Physiology of the choroidal vascular bed.

The choroidal vascular bed has many interesting features such as relatively wide but flat capillaries, fenestrated capillary walls and an enormous blood flow. The high flow rate results in a high oxygen tension in the tissue and is also of importance in the temperature control of the eye. The capillary wall is permeable to plasma proteins which is probably of great importance for the supply of vitamin A to the pigment epithelium. The permeability to low molecular weight substances is very high which results in a tissue fluid similar to plasma with respect to small molecules. It is not clear whether the choriocapillaris is normally reabsorbing fluid transported into the choroid from the retina and from the anterior chamber or if there is a net filtration from the choriocapillaris. Fluid can pass from the choroid through the suprachoroid into the episcleral tissues via the scleral substance and spaces around the blood vessels and nerves.