Permeability of the intraocular blood vessels.

This paper reviews quantitative studies on the permeability of the ocular blood vessels. In the retina, where there are tight junctions between the endothelial cells of the microvessels, there is very low permeability even to sodium ions. Glucose is transported through the capillary wall by a carrier, net glucose extraction amounting to about 12 per cent. In the choroid and ciliary processes, where there are fenestrated capillaries, the permeability to plasma proteins is about five times that in the kidney (another tissue with fenestrated capillaries), and ten to thirty times that in heart and skeletal muscle, which has non-fenestrated capillaries without tight junctions. Capillary permeability to low molecular weight substances is high in the choroid; sodium permeability is probably about thirty times that in heart muscle and fifty times that in skeletal muscle. This high permeability results in a high glucose concentration in the tissue fluid enabling rapid glucose movement into the retina.