Question

Why are capillaries one cell thick?

Answer 1

Hint: Capillaries are very thin in structure. The diameter is approximately $5$ micrometres. They are so thin that the red blood corpuscles need to flow through them in a single file. Capillaries are the smallest and numerous of the blood vessels. The main function of the capillaries is to exchange materials between blood and tissue cells are discussed below:

Complete answer:
Capillaries are the connection of the smallest branches of arteries and veins. The capillary walls are one cell thick. Capillaries allow molecules to pass through across the capillary walls. This exchange of materials is not possible across the walls of other types of blood vessels because the walls are too thick.
The thin wall of the capillary allows oxygen and nutrients to pass from the blood into tissues and allows waste products to pass from tissues into the blood.
The capillaries need to contain the blood, yet to be thin enough that vital materials can quickly and easily diffuse in and out of the bloodstream. These are called the “exchange vessels” of the circulatory system. Because they’re the main point or place at which the blood exchanges substances like oxygen and nutrients with waste materials with the surrounding tissues. Without this the whole circulatory system would collapse. With the thicker walls like we can see even in the smallest arteries and veins, the exchange of substances through the wall would be too slow to support any vertebrate animal’s life. And sometimes in some invertebrates.

Note:
There are estimated of $10$ billion capillaries in a human body. The tiny size of capillaries allows the necessary transfers to take place. But their fragile state can cause capillaries to break so easily. Capillaries can rupture in some right circumstances. And if it is near the surface of the skin then it will look like a red web channel. This condition is known as telangiectasia.