Uveoscleral aqueous outflow in the rhesus monkey: importance of uveal reabsorption.

The uveal absorption of aqueous humor at two different intraocular pressures was measured in rhesus monkeys by sampling vortex vein blood during anterior chamber perfusion of Ringer's solution containing both fluorescein and 125I albumin. At 20 mm. Hg, an excess of fluorescein equivalent in amount to 0.45 microliter/min. of anterior chamber perfusate and an excess of 125I albumin equivalent to 0.18 microliter/min. of perfusate was found in the vortex vein blood compared to systemic blood. At 32 mm. Hg, an excess of 0.86 microliter/min. of fluorescein and 0.26 microliter/min. of 125I albumin was measured. The increase in fluorescein absorption at the higher intraocular pressure was significant (0.025 less than p less than 0.05), with a uveal outflow facility of 0.034 microliter/min./mm. Hg. At an intraocular pressure of 20 mm. Hg, uveal aqueous outflow is less than 10% of total aqueous outflow. The pressure dependence of the uveal uptake of fluorescein implies an aqueous reabsorption into the uveal vessels by ultrafiltration. Uveoscleral aqueous "outflow" appears to consist of intraocular uveal reabsorption of water and small molecules from the aqueous, with only larger molecules like albumin actually leaving the eye through posterior scleral vessel perforations. This reabsorption plays a minor role in intraocular pressure regulation under normal conditions.