Aqueous Humor Dynamics

Aqueous Humor Dynamics: A Review

Abstract: Glaucoma is a family of optic neuropathies which cause irreversible but potentially preventable vision loss. The vision loss in most forms of glaucoma is related to elevated IOP with subsequent injury to the optic nerve. The secretion of the aqueous humor and the regulation of its outflow are physiologically important processes for maintaining the IOP in the normal range. Thus, understanding the complex mechanisms that regulate aqueous humor circulation is essential for management of glaucoma. The two main structures related to aqueous humor dynamics are the ciliary body and the trabecular meshwork (TM). Three mechanisms are involved in the aqueous humor formation: diffusion, ultrafiltration and active secretion. Active secretion is the major contributor to aqueous humor formation. The aqueous humor flow in humans follows a circadian rhythm, being higher in the morning than at night. The aqueous humor leaves the eye by passive flow via two pathways - the trabecular meshwork and the uveoscleral pathway. In humans, 75% of the resistance to the aqueous humor outflow is localized within the TM with the juxtacanalicular portion of the TM being the main site of outflow resistance. Glycosaminoglycan deposition in the TM extracellular matrix (ECM) has been suggested to be responsible for increased outflow resistance at this specific site whereas others have suggested deposition of proteins such as cochlin obstruct the aqueous humor outflow through the TM. The uveoscleral outflow pathway is independent of the intraocular pressure and the proportion of aqueous humor exiting the eye via the uveoscleral pathway varies with age.


Glaucoma is a heterogeneous group of eye diseases from the viewpoint of pathogenesis and clinical expression. Glauoma is characterized by optic nerve damage, leading ultimately to irreversible blindness. Glaucoma is estimated to affect approximately 70 million people worldwide (Thylefors, Negrel et al. 1995; Quigley 1996) and more than 2 million people in the USA. In the years to come, this disease is expected to affect even greater populations, especially as the elderly population grows disproportionately (Friedman, Wolfs et al. 2004). Glaucoma is known to be a multi-factorial in origin, with established genetic and biological risk factors. However, the fundamental causes remain unknown for many types of glaucoma.

Glaucoma is often classified into primary open-angle (POAG), primary angle-closure (PACG), secondary angle-closure, and secondary open-angle, congenital and juvenile glaucomas. The most common type may differ from one region of the world to another. For instance, PACG is more prevalent in Asia, whereas POAG is more equally distributed throughout the world and is the most common form of the disease (Quigley 1996). Vision loss in most forms of glaucoma is related to elevated intraocular pressure (IOP) with subsequent injury to the optic nerve (Hollows and Graham 1966).

The secretion of the aqueous humor and the regulation of its outflow are physiologically important processes for the normal function of the eye. In the healthy eye, flow of aqueous humor against resistance generates an average intraocular pressure of approximately 15mmHg, which is necessary for the proper shape and optical properties of the globe (Millar and Kaufman 1995; Liebovitch 2006). The basic concept that impairment in aqueous humor outflow results in elevation of the IOP is a central tenet of glaucoma pathology and treatment. Therefore, understanding the complex mechanisms that regulate aqueous humor circulation is essential for improved management of glaucoma.

Functional anatomy

The two main ocular structures related to aqueous humor dynamics are the ciliary body (the site of aqueous humor production), and the limbal region, which includes the trabecular meshwork (the principal location of aqueous humor outflow).

The ciliary body attaches to the scleral spur and has the shape of a right triangle. Occupying the innermost and anterior most portion of this structure, in a region called pars plicata, are the ciliary processes. The ciliary processes are the sites of aqueous humor production. The ciliary processes been shown to have increased basal and lateral interdigitations, mitochondria and rough endoplasmic reticulum in the non-pigmented ciliary epithelium, a thinner layer of ciliary stroma, and increased numbers of cellular organelles and gap junctions as compared to other regions of the ciliary body (Hara, Lutjen-Drecoll et al. 1977). The epithelium of the ciliary processes has two layers: an inner, non-pigmented layer in contact with the aqueous humor in the posterior chamber, and an external, pigmented layer in contact with the ciliary process stroma. The apical surfaces of the two layers lie in apposition to each other (Smelser 1966; Tormey 1966). The non-pigmented ciliary epithelium represents the continuation of the retina; the pigmented epithelium, the continuation of the retinal pigmented epithelium (Ozanics and Jakobiec 1982). The posterior part of the ciliary body, called the pars plana, has a flatter inner surface and joins the choroid at the ora serrata. Both sympathetic and parasympathetic nerves supply the ciliary body. Parasympathetic fibers come from the Edinger-Westphal nucleus (Williams and Warwick 1975) and pterygopalatine ganglion (Ruskell 1970). Sympathetic fibers originate from the cervical superior ganglion and from the carotid plexus (Williams and Warwick 1975), and sensory fibers originate from the trigeminal ganglion by way of the ophthalmic nerve.

