Until the early twentieth century, aqueous humor was regarded as a stagnant fluid.⁶⁸ Since that time, however, it has been shown to be continuously formed and drained,⁶⁹ and the associated anatomic drainage portals (Schlemm’s canal, collector channels, aqueous veins, and ciliary muscle interstices) have been described.^70,71,72,73

Three physiologic processes contribute to the formation and chemical composition of the aqueous humor¹: diffusion, ultrafiltration (and related dialysis), and active secretion. The first two processes are passive and therefore require no active cellular participation. Diffusion of solutes across cell membranes occurs down a concentration gradient, and substances with high lipid solubility coefficients that can easily penetrate biological membranes move readily in this way. Ultrafiltration is the term used to describe the bulk flow of blood plasma across the fenestrated ciliary capillary endothelia into the ciliary stroma; it can be increased by augmentation of the hydrostatic driving force (see Fig. 2). These processes (diffusion and ultrafiltration) are responsible for the formation of the “reservoir” of the plasma ultrafiltrate in the stroma, from which the posterior chamber aqueous is derived through active secretion across the ciliary epithelium. Active secretion requires energy, normally provided by the hydrolysis of adenosine triphosphate (ATP). The energy is used to secrete substances against a concentration gradient. Energy-dependent active transport of sodium into the posterior chamber by the nonpigmented ciliary epithelium (NPE) (see Fig. 2) results in water movement from the stromal pool into the posterior chamber. Although controversy has surrounded the relative quantitative roles of ultrafiltration and active secretion, it seems fairly certain that under normal conditions active secretion accounts for perhaps 80% to 90% of total aqueous humor formation.^{1,11,74,75,76,77,78,79,80,81} The observation that moderate alterations in systemic blood pressure and ciliary process blood flow have little effect on aqueous formation rate supports this notion.^11,74,82 Moreover, Bill noted that the hydrostatic and oncotic forces that exist across the ciliary epithelium—posterior aqueous interface favor resorption, not secretion, of aqueous humor.⁷⁴ Active secretion is essentially pressure-insensitive at near-physiologic intraocular pressure (IOP). However, the ultrafiltration component of aqueous humor formation is sensitive to changes in IOP, decreasing with increasing IOP. This phenomenon is quantifiable and is termed facility of inflow or pseudofacility (C_ps)—the latter because a pressure-induced decrease in inflow appears as an increase in outflow when techniques such as tonography and constant-pressure perfusion are used to measure outflow facility.^{1,83,84,85,86,87,88,89} C_ps quantities in monkeys^87,90 and humans⁹¹ are approximately 0.02 μl and 0.06 μl × min^-1 × mmHg^-1, respectively, although the latter may be an overestimate.

In most mammalian species, the turnover constant of the anterior chamber aqueous humor is approximately 0.01 × min^-1, that is, the rate of aqueous humor formation and drainage is about 1% of the anterior chamber volume per minute.^{76,92,93,94,95,96} This is true also in the normal human eye, in which the aqueous formation rate is approximately 2.0 to 2.5 μl × min^-1.^{1,8,97,98,99,100,101}

More comprehensive theoretic analyses of the fluid mechanics of aqueous production can be found elsewhere.^{84,99,102,103,104,105}

The active process of aqueous secretion is mediated through selective transport of certain ions and substances across the basolateral membrane of the NPE against a concentration gradient. Two enzymes abundantly present in the NPE are intimately involved in this process: sodium-potassium-activated adenosine triphosphatase (Na⁺ -K⁺ -ATPase) and carbonic anhydrase (CA).^{78,106,107,108,109,110,111,112,113,114,115,116,117} Na⁺ -K⁺ -ATPase is found predominantly bound to the plasma membrane of the basolateral infoldings of the NPE.^{78,108,109,110,111,113,114116} The enzyme provides the energy for the metabolic pump, which transports sodium into the posterior chamber, by catalyzing the reaction ATP → ADP + Pi + energy,^118,119 where ATP = adenosine triphosphate, ADP = adenosine diphosphate, and Pi = inorganic phosphate.

As a result of active transport, aqueous humor in humans exhibits greater levels of ascorbate, some amino acids, and certain ions such as Cl-.^99,101 There is also a passive transporter for HCO3-.^1,120 A summary of the biochemistry of aqueous secretion is shown in Figure 3.

