Oxygen in the air diffuses across .......... as it follows its partial pressure gradient into the human body.

Options

A. pleural sacs
B. a moist respiratory surface
C. alveolar sacs
D. both (b) and (c)

Explanation

To answer the question, "Oxygen in the air diffuses across .......... as it follows its partial pressure gradient into the human body," we need to identify the correct structure involved in the diffusion of oxygen during the process of respiration.

### Step-by-Step Solution:

1. **Understanding the Context**: The question pertains to the process of gas exchange in the human respiratory system, specifically focusing on how oxygen enters the body.

2. **Identifying the Key Structures**: The primary structures involved in the exchange of gases in the lungs are the alveoli. These are small air sacs where the actual exchange of oxygen and carbon dioxide occurs.

3. **Function of Alveoli**: Alveoli have thin walls that facilitate the diffusion of gases. They are highly vascularized, meaning they have a rich blood supply, which is crucial for transporting oxygen to the body and removing carbon dioxide.

4. **Moist Respiratory Surface**: The walls of the alveoli are also moist, which aids in the diffusion process. This moisture helps gases to dissolve and diffuse more easily across the membrane.

5. **Final Answer**: Given the context and the structures involved, the correct completion of the statement is: "Oxygen in the air diffuses across **the moist respiratory surface (alveolar sac)** as it follows its partial pressure gradient into the human body."

### Conclusion:
Thus, the answer can be either "moist respiratory surface" or "alveolar sac," as both are correct in the context of the question.