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Blood capillaries are made up of endothelial cells. These are where the important functions of circulation takes place.

1. The walls of blood capillaries do not possess tunica externa (connective tissue layer containing lymph vessels), tunica media (a thick network of smooth muscle fibres) but only tunica interna (a single layer endothelium of squamous cells).
2. Blood group A has Antigen-A & antibody ‘b’ or anti A or a, Blood group B has antigen-B & antibody a or anti B or b.
3. Blood group AB has antigen-A & B and no antibody.
4. Blood group O has no antigen & antibodies a & b both.

Correct answer: The walls of blood capillaries do not possess tunica externa (connective tissue layer containing lymph vessels), tunica media (a thick network of smooth muscle fibres) but only tunica interna (a single layer endothelium of squamous cells).

Solution

Capillaries are the smallest vessels and are specialized for diffusion, so their walls are just a single layer of endothelial cells. They lack the thicker muscle and connective tissue layers found in larger blood vessels.

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