carbon dioxide and water vapor.

5. **Determining the Partial Pressure of Oxygen in Alveoli**:
- The partial pressure of oxygen in the alveoli is around 104 mm Hg. This lower pressure allows oxygen to diffuse from the alveoli into the blood, where the concentration is lower.

6. **Conclusion**:
- Therefore, the partial pressure of oxygen in the alveolar air is approximately **104 mm Hg**.