 | Capillary: Encyclopedia - Capillary
Capillary
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 constricting smooth muscle.
Capillary - Types
Capillaries come in 3 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 opening 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.
Capillary - Details
The total length of capillaries in an average adult human is approximately 40,000 km (25,000 mi).
Heart - Aorta - Arteries - Arterioles - Capillaries - Venules - Veins - Venae cavae - Pulmonary arteries - Lungs - Pulmonary veins - Blood
Category: Cardiovascular system
Other related archivesAorta, Arteries, Arterioles, Blood, Cardiovascular system, Heart, Lungs, Pulmonary arteries, Pulmonary veins, Veins, Venae cavae, Venules, arteries, blood, blood vessels, capillary action, carbon dioxide, cells, cytokines, endocytosis, endothelial cells, endothelium, exocytosis, km, lipids, mi, molecules, oxygen, smooth muscle, tissues, urea, veins, water, μm
 Adapted from the Wikipedia article "Capillary", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
