Return of aqueous humor to the systemic circulation is facilitated by the lower pressure of the episcleral venous system 9 mmHg) relative to the anterior chamber 16 mmHg), as aqueous percolates through the trabecular meshwork and collects into the canal of Schlemm (1). A second pressure-independent pathway, called the uveoscleral route, provides an important contribution to aqueous outflow in humans. In contrast, rabbits have virtually no aqueous outflow by this route (9). Resistance to flow, or aqueous humor outflow facility, is used to describe the passive resistance of the trabecular meshwork to the passage of aqueous humor (10,11). The pressure-independent flow pathway behaves like a constant-rate pump; however, no metabolically dependent process has been identified as a driving force for pressure-independent flow (11). The uveoscleral pathway is described as the slow entry of aqueous humor through the face of the ciliary body just posterior to the scleral spur, with movement by bulk flow through the tissue and absorption into the uveal vessels or into periocular orbital tissues (10). There is considerable discussion concerning whether or not a significant energy-dependent component of the outflow pathway exists (10). The cells of the trabecular meshwork have phagocytic activity (12-14), which may contribute to increased facility of outflow. Trabecular meshwork outflow is biologically active, providing biochemical modulation of a passive physical process (10).
The relationship between inflow and outflow provides a means for estimating the intraocular pressure (IOP). This relationship is described as:
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