Capillaries supply blood directly to tissues. Through capillary bed formation, they form a vast blood flow network that ensures no cell is too far away. The exchange of chemicals between blood and body tissues occurs at capillaries.

Three types of capillaries exist, with slightly different structures depending on their function. The continuous capillary is composed of a thin, one-cell thick endothelial membrane, which is surrounded by the basement membrane. This thin membrane ensures the distance between the vessel tissue is small. Fick’s Law says that ‘the rate is proportional both to the concentration and to the area of diffusion available, while being inversely correlated to the diffusion distance’.

The continuous capillaries structure is designed to maximize the effectiveness of diffusion. There are a lot of capillaries in order to maximize the surface area for diffusion. This is possible because of the small lumen. A small lumen helps to slow down blood flow, allowing for efficient exchange of blood across thin endothelial cells. The capillaries are designed to allow only one cell of blood at a time through, thus maximizing surface area. Each capillary is supplied with constant blood to maintain the gradient. This allows the molecules within the capillaries exchange with the tissue. As stated before, diffusion differences are reduced by only having one cell in the wall of capillaries.

The endothelial membrane has pores called fenestrations. These pores make the capillary more permeable for certain molecules. The size and number of pores are determined by the capillary’s location and function. They are usually found in kidneys where they help in filtration.

These capillaries can be found in the bone marrow, lymph nodes and liver. These capillaries feature large intercellular holes, along with pores and an unfinished basement membrane. These increased gaps enable them to transport larger molecules such as plasma protein and whole cells into and out of blood. This is essential in the marrow because it allows new red cell to enter into the bloodstream.

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