Capillaries are tiny blood vessels that pass blood from the arteries into the veins. They are very small, the largest being about 10 micrometers in diameter. Their walls are thin which allows materials to pass into the capillaries. Different types of capillaries exist and perform different functions for the body. Primarily, however, the capillaries are able to profuse the tissues of the body with needed oxygen and important nutrients supplied by blood.

Three types of capillaries exist. These are continuous capillaries, fenestrated capillaries, and sinusoidal capillaries. They vary in construction and in the degree to which they will allow things outside the capillaries to get into the capillaries. All capillaries have an endothelial wall with a differing degree of permeability depending upon type.

Continuous capillaries have the thickest endothelial wall. They allow only water, and ions into their pathways. Fenestrated capillaries have “windows” that lets larger molecules in and out of the capillaries. Sinusoidal capillaries have the greatest amount of permeability, letting red blood cells and proteins in through the endothelial walls.

While capillaries function in one respect as the “communicators” between arteries and veins, they also are the tiny blood vessels that supply blood to organs. Capillaries supplying blood to an organ, when taken in whole, are called a capillary bed. They are numerous, and feed the organ with amino acids, proteins, and most importantly oxygen, without which organ cells could not survive.

In addition to being the transporters of blood products, capillaries allow for waste products to enter. In this way they perform an important function because waste is ultimately transported out of the body through this interchange.

The amount of capillaries in the human body is quite amazing. If one could count and measure the human adult capillaries, one would find about 25,000 miles (40,233.6 km) of them. The extensive supply of capillaries in the human body indicates their extreme importance to our existence and health.