ABSTRACT
The circulating intraocular fluid, the aqueous humor, is produced at the ciliary processes by a combination of energy-dependent secret ion and pressuredependent ultrafiltration. On the other hand, drainage from the eye is entirely a result of pressure-dependent bulk flow. The characteristic of a bulk flow system is that flow depends on a pressure gradient overcoming the intrinsic resistance of the tissue to allow outflow to occur at the same rate as inflow. The analogy of a faucet dripping into a sink with a partially blocked drain is of some value. The sink will fill with water until a pressure head develops that is sufficient to drive water out at the same rate that it drips into the sink. In the normal human population, the inflow of aqueous humor is between 2 and 4ml/min, the average IOP is approximately 16mmHg, and the episcleral venous pressure is of the order of 9mmHg. Therefore, a pressure head of 7mmHg is needed to overcome the intrinsic resistance of the outflow system to allow the egress of aqueous humor at the rate of 2 to 4ml/min and thus maintain ocular equilibrium.
