Sinusoid (blood vessel)

A sinusoid is a small blood vessel that is a type of capillary similar to a fenestrated endothelium. Sinusoids are actually classified as a type of open pore capillary (or discontinuous) as opposed to continuous and fenestrated types. Fenestrated capillaries have diaphragms that cover the pores whereas open pore 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.