A sinusoid is a small blood vessel that is a type of capillary similar to a fenestratedendothelium. Sinusoids are actually classified as a type of Open Pore Capillary (aka discontinuous) as opposed to fenestrated. 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.