The supply of oxygen to the outer third of the retina (avascular) is the responsibility of the choroid, which in turn receives its blood supply from the short and long PCA. Its drainage is carried out into the central retinal vein, which also receives blood from the optic nerve. Occasionally they cross with the corresponding arteries, sharing the adventitial layer with them. Retinal veins (especially venules) are also found in the inner retina. Venous drainage from the retina usually accompanies arterial vascularization. Capillaries are also absent in the extreme periphery of the retina and in the fovea, delimiting the FAZ, with a diameter of 400-500 μm in normal eyes. Capillary-free zones are distinguished around each artery and each larger vein. To ensure adequate perfusion of all cells in the inner retina, the retinal capillaries have a configuration described as a “mesh”. The capillaries of the deep plexus have a regular distribution around the foveal avascular zone (FAZ), with small and complex interconnections of a finely reticular shape, resembling a meshwork. They present a linear and continuous shape, with a homogeneous wall. The vessels of the superficial retinal plexus are evenly distributed, resembling a spider's web. Peripapillary capillaries have a radial distribution, these rarely anastomose (in contrast to other intraretinal capillaries) and have a limited extension to the posterior pole. These capillaries arise from single arterioles around the disc from the short PCA, not from vessels within the disc. Peripapillary radial capillaries are in a "2-D" network in the nerve fiber layer and are superficial to the "3-D" network of capillaries in the inner retina. Several scientific publications have resorted to OCT-A to better clarify this anatomy, as discussed later. Typically, they are both regarded as a single deep capillary plexus. The intermediate and deep capillary plexuses, correspond to the capillary beds on either side of the inner nuclear layer.The plexus of superficial capillaries at the nerve fiber layer, and retinal ganglion cell layer.The peripapillary radial capillary network, is located in the nerve fiber layer and around the optic nerve head.At the histological level, four vascular networks are present in the retina of the posterior pole: Retinal arteries, similar to veins, remain in the nerve fiber layer, allowing capillaries to arrange themselves into various networks in the inner retina. Occasionally (18% to 32% of eyes), there is a cilioretinal artery from the ciliary circulation, which irrigates the macula. The vessel that crosses the horizontal raphe is called a congenital retinal macrovessel. This anatomical distribution is maintained throughout the retina, as normal retinal vessels rarely cross the horizontal raphe. Then, the upper and lower branches divide into their respective temporal and nasal branches. Immediately before leaving the optic nerve, the CRA divides into superior and inferior branches, which measure an approximate diameter of 80 μm at the level of the optic disc edge (contrasting with the average diameter of around 125 μm of the veins at this level). The CRA is considered a “terminal branch” since it does not present significant anastomoses with other arteries in its course. Retinal blood vessels from the CRA supply the inner two-thirds of the retinal thickness. The retina is supplied by two distinct vascular systems from important branches of the ophthalmic artery: The ophthalmic artery, the first intracranial subdivision of the internal carotid artery, gives rise to a series of arteries, responsible for nourishing the various ocular structures.
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