Blood enters the choroid through the posterior ciliary arteries (Fig 1-10). The outer layer of large-caliber choroidal vessels, known as the Haller layer, is relatively thick. The choroidal vessels in this layer divide into smaller-diameter vessels and precapillary arterioles in a layer known as the Sattler layer. These vessels distribute the blood throughout the choroid, reducing arterial pressure to the relatively low pressure found in the choriocapillaris. The choroid has a maximal thickness posteriorly. On histologic examination, it is 0.22 mm thick in the central macular region, becoming progressively thinner anteriorly; at the ora serrata, it is 0.1 mm thick. Subfoveal choroidal thickness measured in vivo among healthy volunteers with a mean age of 50 years by SD-OCT is approximately 287 μm. However, thickness changes with age and disease states of the eye. The presence of thin choroid (leptochoroid) and thick choroid (pachychoroid) is associated with ocular disease.
In the posterior pole, the choriocapillaris forms a plexus of capillaries, even though the capillaries themselves are not arranged strictly into lobules. The capillary arrangement becomes more irregular toward the periphery, where the capillaries are arranged more radially. Interspersed between the vessels of the choroid are loose connective tissue, fibroblasts, and melanocytes.
After passing through the choriocapillaris, the blood is collected in venules, which coalesce into collecting channels, or ampullae, of the vortex veins. Most eyes have 4 or 5 vortex veins, which exit the eye at or posterior to the equator. The vortex veins drain into the superior and inferior ophthalmic veins.
Figure 1-10 Scanning electron micrograph of the choroid. Vascular cast of the choroid from the posterior pole of a 62-year-old man, demonstrating arteries (a), veins (v), and the choriocapillaris (CH). (70×)
(Courtesy of A. Fryczkowski, MD.)
The choroid supplies the metabolic needs of the retina, which has one of the highest metabolic rates per gram of tissue in the body. In some estimates, the choroidal circulation supplies 90% of the oxygen consumed by the retina, primarily by the photoreceptors. The choroid also has the highest blood flow of any tissue, and the venous blood exiting the choroid still has a very high oxygen tension. The RPE cells, which are anatomically closely associated with the choriocapillaris, are exposed to the highest oxygen tensions of any perfused tissue, increasing the risk of oxidative damage. The rapid flow in the choroid also acts as a heat sink, removing thermal energy obtained by light absorption.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.