Capillaries are small vessels that irrigate the tissues themselves. They form a vast blood circulation network, through capillary beds, which ensure that no cell in the body is too far from a blood supply. Capillaries are where chemicals are exchanged between the blood and body tissues. Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get the original essay There are three types of capillaries, each with a slightly different structure depending on their 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 diffusion distance is very small between the vessel and the tissue. Fick's law states that "the diffusion rate is proportional to the concentration difference and the surface area available for diffusion, while being inversely proportional to the diffusion distance." The structure of the continuous capillaries is designed to increase the efficiency of this diffusion. To increase the maximum diffusion surface available to body tissues, capillaries are present in large numbers. A small light facilitates 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, maximizing the exposed surface area. The concentration gradient is maintained by ensuring that each capillary receives a constant supply of blood, allowing molecules in the capillaries to be exchanged with the tissues. As mentioned previously, the diffusion difference is reduced by making the capillary walls only one cell thick. Fenestrated capillaries are similar to continuous capillaries, except that they have pores (fenestrations) embedded in 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, which aids in blood filtration. Keep in mind: this is just a sample. Get a personalized paper now from our expert writers. Get a Personalized Essay Sinusoid capillaries are the least common form of capillary, found in the liver, bone marrow, and lymph nodes. These capillaries have large intercellular spaces, as well as pores and an incomplete basement membrane. This increase in gaps allows them to transfer 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 to enter the bloodstream, cells that would be too large to pass through regular capillaries..