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]
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. 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 pass through 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
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
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