The blood vessels responsible for the transportation of blood from and to the heart have muscular walls to undergo contraction and relaxation. This maintains the blood pressure. So, arteries, arterioles, veins and venules have the muscular walls. However, capillaries are meant to provide the surface for the exchange of nutrients, gases, medicines, waste between the blood and the tissues. So, they must have thin walls to allow such diffusion. Therefore, their walls are made of thin squamous epithelium.

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