The limbus is a transitional zone between the cornea and the sclera. On its inner surface is an identation, the scleral sulcus, which has a sharp posterior margin, the scleral spur and an inclined anterior border that extends to the peripheral cornea (Hogan, Alvarado et al. 1971b; Thoft 1989).

The trabecular meshwork is the structure that overpasses the scleral sulcus and converts it into a circular channel, called Schlemm's canal. The TM is a triangular, porous structure, in cross section, that consists of connective tissue surrounded by endothelium. TM can be divided in three components: uveal meshwork, corneoscleral meshwork and juxtacanalicular meshwork (Van Buskirk 1986). Sympathetic innervation of the TM originates from the superior sympathetic ganglion. Parasympathetic innervation derives from the ciliary ganglion. Sensory nerves originate from the trigeminal ganglion (Gong, Tripathi et al. 1996).

The uveal meshwork forms the lateral border of the anterior chamber, extending from the iris root and ciliary body to the peripheral cornea. The uveal meshwork consists of bands of connective tissue, with irregular openings that measure between 25 to 75µm (Flocks 1956).

The corneoscleral meshwork extends from the scleral spur to the anterior wall of the scleral sulcus and is the most extensive portion of the TM. It is composed of perforated sheets that become progressively smaller nearing the Schlemm's canal (Flocks 1956). The corneoscleral meshwork is organized into four concentric layers, viz. from within outwards connective tissue with collagen fiber layer, elastic fiber layer, “glass membrane” layer (delicate filaments embedded in ground substance) and endothelial layer (Ashton, Brini et al. 1956; Fine 1966; Gong, Trinkaus-Randall et al. 1989).

The outermost part of the trabecular meshwork, composed of a layer of connective tissue lined on either side by endothelium, is called the juxtacanalicular meshwork (Fine 1964). The central connective tissue layer has variable thickness and is non-fenestrated and the outer endothelial layer comprises the inner wall of the Schlemm's canal (Fine 1964; Fine 1966).

Schlemm's canal (SC) is comprised of endothelial cells surrounded by connective tissue like a vein. SC possesses an internal collector channel and is connected to episcleral and conjunctival veins through the external collector channels, the intrascleral venous plexus, the deep scleral plexus and the aqueous veins (Hogan, Alvarado et al. 1971b; Gong, Tripathi et al. 1996).

Aqueous humor - Definition and overview

Aqueous humor is a clear fluid that fills and helps form the anterior and posterior chambers of the eye. The lens and cornea must remain clear to allow light transmission, and therefore cannot be invested within a vasculature. The aqueous humor is analogous to a blood surrogate for these avascular structures and provides nutrition, removes excretory products from metabolism, transports neurotransmitters, stabilizes the ocular structure and contributes to the regulation of the homeostasis of these ocular tissues. Aqueous humor also permits inflammatory cells and mediators to circulate in the eye in pathological conditions, as well as drugs to be distributed to different ocular structures (Sires 1997).

Aqueous humor provides a transparent and colorless medium between the cornea and the lens and constitutes an important component of the eye's optical system. Aqueous humor is secreted by the ciliary epithelium lining the ciliary processes and enters the posterior chamber. Initially, to reach the posterior chamber, the various constituents of aqueous humor must traverse the three tissue components of the ciliary processes - the capillary wall, stroma, and epithelial bilayer. The principal barrier to transport across these tissues is the cell membrane and related junctional complexes of the non-pigmented epithelial layer (Hogan, Alvarado et al. 1971a). Circulating aqueous humor flows around the lens and through the pupil into the anterior chamber. Within the anterior chamber, a temperature gradient creates a convective flow pattern, which is downward close to the cornea where the temperature is cooler, and upward near the lens where the temperature is warmer (Heys and Barocas 2002).

