The choroid lacks autoregulation, that is, it lacks metabolic regulation. We have found that when neonatal dogs are exposed to 100 percent oxygen for 4 days the choroidal vasculature does not constrict. In cat the Po2 in choroid is normally around 70 mmHg, whereas in hyperoxia it increases to 250 mmHg [5]. As a consequence, systemic hypoxia and elevated intraocular pressure lead to decreases in choroidal Po2. The choroidal vasculature supplies oxygen to outer retina while retinal vasculature supplies oxygen to inner retina. From the measurements and modeling of Linsenmeier's lab [5], the photoreceptors live at the edge. The retina consumes more oxygen per gram of tissue than many other tissues, including brain, barely receiving enough oxygen for maintenance of its normal metabolic function. All of the oxygen for photoreceptor metabolism is supplied by the choroidal vasculature in light conditions and 90 percent of the oxygen consumed by photoreceptors in the dark [5]. As a consequence, choroidal nonperfusion is extremely detrimental to photoreceptor function and viability and retinal detachment from choroid also results in photoreceptor death.

Blood flow rate in cat choroid is said to be at least 10 times higher than retinal blood flow as measured by radioactive microspheres [6]. High blood flow was thought necessary to provide enough oxygen to the retina. However, our recent study using fluorescently labeled erythrocytes demonstrates that, at least in rat, the RBC velocity in chori-ocapillaris is actually four times slower than in retinal capillaries. The difference in these two values may be due to species differences, but it is more likely due to techniques where the former values are based on blood flow in all choroidal vessels whereas, the RBC velocities where measured directly in capillaries.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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