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A 'sinusoid' is a small blood vessel similar to a capillary but with a discontinuous endothelium.
Sinusoids are found in the liver, lymphoid tissue, endocrine organs, and hematopoietic organs such as the bone marrow and the spleen.
Their highly permeable nature is due to larger inter-cellular clefts, fewer tight junctions, and discontinous endothelial cells, allows small and medium-sized proteins such as albumin to enter and leave the blood stream. Some spaces are large enough for blood cells to pass.
The sinusoids of the liver are of particular importance to the function of that organ, and are discussed in more detail at liver sinusoid.