Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 84

The retina receives 4% of the ocular blood flow in the monkey. In cats, 20% of the oxygen consumed by the retina is delivered through the retinal circulation and the remaining 80% is delivered via the choroidal circulation. Similar data are not available for other domestic animals. Blood flow in the innermost retina is practically unaffected by moderate changes in perfusion pressure. Autoregulation of retinal blood flow is extensive in the cat, monkey, and pig, and protects the retinal circulation from large variations in perfusion pressure. Both metabolic and myogenous autoregulation are present in the eye. Metabolic control of retinal blood flow is similar to that of blood flow to the brain. In the cat, maximum retinal vasodilation occurs with an increased of 75–80 mmHg, so as to increase flow from 15 to 50 ml/min. Neural control of retinal blood flow is limited to those vessels indirectly affecting retinal blood flow. Retinal vessels have α‐adrenergic binding sites that, when stimulated, cause vasoconstriction, thus increasing retinal vascular resistance. Retinal arteries most likely autoregulate through a myogenic mechanism, which is activated based on stretch. During sympathetic stimulation, myogenic autoregulatory responses appear to increase. Opening and closing of capillary beds in many tissues occur with varying metabolic needs. The vascular endothelial cells may produce nitric oxide, endothelins, prostaglandins, and renin–angiotensin products in response to chemical stimuli (e.g., acetylcholine and bradykinin), changes in blood pressure and blood vessel wall stress, changes in local oxygen levels, and other stimuli. As the mechanisms of local autoregulation become better understood, pharmacological modulation of these processes may become possible. The theoretical oxygen diffusion maximum of 143 μm plays a significant role in animal species with avascular retinas; as a result, avascular retinas are usually very thin, and have short photoreceptors, no tapeta, high glycogen levels in the Müller cells, and no retinal taper.