The aqueous humor leaves the eye by passive flow via two pathways at the anterior chamber angle, anatomically located at the limbus. The conventional pathway consists of the aqueous humor passing through the trabecular meshwork, across the inner wall of the Schlemm's canal, into its lumen, and into draining collector channels, aqueous veins and episcleral veins (Goldmann 1950; Ascher 1954). The non-conventional route is composed of the uveal meshwork and anterior face of the ciliary muscle. The aqueous humor enters the connective tissue between the muscle bundles, through the suprachoroidal space, and out through the sclera (Bill and Hellsing 1965; Johnson and Erickson 2000).

Equilibrium exists between the production and drainage of aqueous humor. Disruption of aqueous outflow, usually through the conventional pathway, results in elevation of IOP, which is a major risk factor in the pathogenesis of glaucoma (Kass, Hart et al. 1980).

Aqueous Humor – Formation and Composition

Three mechanisms are involved in aqueous humor formation: diffusion, ultrafiltration and active secretion (Millar and Kaufman 1995). The first two processes are passive and do not entail active cellular participation.

Diffusion occurs when solutes, especially lipid soluble substances, are transported through the lipid portions of the membrane of the tissues between the capillaries and the posterior chamber, proportional to a concentration gradient across the membrane (Civan and Macknight 2004).

Ultrafiltration is the flow of water and water-soluble substances, limited by size and charge, across fenestrated ciliary capillary endothelia into the ciliary stroma, in response to an osmotic gradient or hydrostatic pressure (Civan and Macknight 2004).

Diffusion and ultrafiltration are responsible for the accumulation of plasma ultrafitrate in the stroma, behind tight junctions of the non-pigmented epithelium, from which the posterior chamber aqueous humor is derived (Smith and Rudt 1973; Uusitalo, Palkama et al. 1973).

Active secretion is thought to be the major contributor to aqueous formation, responsible for approximately 80% to 90% of the total aqueous humor formation (Mark; Gabelt and Kaufman 2003). The main site for active transport is believed to be the non-pigmented epithelial cells. Active transport takes place through selective transcellular movement of anions, cations, and other molecues across a concentration gradient in blood-aqueous barrier. This is mediated by protein transporters distributed in the cellular membrane. Aquaporins (AQPs) are molecular water channels which aid with rapid bulk transport of fluid or transport of fluids against an insufficient osmotic pressure gap. Two AQP's, AQP1 and AQP4, have been shown to contribute to aqueous humor secretion (Yamaguchi, Watanabe et al. 2006). The energy required for the transport is generated by hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP), which is activated by Na+ and K+ (66) mediated by Na+-K+-ATPase, an enzyme located in both the non-pigmented and pigmented ciliary epithelia (Coca-Prados and Sanchez-Torres 1998). Na+-K+-ATPase can be inhibited by many different molecules, including cardiac glycosides, dinitrophenol (Cole 1984)), vanadate (Becker 1980), and possibly acetazolamide through pH changes (Maren 1976). Thus, Na+-K+-ATPase is of particular interest in pharmacological studies of aqueous humor dynamics.

Another enzyme, carbonic anhydrase, found in the non-pigmented and pigmented ciliary epithelia (Dobbs, Epstein et al. 1979), mediates the transport of bicarbonate across the ciliary epithelium by the reversible hydration of CO2 to form HCO3-and protons through the following reaction: CO2 + H2O H2CO3 HCO3- + H+ (Wistrand 1951). Bicarbonate formation influences fluid transport by affecting Na+, possibly by regulating the pH for optimal active ion transport (Maren 1976).

