Question

why are capillaries so thin.

Answer 1

A single capillary is so small that it allows only one blood cell to flow through it at a time. The capillary walls are also very small, only one cell thick. These thin walls easily allow water, oxygen, carbon dioxide, and other nutrient and waste substances to exchange between blood cells and the surrounding tissue.

Answer 2

capillaries are so thin to increase the pressure because if area of capillaries is bigger than normal then the pressure will decreases and blood could not able to reach to all organs of the body.