FIGURE 3-21

Schematic showing possible mechanisms of ion flow through the PE and NPE.

aqueous humor.23,35 The current belief is that the movement of Na+ and Cl− primarily drives secretion into the posterior chamber with bicarbonate ions having an indirect role by moderating Cl− flux.23 Figure 3-21 shows an abreviated schematic of ion flow.

Function and Rate of Production

The aqueous provides nutrients to the avascular cornea and lens. The primary difference between blood plasma and aqueous is in the concentration of ascorbate and of protein. Ascorbate concentration is approximately 20 times higher in aqueous than in blood plasma and must be actively transported into the aqueous. Ascorbate is thus supplied to both cornea and lens and is important as a free radical scavenger helping to guard these tissues against oxidative damage. The protein content in plasma is 200 times greater than in aqueous, a consequence of the tight junctional barrier. The low concentration of protein causes minimal light scatter and thus maximum light transmission is maintained through the aqueous. The aqueous also carries waste products from the cornea and lens and therefore has a high concentration of lactate, a metabolic waste product of the anaerobic glycolysis of the lens and cornea.

Approximately 2.5 μl of aqueous is produced per ­minute.15 Aqueous production follows the circadian rhythm with a higher rate during the day, that rate is decreased by about 50% during the night.23 It is unclear whether the fluctuation of IOP coincides with this production cycle. During sleep circulating epinephrine is decreased which may in part account for the reduction in production but is unlikely the sole factor for the ­circadian cycle.36

Although the ultrafiltration process can be influenced by changes in IOP the effect on the rate of formation is slight.15 Since active secretion is the primary mechanism for aqueous formation moderate changes in blood pressure have little effect on the rate of formation.31 Autonomic nerves located within the ciliary body can influence aqueous production by acting on the blood vessels, dilating them and increasing blood volume or decreasing volume by constricting the vessels. Although no anatomic evidence has been found identifying autonomic innervation to the epithelia, animal studies have found some alteration in production volumes in response to manipulations of the autonomic signal. Further information on the effect of aqueous production on intraocular pressure and drug treatments that reduce aqueous production will be found in Chapter 6.

Vitreous Production

Investigators recently have suggested that similar processes may occur in the epithelium of the pars plana region and have a significant role in the production and secretion of various connective tissue macromolecules located in the vitreous body.37

Blood-Aqueous Barrier

The blood-aqueous barrier selectively controls the secreted substance—aqueous humor. The fenestrated ciliary body capillaries permit large molecules to exit the blood. However, the tight zonular junctions of the nonpigmented epithelium prevent the molecules from passing between the cells, forcing them instead to pass through the cell to enter the posterior chamber. One of the substances thus controlled is protein. The protein content of aqueous humor is very small compared with that of blood.38 Proteins pass easily out of the ciliary vessels through the fenestrations but do not pass into the posterior chamber because of the tight junction barrier of the nonpigmented epithelium.15,39-41

The iris is freely permeated by the aqueous humor, which readily enters the stroma through the surface crypts.1 To prevent large molecules from leaking out of the iris blood vessels and altering the content of the aqueous fluid, the iris capillaries have no fenestrations, and their endothelial cells maintain the barrier function through their zonula occludens junctions.21,41-43

Clinical Comment: Tyndall

Phenomenon

Clinical examination of the aqueous with the biomicroscope is accomplished by focusing a conical beam within the anterior chamber and with high magnification and a dark room watching for movement within the beam. In normal