The movement of electrolytes across the ciliary epithelium is regulated by electrochemical gradients and, although there is a net direction of secretion across the epithelium (Gabelt and Kaufman 2003), hydrostatic and oncotic forces favor resorption of aqueous humor (Bill 1973). Chloride ion is the major anion transported across the epithelium through Cl- channels (Coca-Prados and Sanchez-Torres 1998). Other molecules are also actively transported, including ascorbic acid, which is secreted against a concentration gradient by sodium-dependent vitamin C transporter 2 (SVCT2) (Tsukaguchi, Tokui et al. 1999) and certain amino acids, which are secreted by at least three different solute carriers (Reddy 1979). Active transport produces an osmotic gradient across the ciliary epithelium, which promotes the movement of other plasma constituents by ultrafiltration and diffusion (Tornquist, Alm et al. 1990).

The rate of aqueous humor turnover is estimated to be 1.0% to 1.5% of the anterior chamber volume per minute (Gabelt and Kaufman 2003), which is 2.4 ± 0.6μl/min (mean ± SD, daytime measurements in adults aged 20–83 years) (Brubaker 1989). These values were obtained by fluorophotometry and diurnal variations were observed, reflecting a pattern known as the circadian rhythm of aqueous humor flow in humans. Aqueous humor flow is higher in the morning than at night. Aqueous humor flow is normally about 3.0μl/min in the morning, 2.4μl/min in the afternoon, and drops to 1.5μl/min at night (Brubaker 1989). The mechanism that controls this biologic rhythm is poorly understood. Circulating epinephrine available to the ciliary epithelia may be a major driving force (Brubaker 1998). The effect of timolol, epinephrine and acetazolamide on the rate of aqueous humor flow through the anterior chamber has been studied. Epinephrine increased the rate of aqueous flow in sleeping subjects to a greater extent than it did in awake subjects. Timolol reduced the rate in awake individuals, but not in sleeping ones and acetazolamide reduced the rate of flow in both awake and epinephrine-stimulated subjects (Topper and Brubaker 1985). Norepinephrine has also been shown to stimulate aqueous flow, but not as effectively as epinephrine (Brubaker 1998). Another hypothesis supporting epinephrine influence on circadian rhythm could be a ciliary production of this hormone. However, epinephrine concentration in human aqueous humor appears to be very low, ranging from 0 to 0.1 ng/ml (Cooper, Constable et al. 1984). Moreover, in patients with surgical adrenalectomy or Horner syndrome (reduced or absent sympathetic innervation on one side), the circadian flow pattern was observed to be normal (Larson and Brubaker 1988; Maus, Young et al. 1994). Other hormones, such as melatonin, hormones related to pregnancy, and antidiuretic hormones, do not appear to alter the normal circadian rhythm of the aqueous flow (Brubaker 1998)

Measuring the relative concentrations of substances in the aqueous humor and the plasma, as well as in the anterior and posterior chambers, separately, make it possible to obtain information about the composition of the aqueous humor (Kinsey and Reddy 1964). Aqueous humor composition depends not only on the nature of its production, but also on the metabolic interchanges that occur with various tissues throughout its intraocular route (De Berardinis, Tieri et al. 1966).

The major components of the aqueous humor are organic and inorganic ions, carbohydrates, glutathione, urea, amino acids and proteins, oxygen, carbon dioxide and water. Aqueous humor is slightly hypertonic to plasma in a number of mammalian species (Benham, Duke-Elder et al. 1938; Kinsey 1951; Levene 1958), except from eyes of rhesus monkeys, in which no significant difference was observed (Gaasterland, Pederson et al. 1979). When comparing anterior chamber and posterior chamber fluids separately, no differences were found in osmolarity, in the total concentration of dissolved substances or in pH (Kinsey 1953). Most studies have shown the Na+ concentration in plasma and aqueous humor to be similar (Kinsey 1953; Sears 1981). The greatest differences in aqueous humor relative to plasma, are the concentrations of protein (200 times less) and ascorbate (20 to 50 times higher) (Reiss, Werness et al. 1986). The protein content of aqueous humor has both quantitative and qualitative differences compared to plasma. Most aqueous humor proteins are intrinsic glycoproteins of the vitreous, which are secretory products of the inner epithelial layer of the ciliary body (Haddad, Laicine et al. 1991). Specific classes of proteins, such as IgG, were found in highest concentrations in the aqueous humor, but IgM and IgA were not detectable. Relative concentrations of free amino acids vary, with ratios to plasma concentration ranging from 0.08 to 3.14, reinforcing the concept of active transport of amino acids (Dickinson, Durham et al. 1968). Glucose and urea in the aqueous humor are approximately 80% of the plasma levels. Important anti-oxidant substances can also be found in the aqueous humor, such as glutathione (derived by diffusion from the blood) and ascorbate (which helps protect against light-induced oxidative damage) (Gabelt and Kaufman 2003). A number of molecules involved in the maintenance of the extracellular matrix, such as collagenase, have been identified in human aqueous humor, which may influence TM outflow resistance and, consequently, the IOP (Vadillo-Ortega, Gonzalez-Avila et al. 1989). In addition, growth factors have been detected in the aqueous humor, as well as receptors for many of these factors on target tissues, including transferrin (Tripathi, Borisuth et al. 1992), transforming growth factors (Cousins, McCabe et al. 1991), endothelin-1 (Lepple-Wienhues, Becker et al. 1992) and indoleamine 2,3-dioxygenase (Malina and Martin 1993).

