The influence of the intraocular pressure on the rate of drainage of aqueous humor. Stabilization of intra-ocular pressure or of aqueous flow?
Brit Jour Ophthalmol 31(3): 160-176
Expts. described wherein the equilibrium level of intra-ocular pressure was changed by unilateral closure of the common carotid artery in rabbits. The other eye was used as a control. Radiosodium was the test substance and its concn. in aqueous samples detd. Under equilibrium conditions the composition of aqueous is constant, thus Eo = Co X Fo where Eo = Rate of entrance of substance into aqueous. Co = Concentration of substance in aqueous and Fo = Rate of flow of aqueous. If equilibrium level of intra-ocular pressure is changed, then E1 = C1 X F1 and F1/F0 = (E1/E0)/(C1/C0). No difference occurred in the radiosodium content of the aqueous between the occluded side and the control eye. That is, C1 = Co and therefore F1/F0 = E1/E0. Thus the relative change in aqueous outflow equalled the relative change in rate of entrance of radiosodium, and this latter was only 7% less on the occluded side despite a 10-15% reduction in intraocular pressure. The theory is advanced that two forces control aqueous outflow[long dash]a hydrostatic one dependent on intra-ocular pressure and an osmotic one from the plasma colloids in the trabecular veins. When the hydrostatic mechanism is strong, the colloid-osmotic attraction is reduced due to a dilution effect. A "basal" flow of aqueous occurs even when the hydrostatic pressure is zero. This is contrary to the "safety-valve" theory of aqueous drainage. The maintenance of a constant circulation of aqueous to the lens takes precedence over the stabilization of the intra-ocular pressure by the drainage mechanism. The findings are not directly applicable to man, but plasma has been demonstrated in Schlemm's canal probably derived from special arterial connections.