A major physiologic function for the high-flow choroidal circulation is the stabilization of the temperature environment of the retina. This is especially important in the macular region, which lies at the focal point of the eye's optical system. Observations of temperature changes in the macula under varying conditions of ambient illumination and choroidal blood flow have established that the choroidal circulation serves a dual purpose in stabilizing retinal tissue temperature, acting both as a heat source and a heat sink, depending on prevailing tissue temperatures.