AskDefine | Define capillary

Dictionary Definition

capillary adj
1 of or relating to hair
2 long and slender with a very small internal diameter; "a capillary tube" [syn: hairlike]


1 a tube of small internal diameter; holds liquid by capillary action [syn: capillary tube, capillary tubing]
2 any of the minute blood vessels connecting arterioles with venules [syn: capillary vessel]

User Contributed Dictionary



From capillaris.


  • (UK, Canada) /kəˈpɪləri/
  • (US, Canada) /ˈkæpɪˌlɛri/


  1. Any of the small blood vessels that connect arterioles to venules. Their walls are composed of a single layer of cells which allows molecules such as oxygen, water and lipids to 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.

Derived terms


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Extensive Definition

Capillaries are the smallest of a body's blood vessels, measuring 5-10 μm in diameter, which connect arterioles and venules, and enable the interchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissues.


When you get hot and go outside, it feels cold because your capillaries are constricting due to the overwhelming change in external temperature. The osmotic pressure is trying to increase, but the arteriole-venule anastomosis that originally shunted blood away from the capillaries will not let this process to develop, and so pain and heat become evident as tissue is deprived of oxygen.
The walls of capillaries are composed of only 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. Capillaries are so small the red blood cells need to partially fold into bullet-like shapes in order to pass through them in single file.
Capillary permeability can be increased by the release of certain cytokines, such as in an immune response.

Immune response

In an immune response, the endothelial cells of the capillary will upregulate receptor molecules, thus it signals the need for an immune response by the site of infection and aids extravasion of these cells into the tissue.

Capillary Bed

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 auto regulation by inducing relaxation of smooth muscle.
The capillaries do not possess this smooth muscle in their own walls, and so any change in their diameter is passive. Any signaling 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.
Capillaries have a wall consisting of endothelium and basement membrane only. Metarterioles provide direct communication between arterioles and venules. True capillaries branch mainly from metarterioles and provide exchange between cells and the circulation. The internal diameter of 8uM will just accommodate a red blood cell. Precapillary sphincters are rings of smooth muscles at the origin of true capillaries that regulate blood flow into true capillaries and thus control blood flow through a tissue.


Capillaries come in three types:
  • Continuous - Continuous capillaries have a sealed endothelium and only allow small molecules, water and ions to diffuse. Continuous capillaries can be further divided into two subtypes: 1. containing numerous transport vesicles and with macula occludens junction (found in skeletal muscles, lung, gonads, and skin) 2. characterized with few vesicles and with zonula occudens junctions (primarily found in central nervous system).
  • Fenestrated - Fenestrated capillaries (derived from "fenestra," the Latin word for "window") have openings that allow small molecules and limited amounts of protein to diffuse.
  • Sinusoidal - Sinusoidal (aka discontinuous) capillaries are special forms of fenestrated capillaries that have larger openings in the epithelium allowing red blood cells and serum proteins to enter.


Ibn al-Nafis theorized a "premonition of the capillary circulation in his assertion that the pulmonary vein receives what comes out of the pulmonary artery, this being the reason for the existence of perceptible passages between the two."
Marcello Malpighi was the first to physically observe capillaries and accurately explain them in 1661.


External links

capillary in Arabic: شعيرة دموية
capillary in Bosnian: Kapilari
capillary in Danish: Kapillær
capillary in German: Kapillare
capillary in Spanish: Capilar sanguíneo
capillary in Persian: مویرگ
capillary in French: Capillaire sanguin
capillary in Icelandic: Háræð
capillary in Italian: Capillare
capillary in Hebrew: נים (כלי דם)
capillary in Latvian: Kapilāri
capillary in Lithuanian: Kapiliaras
capillary in Dutch: Haarvat
capillary in Japanese: 毛細血管
capillary in Norwegian: Kapillær
capillary in Low German: Kapillar
capillary in Polish: Naczynie włosowate
capillary in Portuguese: Capilar sanguíneo
capillary in Russian: Капилляр
capillary in Simple English: Capillary
capillary in Slovak: Vlásočnica
capillary in Serbian: Капилар
capillary in Finnish: Hiussuoni
capillary in Swedish: Kapillär
capillary in Turkish: Kılcal damar
capillary in Chinese: 微血管

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