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Capillaries are small vessels that supply blood to the tissues themselves. The form an extensive blood flow network, through capillary beds, that ensure that no cells in the body are too far away from a blood supply. Capillaries are the site of exchange of chemical substances between the blood and tissues of the body.
There are three types of the capillary, each of which has a slightly different structure according to its function. Continuous Capillaries consist of a thin endothelial wall, usually only one cell thick, surrounded by a basement membrane. This thin layer ensures that the distance for diffusion is very small between the vessel and the tissue. Fick’s Law states that‘the rate of diffusion is proportional to the concentration difference and area available for diffusion, whilst being inversely proportional to the diffusion distance.’
Continuous Capillaries structure are very much designed to increase the efficiency of this diffusion. To increase the maximum area of diffusion available to tissues in the body, capillaries are present in great numbers. A small lumen aids in this ability to have many capillaries present in a small space. The small lumen also aids in slowing down the flow of blood, enabling efficient exchange across the thin endothelial walls. In fact, only one blood cell at a time can pass through the capillaries, again maximizing exposed surface area. The concentration gradient is maintained by having each capillary receive a constant supply of blood, allowing molecules in the capillaries to be exchanged with the tissue. As mentioned before, diffusion difference is reduced by having the walls of the capillaries be only one cell thick.
Fenestrated Capillaries are similar to continuous, except that they have pores (fenestrations) built into the endothelial layer. These make the capillary permeable to certain larger molecules. The number and size of these pores depend on the location of the capillary and its function. They are often found in the kidneys, aiding in the filtration of blood.
Sinusoid capillaries are the least common form of the capillary, found in the liver, bone marrow, and lymph nodes. These capillaries have large intercellular gaps, alongside pores and an incomplete basement membrane. This increase in gaps allows them to transfer larger molecules into and out of the blood, including plasma proteins and even whole cells. This is vital in the bone marrow, to allow new red blood cells to enter the bloodstream, cells who would be too large to pass through regular capillaries.
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