Fig. 2.

Diagrams illustrating the distribution of oxygen in the normal eye and following vitrectomy or advanced vitreous degeneration. a An eye with an intact vitreous gel. Oxygen diffuses into the vitreous gel from the large retinal vessels. The curved arrows show that much of this oxygen is subsequently consumed by the adjacent retina. The result is a ‘standing gradient’ of oxygen near the retina and relatively low oxygen in the midvitreous and near the lens. The lowest oxygen partial pressure is in the midvitreous, due to the consumption of oxygen by the high concentration of ascorbic acid in the intact vitreous. b An eye with advanced vitreous degeneration (posterior vitreous detachment) or after vitrectomy. The posterior of the globe is filled with liquid, instead of gel. This fluid circulates when the eye moves or as a result of convection, as illustrated by the black, curved arrows. When oxygen is carried away from the retina, it can no longer be consumed by retinal metabolism. The increased mixing of oxygen with the ocular fluids delivers more oxygen to the lens (short arrow), where it leads to nuclear opacification.