Capillary

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The word capillary is used to describe any very narrow tube or channel through which a fluid can pass. See capillary action for details.

Capillaries, are the smallest of a body's blood vessels, measuring 5-10 μm. They connect arteries and veins, and most closely interact with tissues. Capillaries have walls composed of a single layer of cells, the endothelium. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. Capillary permeability can be increased by the release of certain cytokines.

The endothelium also actively transports nutrients, messengers and other substances. Large molecules may be too big to diffuse across endothelial cells. 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 (e.g. active muscle cells) by inducing relaxation of smooth muscle. Note that the capillaries do not possess this smooth muscle in their own walls, and so any change in their diameter is passive. Any signalling molecules they release (such as endothelin for constriction and Nitric oxide for dilation) act on the smooth muscle cells in the walls of nearby, larger vessels, e.g. arterioles.

Types

Capillaries come in three types:

Continuous - Continuous capillaries have a sealed epithelium and only allow small molecules, water and ions to diffuse.
Fenestrated - Fenestrated capillaries (as their name implies "fenster") have openings that allow larger molecules to diffuse.
Sinusoidal - Sinusoidal capillaries are special forms of fenestrated capillaries that have larger opening allowing RBCs and serum proteins to enter.

Details

The total length of capillaries in an average adult human is approximately 40,000 km (25,000 mi).

Cardiovascular system - [http://encycl.opentopia.com/ edit]
Blood > Heart → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → Heart → Pulmonary arteries → Lungs → Pulmonary veins → Heart

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