Aqueous Humor – Outflow

As mentioned earlier, the aqueous humor exits the eye through both conventional and unconventional pathways. The physiology of these two routes differs in several important ways, although regulation of the extracellular matrix (ECM) composition appears to influence aqueous humor outflow resistance (Morrison and Acott 2003).

Another mechanism reported to influence aqueous humor outflow involves age-dependent changes. For instance, in the healthy aging human eye, a reduction in the production of aqueous humor is balanced by a reduction in its drainage through the uveoscleral outflow pathway, thereby leaving intraocular pressure relatively unchanged (Toris, Yablonski et al. 1999).

Fluid movement takes place down a pressure gradient from the TM into the Schlemm's canal and through the inner wall of the Schlemm's canal, following the conventional route, and appears to be a passive pressure-dependent transcellular mechanism, frequently associated with paracellular routes, such as giant vacuoles and pores acting as one-way valves (Bill and Svedbergh 1972). These pores range in size from 0.1 to 3µm in diameter, and are the main passageway not only for aqueous humor, but also for particulate materials, such as cells, ferritin and microspheres (Inomata, Bill et al. 1972; Shabo and Maxwell 1972; Epstein and Rohen 1991). Changes in IOP brings about changes in the structure of the endothelium lining the Schlemm's canal. Elevated IOP leads to an increase in the number and size of these vacuoles, and vice versa (Johnstone and Grant 1973; Grierson and Lee 1978). The inner wall of Schlemm's canal is a complex tissue that is poorly understood - there is still doubt if it influences outflow facility in normal or glaucomatous eyes, even though circumstantial evidence points in that direction (Ethier 2002).

After leaving Schlemm's canal, the aqueous humor reaches the aqueous veins and, subsequently, mixes with blood in the episcleral veins, where the pressure is approximately 8–10 mmHg (Brubaker 1967; Phelps and Armaly 1978), and the resistance of the conventional aqueous drainage tissues is approximately 3–4 mmHg/µl/min. This results in an average IOP of 15.5 ± 2.6 mmHg (mean ± SD) for the general population (Schottenstein 1989).

In humans, 75% of the resistance to the aqueous humor outflow is localized to the TM, and 25% occurs beyond Schlemm's canal (Grant 1958). On this basis, trabeculotomy and trabeculectomy were proposed as surgical therapies for treatment of POAG (Sugar 1961). The major site of resistance within the TM structure has not yet been well characterized, but direct pressure measurements (Maepea and Bill 1989; Maepea and Bill 1992) and circumstantial evidence (Ethier 2002) indicate that it resides in the juxtacanalicular portion (Seiler and Wollensak 1985; Ethier, Kamm et al. 1986).

