Oxygen supply to the retina from the retinal and choroidal circulations at normal and increased arterial oxygen tensions.

Abstract

During oxygen breathing small retinal vessels constrict severely and retinal blood flow drops to less than half the value on air. This paper contains a quantitative explanation of why this occurs based on published figures for the arteriovenous oxygen difference and blood flow in the retinal and choroidal circulations. The choroid has a high blood flow and a low arteriovenous oxygen difference. Once the arterial oxygen tension, paOt, rises to about 270 mm. Hg, the intake of the oxygen extraction from the choroidal blood can be met from dissolved oxygen. When pa0t rises above this level the oxygen tension in the choroidal tissue and venous blood rises in step xoith it because the capacity of reduced hemoglobin to buffer changes in oxygen pressure is lost. At a paO- of 400 mm. Hg, 97 per cent of the retina between the choriocapillaris and the deep retinal capillaries is supplied with oxygen from the choroid compared with 60 per cent on air. At higher values of pa0s there is a delivery of oxygen from the choroid to the deep retinal capillaries and under hyperbaric oxygen almost the whole thickness of retina could be supplied with oxygen from the choroid alone.