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 nutrientand waste chemicalsubstances between bloodand surrounding tissues. [cite book
last = Maton
first = Anthea
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
isbn = 0-13-981176-1]
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.
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, like water and ions to diffuse. Continuous capillaries can be further divided into two subtypes: :# Those with numerous transport vesicles and
tight junctions that are primarily found in skeletal muscles, lungs, gonads, and skin. :# Those with few vesicles and tight junctions that are primarily found in the central nervous system.
*Fenestrated - Fenestrated capillaries (derived from "fenestra," the
Latinword for "window") have pores in the the endothelial cells (60-80 nm in diameter) that are spanned by a diaphragm of radially oriented fibrils and allow small molecules [BUHistology|22401lba] [cite book | title = Functional Ultrastructure: An Atlas of Tissue Biology and Pathology| author = Pavelka, Margit; Jürgen Roth| publisher = Springer | year = 2005 | page = 232] and limited amounts of protein to diffuse. In the renal glomerulus there are larger fenestrae which have no diaphragms. Both types of fenestrated blood vessels have continuous basal lamina and are primarily located in the endocrine glands, intestines, pancreas and glomeruli of kidney.
*Sinusoidal - Sinusoidal or discontinuous capillaries are special fenestrated capillaries that have larger openings (30-40 μm in diameter) in the epithelium to allow
red blood cells and serum proteins to enter, a process that is aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles and gaps may be present in cell junctions permitting leakage between endothelial cells. Sinusoid blood vessels are primarily located in the liver, spleen, bone marrow, lymph nodes, and adrenal cortex.
The capillary wall is a one-layer endothelium so thin that gas and molecules such as
oxygen, water, proteinsand lipidscan pass through them driven by osmotic and hydrostatic gradients. Waste products such as carbon dioxideand ureacan diffuse back into the bloodto 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 regulationby inducing relaxation of smooth musclein 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
diameteris passive. Any signaling molecules they release (such as endothelinfor constriction and nitric oxidefor 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.
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
infectionand aids extravasionof these cells into the tissue.
Ibn al-Nafistheorized a "premonition of the capillary circulation in his assertion that the pulmonary veinreceives what comes out of the pulmonary artery, this being the reason for the existence of perceptible passages between the two."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")] Marcello Malpighiwas the first to observe and correctly describe capillaries when he discovered them in a frog's lung in 1661. [cite book | title = Blood Vessels | author = John Cliff, Walter | year = 1976 | publisher = CUP Archives | page = 14]
Blood brain barrier
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