Capillaries are small vessels that supply blood to the tissues themselves. They form an extensive network of blood flow, through capillary beds, which ensure that no cell in the body is too far from the blood supply. Capillaries are the site of exchange of chemicals between the blood and body tissues. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay There are three types of capillary, each of which has a slightly different structure depending on its function. Continuous capillaries consist of a thin endothelial wall, usually a single cell thick, surrounded by a basement membrane. This thin layer ensures that the diffusion distance between the vessel and the tissue is very small. Fick's law states that "the rate of diffusion is proportional to the concentration difference and the area available for diffusion, while it is inversely proportional to the diffusion distance". The continuous structure of the capillaries is designed to increase the efficiency of this diffusion. To increase the maximum diffusion area available to body tissues, capillaries are present in large numbers. A small lumen aids in this ability to have many capillaries present in a small space. The small lumen also helps slow blood flow, allowing efficient exchange across the thin endothelial walls. In fact, only one blood cell at a time can pass through the capillaries, once again maximizing the exposed surface area. The concentration gradient is maintained by ensuring that each capillary receives a constant supply of blood, allowing the exchange of molecules in the capillaries with the tissue. As mentioned above, the diffusion difference is reduced by having the capillary walls only one cell thick. Fenestrated capillaries are similar to continuous ones, except that they have pores (fenestrations) embedded in the endothelial layer. These make the capillary permeable to some larger molecules. The number and size of these pores depend on the position of the capillary and its function. They are often found in the kidneys, aiding in blood filtration. Please note: this is just a sample. Get a custom paper from our expert writers now. Get a Custom Essay Sinusoidal capillaries are the least common form of capillary, found in the liver, bone marrow, and lymph nodes. These capillaries have large intercellular spaces, along with pores and an incomplete basement membrane. This increase in empty spaces allows them to move larger molecules in 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, cells that would be too large to pass through regular capillaries, to enter the bloodstream.