Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 83

The outer retina (and the entire retina in some species) depends heavily on choroidal blood flow for nutrients and waste removal. In the animal species studied, most of the blood supply to the choroid is supplied by the short posterior ciliary arteries, but some of the peripheral choroid receives blood from the major arterial circle of the iris. The choroidal capillaries are fenestrated and large (diameter 15–50 μm). These vessels are highly permeable and permit glucose, proteins, and other substances (including fluorescein) of the blood to enter the choroid.

Within the choroid, these proteins create a high osmotic pressure gradient that assists in removal of fluids from the retina. The short posterior ciliary arteries appear to supply well‐defined territories within the choroid. As a result, these “watershed zones” can develop with marked elevations of IOP (often >50 mmHg), and appear in the dog and nonhuman primate as pyramidal‐shaped areas of choroidal and retinal degeneration extending from the optic nerve head.

The rate of uveal blood flow is rapid (1.2 ml/min in the cat), with a mean combined retinal and choroidal circulation time of 3–4 s. In monkeys, 95% of the ocular blood flows through the uveal tract, of which 85% is through the choroid. With this high rate of blood flow, oxygen extraction from each millimeter of blood is low (∼5–10%). The oxygen content of choroidal venous blood is 95% of that in arterial blood. Reduced flow rates result in higher oxygen extraction, so that total extraction is reached. This protects the oxygen supply to the retina, and it also protects the eye from light‐generated thermal damage. Choroidal vessels have little to no autoregulatory mechanisms, but carbon dioxide is a potent vasodilator of choroidal vessels. Choroidal vessels are under the strong influence of sympathetic stimulation, which can result in a 60% reduction of choroidal blood flow. The α‐adrenergic drugs cause vasoconstriction of choroidal vessels, but β‐adrenergic drugs have no effect.