capillary - Types, Trivia

A minute, thin-walled blood vessel situated between arterioles and venules. It is the site of the exchange of materials (oxygen, nutrients, carbon dioxide, and other waste products) between capillary blood and surrounding tissues, which occurs by diffusion across the capillary wall. The term is also used to denote a small lymphatic channel.

The walls of capillaries are composed of only a single layer of cells, the endothelium. In some cases, vesicles contained in the capillary membrane use endocytosis and exocytosis to transport material between blood and the tissues.

In an immune response, the endothelial cells of the capillary will upregulate receptor molecules, thus "catching" immune cells as they pass by the site of infection and aiding extravasation of these cells into the tissue.

The "capillary bed" is the network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries it will require to supply nutrients. The capillary bed usually carries no more than 25% of the amount of blood it could contain, although this amount can be increased through autoregulation by inducing relaxation of smooth muscle.

Types

Capillaries come in three types:

Continuous - Continuous capillaries have a sealed endothelium and only allow small molecules, water and ions to diffuse. Fenestrated - Fenestrated capillaries (derived from "fenestra," the Latin word for "window") have openings that allow larger molecules to diffuse. Sinusoidal - Sinusoidal capillaries are special forms of fenestrated capillaries that have larger openings in the epithelium allowing red blood cells and serum proteins to enter.

Trivia

The total length of capillaries in an average adult human is approximately 42,000 km (25,000 mi), which if laid out in full would encircle the entire equator of planet Earth with some 1000 miles to spare.