PHYSIOLOGICAL CONSIDERATIONS OF THE ANTERIOR SEGMENT

II. PHYSIOLOGICAL CONSIDERATIONS OF THE ANTERIOR SEGMENT

Aqueous humor, the watery solvent produced by the ciliary body in the posterior chamber, is, in part, an ultrafiltrate of plasma (1). However, a number of the electrolytes are present in higher concentration in aqueous humor than in blood, providing evidence of active secretory and metabolic components to aqueous formation. For example, ascorbate and lactate are 20-fold and 2-fold higher in concentration in aqueous relative to plasma, respectively (2). Aqueous humor serves a nutritive role for avascularized ocular tissues such as the cornea, trabecular meshwork, and lens (2).

A. Aqueous Humor Formation and Turnover

1. Inflow Dynamics Blood, presented at the ciliary body arterioles at relatively high hydrostatic

complex and not completely characterized ways. The protein concentration in aqueous humor is less than 1% of that present in plasma (1). Plasma proteins are prevented from entry into aqueous humor by the tight junctions located at the nonpigmented ciliary epithelium, a component of the so-called blood-aqueous barrier, analogous to the blood-brain barrier (1). Active secretion of electrolytes such as sodium, deposited at the intercellular clefts of the tight junction regions of the nonpigmented ciliary epithelium, provide for a concentration gradient favoring fluid flow from the ciliary processes to the posterior chamber (6). A number of active secretory pathways have been identified (7,8) with specific active transport systems such as Na þ /K þ - ATPase and others providing a major contribution. The formed aqueous humor flows into the posterior chamber, down a pressure gradient, and is transported via convective bulk flow through the pupil into the anterior

2. Outflow Dynamics Return of aqueous humor to the systemic circulation is facilitated by the

cular meshwork and collects into the canal of Schlemm (1). A second pres- sure-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

Anterior Segment Microdialysis 225 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 driv- ing 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:

IOP ¼ þ Pv

C where F is aqueous humor formation, or flow, U is pressure-insensitive flow,

C is the facility of inflow or pressure sensitive flow, and Pv is the episcleral venous pressure (2).