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A sinusoid is a type of capillary that is similar to a fenestrated capillary. Sinusoids are classified as a type of open pore capillary or discontinuous capillary as opposed to the continuous types. Fenestrated capillaries have diaphragms that cover the pores whereas sinusoid capillaries lack a diaphragm and just have an open pore. The open pores of endothelial cells greatly increase their permeability. In addition, permeability is increased by large inter-cellular clefts and fewer tight junctions. The level of permeability is such as to allow small and medium-sized proteins such as albumin to readily enter and leave the blood stream.

Scanning electron micrograph of a liver sinusoid with fenestrated endothelial cells. Fenestræ are approx 100 nm in diameter.
Latinvas sinusoideum
Anatomical terminology

Sinusoids are found in the liver, lymphoid tissue, endocrine organs, and hematopoietic organs such as the bone marrow and the spleen. Sinusoids found within terminal villi of the placenta are not comparable to these because they possess a continuous endothelium and complete basal lamina. This word was first used in 1893.

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