Inhibition of the ciliary process Na⁺ -K⁺ -ATPase by cardiac glycosides (e.g., ouabain) or vanadate (VO₃-, VO₄³-) significantly reduces the rate of aqueous humor formation and consequently IOP in experimental animals^{108,113,119,121,122,123,124} and humans.¹²³ Cardiac glycosides appear to act at the extracellular aspect of the membrane-bound enzyme, but vanadate acts at the cytoplasmic surface.^124,125 Cardiac glycosides given topically are ineffective as ocular hypotensives and may cause corneal edema by interfering with the Na⁺ -K⁺ -ATPase—dependent sodium pump in the corneal endothelium.¹²¹ Intravitreal and systemic administration are effective but carry unacceptable ocular and cardiovascular risks, respectively.^121,126 Vanadate is effective topically in both rabbits and monkeys, apparently without producing acute corneal edema.^119,122,123

CA is abundantly present in erythrocytes, renal tubules, and the basal and lateral membranes and cytoplasm of the pigmented epithelium and NPE of the ciliary processes.^{80,127,128,129,130,131,132} CA catalyzes reaction I of the sequence

CO₂ + H₂O →I H₂CO₃ →II H⁺ + HCO₃–

Reaction II is a spontaneous, virtually instantaneous ionic dissociation.^118,131 The actual sequence may be far more complex, involving energy-dependent separation of H⁺ and OH- at membrane boundaries within the NPE cell, and formation of HCO₃– by CA-catalyzed association of OH- with CO₂.^80,133 The reaction sequence shown above provides the HCO₃-, which is essential for the active secretion of aqueous humor. Although the exact roles of CA and HCO₃– are still debated,^{76,78,80,120,133,134,135,136} it has been demonstrated that inhibition of the production of HCO₃– also leads to an inhibition of the active transport of Na⁺ across the NPE into the first-formed aqueous, thereby reducing active aqueous humor formation (AHF). Several hypotheses explain the relationship between reduction in NPE intracellular HCO₃– and inhibition of Na⁺ transport,¹ including: (1) inhibition of CA causes a decrease in HCO₃– available for transport with Na⁺ from the cytosol of the NPE to the aqueous, which is required to maintain electroneutrality; (2) reduction in intracellular pH inhibits Na⁺ -K⁺ -ATPase, and (3) decreased availability of H⁺ produced by reaction II decreases H⁺ /Na⁺ exchange and reduces the availability of intracellular Na⁺ for transport into the intercellular channel. In addition, inhibition of renal and erythrocyte CA leads to a systemic acidosis that promotes inhibition of AHF.¹³⁷

CA inhibitors (e.g., acetazolamide,¹³⁸ methoxazolamide, ethoxzolamide, dichlorphenamide,^129,139 aminozolamide,¹⁴⁰ trifluormethazolamide¹⁴¹) given systemically can reduce secretion by as much as 50%^{127,128,129,132,139,140,141,142,143,144,145,146,147,148} and have been in use for clinical glaucoma therapy for more than 40 years.^{135,136,139,149} With low doses of certain CA inhibitors (e.g., methazolamide), it is possible to inhibit the ocular enzyme without affecting the renal and erythrocyte enzymes, thus producing submaximal secretory suppression without systemic acidosis.^{80,129,139,150,151} It was once thought that the drug concentration at the ciliary epithelium required to produce the almost continuous, total ciliary CA inhibition necessary to achieve adequate and sustained reduction in AHF (over 99% of the ciliary enzyme must be inhibited to achieve significant secretory suppression) might never be attainable through the eye-drop route,^{80,129,136,139,151}, but the topically effective CA inhibitor dorzolamide,^{146,147,152,153,154,155} which may achieve nearly the same reduction in IOP as the earlier CA inhibitors but without their systemic side effects, is currently available.¹³⁷

The composition of aqueous humor differs from that of plasma as a result of two important physiological characteristics of the anterior segment: a mechanical epithelial/endothelial blood-aqueous barrier, and active transport of various organic and inorganic substances by the ciliary epithelium. The greatest differences are the low protein and high ascorbate concentrations in the aqueous relative to plasma (about 200 times less and 20 times greater, respectively).^{156,157,158,159,160,161,162} The high ascorbate concentration may help protect the anterior ocular structures from ultraviolet light-induced oxidative damage. When the aqueous protein concentration rises significantly above its normal level of approximately 20 mg/100 ml,⁷⁶ as in uveitis, the resultant light scattering (Tyndall effect) makes visible the slit-lamp beam as it traverses the anterior chamber (a phenomenon known as “flare”). Lactate also is normally in excess in the aqueous, presumably as a result of glycolytic activity of the lens, cornea, and other ocular structures.^76,143 Other compounds or ions in excess in the aqueous relative to the plasma are Cl- and certain amino acids.¹⁰¹

The blood-aqueous barrier (BAB)is a functional concept, rather than a discrete structure, invoked to explain the degree to which various solutes are relatively restricted in travel from the ocular vasculature into the aqueous humor. The capillaries of the ciliary processes and choroid are fenestrated, but the interdigitating surfaces of the retinal pigment epithelia and the ciliary process NPE respectively are joined to each other by tight junctions (zonulae occludens) and constitute an effective barrier to intermediate- and high-molecular weight substances, such as proteins.^{163,164,165,166,167,168,169,170,171,172,173} The endothelia of the inner wall of Schlemm’s canal are similarly joined,^166,174 preventing retrograde movement of solutes and fluid from the canal lumen into the TM and anterior chamber. The iris and retina have no similar epithelium between their vasculature and the ocular fluids, but their capillaries are of the nonfenestrated, impermeable type.^{11,76,101,165,168} For present purposes, one may say that the BAB comprises the tight junctions of the ciliary process NPE, the inner wall endothelium of Schlemm’s canal, the iris vasculature, and the outward-directed active transport systems of the ciliary processes. A more universal concept of the BAB must explain the movement of smaller molecules, lipid-soluble substances, and water into the eye.⁷⁶

