Cellular barriers between alveolar air and capillary blood are -

Options

A. 2
B. 4
C. 3
D. 1

Answer

To answer the question about the cellular barriers between alveolar air and capillary blood, we can break down the solution into clear steps:

### Step-by-Step Solution:

1. **Understanding the Structure of the Alveoli**:
- The alveoli are tiny air sacs located in the lungs where the exchange of gases (oxygen and carbon dioxide) occurs. They are surrounded by capillaries, which are small blood vessels.

2. **Identifying the Cellular Barriers**:
- There are two main cellular layers that separate the alveolar air from the capillary blood:
- **Alveolar Epithelium**: This is the thin layer of cells that lines the alveoli. It is primarily composed of type I and type II alveolar cells. Type I cells are involved in gas exchange, while type II cells produce surfactant to reduce surface tension.
- **Endothelium of Capillaries**: This is the thin layer of cells that lines the blood capillaries. The endothelial cells facilitate the exchange of gases and nutrients between the blood and surrounding tissues.

3. **Function of the Barriers**:
- These two layers (alveolar epithelium and capillary endothelium) are crucial for efficient gas exchange. They must be thin enough to allow oxygen to diffuse into the blood and carbon dioxide to diffuse out into the alveoli.

4. **Conclusion**:
- Therefore, the cellular barriers between alveolar air and capillary blood are the **Alveolar Epithelium** and the **Endothelium of the Capillaries**.

### Final Answer:
The cellular barriers between alveolar air and capillary blood are the **Alveolar Epithelium** and the **Endothelium of the Capillaries**.