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A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the innermost layer of an artery or vein), consisting of a thin wall of simple squamous endothelial cells. [2]
In continuous capillaries the endothelial cells are tightly spaced, allowing only small molecules like ions or water to diffuse through the intercellular clefts (the gaps between the endothelial cells). In fenestrated and sinusoidal capillaries there is more space between the cells, allowing the diffusion of macro-molecules and some proteins.
Diffusion through the capillary walls depends on the permeability of the endothelial cells forming the capillary walls, which may be continuous, discontinuous, and fenestrated. [4] The Starling equation describes the roles of hydrostatic and osmotic pressures (the so-called Starling forces ) in the movement of fluid across capillary endothelium .
Continuous blood capillaries have the smallest intercellular clefts, with discontinuous blood capillaries having the largest intercellular clefts, commonly accompanied with gaps in the basement membrane 6.Often, fluid is forced out of the capillaries through the intercellular clefts. Fluid is push out through the intercellular cleft at the ...
Glomerular capillaries have a continuous glycocalyx layer in health and the total transendothelial filtration rate of solvent to the renal tubules is normally around 125 ml/ min (about 180 litres/ day). Glomerular capillary is more familiarly known as the glomerular filtration rate (GFR).
Pericytes are embedded in the basement membrane of blood capillaries, where they communicate with endothelial cells by means of both direct physical contact and paracrine signaling. [3] The morphology, distribution, density and molecular fingerprints of pericytes vary between organs and vascular beds.
The barrier is formed by the continuous blood capillaries in the thymic cortex, reinforced by type 1 epithelial reticular cells (sometimes called thymic epithelial cells) and macrophages. The existence of this barrier was first proposed in 1961 and demonstrated to exist in mice in 1963. [1]
Lymph capillaries or lymphatic capillaries are tiny, thin-walled microvessels located in the spaces between cells (except in the central nervous system and non-vascular tissues) which serve to drain and process extracellular fluid. Upon entering the lumen of a lymphatic capillary, the collected fluid is known as lymph.