Capillary

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Capillaries are the smallest of a bodies 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.

Structure

When you get cold and come inside, it feels hot because your capillaries are constricting due to the overwhelming change in external temperature. The osmotic pressure is trying to increase, but the arteriole venule will not let this process to develop, and so pain and heat become evident.

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 autoregulation 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 signalling 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.

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 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.

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."

Marcello Malpighi was the first to physically observe capillaries and accurately explain them in 1661.

References

External links

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Last updated on Wednesday March 12, 2008 at 15:13:25 PDT (GMT -0700)
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