Microcirculation of the Ocular Fundus

The eye fundus in higher mammals is supplied by two distinct vascular systems, namely the retinal and the uveal systems. Both systems are derived from the ophthalmic artery (OA), a branch of the internal carotid. The uveal system includes the anterior uvea, which comprises the iris and the ciliary body, and the choroid. The retina has a dual blood supply, the inner two-thirds being nourished by branches from the central retinal artery (CRA), whilst the outer third is nourished by the choroidal circulation. Quantitative information on blood velocity and flow in the normal human eye under physiological conditions has been obtained from the various ocular vascular systems using a number of different techniques. In the human eye, two vascular systems are present which differ anatomically and physiologically: the retinal vessels, which supply the neural region of the retina and the prelaminar portion of the ON and the uveal or ciliary blood vessels, which supply the rest of the eye. The rate of blood flow in the intraocular tissues is determined by the ocular perfusion pressure (OPP) and the vascular resistance (R). R depends upon the blood viscosity, length, and diameter of the blood vessels. The diameter of the resistance vessels is modulated by the interaction of multiple control mechanisms affecting the smooth muscle and vascular pericytes: myogenic, metabolic, neurogenic, humoral, and endothelium-derived factors. The determinants of ocular blood flow include systemic, local, neural, endothelial, endocrine, and paracrine factors.