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20-September-2008 10:21:21 - Capillary Redirected from Capillaries 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. 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. 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.1 Contents 1 Anatomy 1.1 Structure 1.2 Types 2 Physiology 2.1 Immune response 3 History 4 See also 5 References 6 External links Anatomy Blood flows from the heart to arteries, which branch and narrow into arterioles, and then branch further still into capillaries. After the tissue has been perfused, capillaries join and 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. The more metabolically active the cells, the more capillaries it will require to supply nutrients and carry away waste products. Metarterioles 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 cells to partially fold into bullet-like shapes in order to by pass them in single file. 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. Structure The walls of capillaries are composed of only a single layer of cells, the endothelium. This cell wall enables the exchange of molecules, 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 2 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. Physiology 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 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. 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.3 Marcello Malpighi was the first to physically observe capillaries and accurately explain them in 1661.4 See also Alveolar-capillary barrier Blood brain barrier Capillary action Hagen-Poiseuille equation References ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright 1993. Human Biology and Health. Englewood Cliffs, New Jersey: Prentice Hall. ISBN 0-13-981176-1. ^ Histology at BU 22401lba ^ 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. The West denies Ibn Al Nafis's contribution to the discovery of the circulation, Encyclopedia of Islamic World ^ The history of the capillary wall: doctors, discoveries, and debates External links Look up Capillary in Wiktionary, the free dictionary. Histology at BU 00903loa v d e Cardiovascular system Systemic circulation Heart → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → Heart Pulmonary circulation Heart → Pulmonary arteries → Lungs → Pulmonary vein → Heart Blood vessels Endothelium - Tunica intima - Tunica media - Tunica externa Retrieved from http://en..org/wiki/Capillary Categories: Cardiovascular system Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية Bosanski Dansk Deutsch Español Ù?ارسی Français í•œêµì–´ Ã?slenska Italiano עברית LatvieÅ¡u Lietuvių Nederlands 日本語 ‪Norsk bokmÃ¥l‬ Plattdüütsch Polski Português РуÑ?Ñ?кий Simple English SlovenÄ?ina СрпÑ?ки / Srpski Suomi Svenska Türkçe УкраїнÑ?ька ä¸æ–‡ This page was last modified on 13 August 2008, at 11:5
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