In alveoli of the lungs the air at the site of gas exchange is separted from the blood by

Options

A. alveolar epithelium only
B. alveolar epithelium and capillary endothelium
C. alveolar epithelium capillary endothelium and tunica adventitia
D. alveloar epithelium capillary endothelium a thin layer of tunica media and tunica adventitia

Explanation

To answer the question: "In alveoli of the lungs, the air at the site of gas exchange is separated from the blood by," we need to understand the structure involved in gas exchange in the lungs.

### Step-by-Step Solution:

1. **Identify the Components of Gas Exchange:**
- The gas exchange in the lungs occurs in the alveoli, which are tiny air sacs where oxygen and carbon dioxide are exchanged.

2. **Understand the Structure of the Alveoli:**
- The walls of the alveoli are made up of a type of tissue called squamous epithelium. This tissue is very thin, allowing for efficient gas exchange.

3. **Identify the Blood Vessel Structure:**
- The blood vessels surrounding the alveoli have a lining called endothelium, which is also made of squamous cells. This layer is crucial for the exchange of gases.

4. **Recognize the Membrane Between Air and Blood:**
- The air in the alveoli is separated from the blood in the capillaries by two layers: the alveolar epithelium and the capillary endothelium. Between these two layers, there is a thin basement membrane or interstitial fluid.

5. **Determine the Correct Answer:**
- The air in the alveoli is separated from the blood by the combined layers of the alveolar epithelium and the capillary endothelium, along with the basement membrane. Therefore, the correct answer is that the air is separated from the blood by the alveolar epithelium and the capillary endothelium.

### Final Answer:
In the alveoli of the lungs, the air at the site of gas exchange is separated from the blood by the alveolar epithelium and the capillary endothelium.

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