Capillaries are the only blood vessels that allow diffusion between the blood and interstitial fluids. One of the reasons for this is that the walls of capillaries are extremely thin - about the diameter of one red blood cell. Capillaries are generally divided into two groups - continuous and fenestrated.
Continuous capillaries are found in all body tissue except cartilage and epithelia. Continuous capillaries allow water and other small solutes and lipids to move into the interstitial fluid. Continuous capillaries are part of the blood-brain barrier.
Fenestrated capillaries have pore-like openings in the endothelial lining and allow water and larger solutes like small peptids to cross into the interstitial fluid.Fenestrated capillaries can be found in the choroid plexus of the brain.
Sinusoids are similar to fenestrated capillaries but they have bridge-like gaps between endothelial cells Sinusoids are commonly found in organs like the liver and spleen and endocrine glands.
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