Capillary
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Discospinster
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Reverted edits by [[Special:Contributions/121.54.66.21|121.54.66.21]] to last version by ClueBot (using [[WP:HG|Huggle]])
[[Image:Illu capillary.jpg|thumb|300px|Blood flows from the heart to [[artery|arteries]], which narrow into [[arteriole]]s, and then narrow further still into capillaries. After the tissue has been [[perfusion|perfused]], capillaries widen to become venules and then widen more to become veins, which return blood to the heart.]]
'''Capillaries''' are the smallest of a body's [[blood vessel]]s, measuring 5-10 [[micrometre|μm]] in diameter, which connect [[arteriole]]s and [[venule]]s, and enable the interchange of [[water]], [[oxygen]], [[carbon dioxide]], and many other [[nutrient]] and [[waste]] [[chemical]] substances between [[blood]] and surrounding [[tissue (biology)|tissue]]s.<ref>{{cite book
| last = Maton
| first = Anthea
| authorlink =
| coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
| title = Human Biology and Health
| publisher = Prentice Hall
| date = 1993
| location = Englewood Cliffs, New Jersey
| pages =
| url =
| doi =
| id =
| isbn = 0-13-981176-1}}</ref>
==Anatomy==
Blood flows from the heart to arteries, which narrow into arterioles, and then narrow further still into capillaries. After the tissue has been perfused, capillaries widen to become venules and then widen more to become veins, which return blood to the heart.
The "capillary bed" is the network of capillaries supplying an [[organ (biology)|organ]]. The more [[Metabolism|metabolically]] active the cells, the more capillaries it will require to supply nutrients and carry away waste products.
[[Metarteriole]]s provide direct communication between arterioles and venules and are important in bypassing the bloodflow through the capillaries. True capillaries branch mainly from metarterioles and provide exchange between cells and the circulation. The internal diameter of 8 μm forces the red [[blood cell]]s to partially fold into bullet-like shapes in order to pass through them in single file.
Precapillary [[sphincter]]s are rings of [[smooth muscle]]s at the origin of true capillaries that regulate blood flow into true capillaries and thus control blood flow through a tissue.
===Structure===
The walls of capillaries are composed of only a single layer of [[cell (biology)|cells]], the [[endothelium]]. This cell wall enables the exchange of [[molecule]]s, water and [[gas]] driven by osmotic and hydrostatic gradients.
===Types===
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 <ref>{{BUHistology|22401lba}}</ref> and limited amounts of protein to diffuse.
*[[Sinusoid (blood vessel)|Sinusoidal]] - Sinusoidal (aka discontinuous) capillaries are special forms of fenestrated capillaries that have larger openings in the epithelium allowing [[red blood cell]]s and serum proteins to enter.
==Physiology==
[[Image:Illu_capillary_microcirculation.jpg|right]]
The capillary wall is a one-layer endothelium so thin that gas and molecules such as [[oxygen]], [[water]], [[proteins]] and [[lipids]] can pass through them driven by osmotic and hydrostatic gradients. Waste products such as [[carbon dioxide]] and [[urea]] can diffuse back into the [[blood]] to be carried away for removal from the body. The physics of this exhange is explained by the [[Starling equation]].
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]] in the arterioles that lead to the capillary bed as well as constriction of the metarterioles.
The capillaries do not possess this smooth muscle in their own wall, 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]].
Capillary [[vascular permeability|permeability]] can be increased by the release of certain [[cytokines]], such as in an [[immune response]].
===Immune response===
In an [[immune system|immune]] response, the [[endothelium|endothelial cells]] of the capillary will upregulate [[cell receptor|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.
==History==
[[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."<ref name=Paul>Dr. Paul Ghalioungui (1982), "The West denies Ibn Al Nafis's contribution to the discovery of the circulation", ''Symposium on Ibn al-Nafis'', Second International Conference on Islamic Medicine: Islamic Medical Organization, Kuwait ([[cf.]] [http://www.islamset.com/isc/nafis/drpaul.html The West denies Ibn Al Nafis's contribution to the discovery of the circulation], ''Encyclopedia of Islamic World'')</ref>
[[Marcello Malpighi]] was the first to physically observe capillaries and accurately explain them in 1661.<ref>[http://ajpheart.physiology.org/cgi/content/abstract/293/5/H2667 The history of the capillary wall: doctors, discoveries, and debates]</ref>
==See also==
*[[Alveolar-capillary barrier]]
*[[Blood brain barrier]]
*[[Capillary action]]
==References==
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{{reflist}}
==External links==
{{wiktionary}}
* {{BUHistology|00903loa}}
{{cardiovascular_system}}
[[Category:Cardiovascular system]]
[[ar:شعيرة دموية]]
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