Cellular barriers between alveolar air and capillary blood are -

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding the Context**: The question asks about the cellular barriers between alveolar air and capillary blood. This involves understanding the respiratory system and the process of gas exchange that occurs in the lungs.

2. **Identifying the Process**: The primary process occurring between the alveoli (air sacs in the lungs) and the capillaries (small blood vessels) is diffusion. This is the spontaneous movement of gases (oxygen and carbon dioxide) from areas of higher concentration to areas of lower concentration.

3. **Recognizing the Barriers**: There are two main cellular barriers involved in this process:
- **Alveolar Epithelium**: This is the thin layer of cells that line the alveoli.
- **Capillary Epithelium**: This is the layer of cells that line the capillaries adjacent to the alveoli.

4. **Considering the Basement Membrane**: Between these two epithelial layers, there is a basement membrane that further contributes to the barrier. However, the question specifically asks about cellular barriers, so we focus on the epithelial layers.

5. **Conclusion**: Therefore, the total number of cellular barriers between alveolar air and capillary blood is two: the alveolar epithelium and the capillary epithelium.

6. **Final Answer**: The cellular barriers between alveolar air and capillary blood are two.

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