Question

In the lungs, the alveolar air is separated from blood by ______.

• A. a layer of squamous epithelium and smooth muscles.
• B. a layer of squamous epithelium and a layer of transitional epithelium
• C. two layers of simple squamous epithelium
• D. two layers of tunica externa, interna and tunica media

Hint:

Think of the thin diffusion membrane between the alveolus and the capillary.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
Gas exchange occurs across a very thin respiratory membrane between the alveolus and the pulmonary capillary.

Step 2: Layers of the membrane
The alveolar wall is a single layer of simple squamous epithelium. The wall of the capillary is made of endothelium, which is also simple squamous. A thin basement substance lies between them.

Step 3: Conclusion
Muscle, transitional epithelium or three-layered vessel walls are not present. The barrier is kept thin so that diffusion is fast.

Final Answer:
The alveolar air is separated from the blood by two layers of simple squamous epithelium. \[ \boxed{\text{(C) }\text{two layers of simple squamous epithelium}} \]