With disease-, drug-, or trauma-induced breakdown of the BAB (Table 1), plasma components enter the aqueous humor. Net fluid movement from blood to aqueous increases, but so does its IOP-dependence (C_ps).¹⁷⁵ Total facility, as measured by IOP-altering techniques, cannot distinguish C_ps from total outflow facility (C_tot) and therefore erroneously records the C_ps component as increased C_tot (hence the term, ‘pseudofacility’) and underestimates the extent to which the outflow pathways have been compromised by the insult. Under these circumstances, increased C_ps provides some protection against a precipitous rise in IOP; as IOP rises, aqueous inflow by ultrafiltration is partly suppressed, blunting (but not completely suppressing)^88,176 further IOP elevation. Additionally, the inflammatory process that occurs during BAB breakdown leads to a reduction in active secretion of aqueous humor, possibly through interference with active transport mechanisms.¹⁷⁷ This in turn may actually produce ocular hypotony, despite compromised outflow pathways (because of plasma protein blockage of the TM). Prostaglandin release during inflammation may contribute to the hypotony by increasing aqueous outflow through the uveoscleral route.^178,179 When the noxious stimulus is removed, however, the ciliary body may recover before the TM, and the resulting normalization of AHF rate in the face of still-compromised outflow pathways leads to elevated IOP, as seen from the modified Goldmann equation IOP = [(F – U)/C_trab] + P_e, where F = aqueous humor flow, C_trab = facility of outflow from the anterior chamber through the trabecular meshwork and Schlemm’s canal, IOP = intraocular pressure, P_e = episcleral venous pressure (the pressure against which fluid leaving the anterior chamber through the trabecular-canalicular route must drain), and U = uveoscleral outflow.¹⁸⁰

The ciliary processes possess the ability to actively transport (or exclude) a variety of organic and inorganic compounds and ions from the eye, that is, to move them from the aqueous or vitreous to the blood against a concentration gradient. Para-aminohippurate (PAH), diodrast, and penicillin are examples of large anions that are actively transported out of the eye. These systems are similar to those in the renal tubules and satisfy all the criteria for active transport, including saturability, energy and temperature dependence, Michaelis-Menten kinetics, and inhibition by ouabain and probenecid among others.^{101,181,182,183,184,185,186,187,188,189,190,191,192} In addition, another system actively excretes injected iodide from the aqueous, resembling iodide transport in the thyroid and salivary glands.¹⁸⁷ The physiologic role of these outward-directed systems is unknown. With the discovery that prostaglandins may be actively transported out of the eye,^193,194 some workers have suggested that such outward-directed mechanisms may rid the eye of biologically active substances that are no longer needed or may even be detrimental.^{186,188,195,196,197} Other outwardly-directed ion-uptake mechanisms are present in the eye. The anterior uvea of the rabbit eye, for example, accumulates the anions cholate, glycocholate, deoxycholate, chenodeoxycholate, iodipamide, and o-iodohippurate.^181,185 At least one outwardly-directed cationic pump also has been reported; iris-ciliary body (ICB) preparations accumulate the cation emepronium,¹⁹⁸ although one report¹⁹⁹ questioned whether any other cations are actively eliminated from the eye.

Bárány,¹⁸³ making an analogy to the ion pump located at the renal peritubular cell border adjacent to the blood, which pumps simple cations from the blood to the kidney tubule, investigated whether there are any inwardly-directed complex cation pump systems from blood to aqueous, but concluded that such mechanisms probably do not exist, at least in the rabbit. All of these transport systems (inward- and outward-directed) are thought to be located in the NPE.⁷⁶

Sympathetic and parasympathetic nerve terminals are present in the ciliary body^{200,201,202,203,204} and arise from branches of the long and short posterior ciliary nerves. These nerve fibers are of both myelinated and nonmyelinated variety. Parasympathetic fibers originate in the Edinger-Westphal nucleus of the third cranial nerve, run with the inferior division of this nerve in the orbit, and synapse in the ciliary ganglion.²⁰⁵ Sympathetic fibers synapse in the superior cervical ganglion and are distributed to the muscles and blood vessels of the ciliary body. Numerous unmyelinated nerve fibers surround the stromal vessels of the ciliary processes; these are most likely noradrenergic and subserve vasomotion.⁷⁶ Sensory fibers arise from the ophthalmic division of the trigeminal nerve and enter the ciliary body, but their distribution and function have not been studied thoroughly. No innervation of the ciliary epithelium has been found anatomically,¹⁶⁶ but stimulation of the ciliary ganglion leads to an increase in AHF in the enucleated arterially perfused cat eye,²⁰⁶ suggesting that neurotransmitters released in the ciliary stroma might diffuse toward the epithelia.