Squamous epithelium + tunica externa of blood vessel
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B
Squamous epithelium + endothelium of blood vessel
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C
Transitional epithelium + tunica media of blood vessel
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D
Columnar epithelium + 3 layered wall of blood vessel
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Solution
The correct option is A Squamous epithelium + endothelium of blood vessel Alveoli are the primary sites of exchange of gases. Exchange of gases also occurs between blood and tissues. Oxygen and carbon dioxide are exchanged at these sites by simple diffusion mainly based on pressure/ concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are some important factors that can affect the rate of diffusion. The diffusion membrane is made up of three major layers namely, the thin squamous epithelium of alveoli, the endothelium of the alveolar capillaries. However, its total thickness is much less than a millimetre.