Some studies suggest that glycosaminoglycans, which constitute the fundamental substance of the ECM of the TM (Berggren and Vrabec 1957; Zimmerman 1957; Southren, Gordon et al. 1985), are partly responsible for increased resistance to outflow. The osmotic forces exerted by glycosaminoglycans may induce the hydration (edema) of the TM, which can cause obstruction of the trabecular structure (Francois 1977). Catabolic enzymes released from lysosomes depolymerizes glycosaminoglycans and prevents this obstruction. This effect is blocked by corticosteroids, which inhibit the release of the enzymes by stabilizing the lysosomal membranes and has been associated with a role in outflow obstruction and glaucoma pathogenesis (Francois 1977; Johnson, Bradley et al. 1990). In glaucomatous eyes, an increase in the ECM thickness beneath the inner wall of the Schlemm's canal and in the juxtacanalicular meshwork compared with age-matched healthy controls has been observed (Lütgen-Drecoll and Rohen 1989). Other studies suggest that the interaction of ECM components with different proteins may induce formation of deposits that obstruct aqueous humor outflow through the TM. For instance, proteomic analyses has identified cochlin, a protein of incompletely understood function, in the glaucomatous TM but not in healthy controls. Functionally, cochlin undergoes multimerization induced by shear stress and other changes in the microenvironment. Cochlin, along with mucopolysaccharide deposits, have been found exclusively in glaucomatous TM (Bhattacharya, Annangudi et al. 2005a; Bhattacharya, Rockwood et al. 2005b).

The influence of the iris and ciliary muscle, two contractile structures innervated with cholinergic nerves, on the resistance to aqueous outflow has also been contemplated. The anterior tendons of the ciliary muscle insert into the outer portion of the corneoscleral meshwork and into the juxtacanalicular meshwork (Rohen, Lutjen et al. 1967). During contraction, the ciliary muscle moves in an anterior and inward direction, resulting in spreading of the TM and dilation of Schlemm's canal, thus decreasing outflow resistance. During relaxation, the opposite occurs, thereby increasing outflow resistance (Barany 1966). Studies in various animal species demonstrated that voluntary accommodation, electrical stimulation of the trigeminal nerve, and local or systemic administration of cholinergic agents decrease outflow resistance (Armaly and Burian 1958; Armaly 1959b; Armaly 1959c; Armaly 1959a; Barany and Christensen 1967).

Direct and indirect acting muscarinic cholinergic agonists have been used in the medical management of primary open angle glaucoma. Recent studies demonstrate the presence of at least two different subtypes of muscarinic receptors in the ciliary muscle and in the trabecular meshwork tissue or cell cultures from human eyes (Gupta, Drance et al. 1994a; Gupta, McAllister et al. 1994b). In addition, cholinergic agonists, such as oxotremorine, induce the contraction of the ciliary muscle by binding selectively to receptors located in the longitudinal portion of the muscle, indicating that these agents may modulate the outflow facility independently from accommodation and miosis (Gupta, McAllister et al. 1994). Ultrastructural and histochemical differences between the longitudinal (more relevant to outflow facility) and circular (more relevant to accommodation) portions of the ciliary muscle have been observed in monkey eyes. However, these differences in muscarinic receptor subtypes do not appear to play a role in dissociation of accommodative and outflow resistance responses (Gabelt and Kaufman 1994; Poyer, Gabelt et al. 1994).

Uveoscleral outflow was described by observing the exit of radioactive tracers into the anterior chamber of the cynomologous monkey eye (Bill 1965). Further characterization of this pathway derives mostly from animal experiments and from mathematical calculations of an expanded Goldmann equation (i.e. F= (Pi-Pe) X C+U where F is the rate of aqueous humor formation, Pi is the intraocular pressure, Pe is the episcleral venous pressure , C is the tonographic facility of outflow and U is the pressure insensitive parameter to symbolize uveoscleral outflow) (Bill 1965). This model has been utilized for many years and, in the opinion of many investigators, views the aqueous outflow as passive fluid movement down a pressure gradient (Kupfer, Gaasterland et al. 1971; Kupfer and Ross 1971). The main obstacles in the assessing more accurately aqueous outflow values with the Goldman equation are: the pressure of the episcleral venous plexus into which aqueous humor flows, and the fact that the uveoscleral flow must be independent of the IOP for the equation to be relevant. Although the relationship established is acceptable, it is oversimplified, since this formula implies the recipient pressure for all pressure-dependent outflows can be represented by a single episcleral vessel (Brubaker 2001). In monkeys, uveoscleral flow is not truly pressure-independent and the relationship between flow and IOP is not linear (Bill 2003). Ciliary muscle contraction greatly affects uveoscleral outflow (Crawford and Kaufman 1987), and prostaglandin F2α greatly increases uveoscleral outflow by decreasing the flow resistance of the interstitial spaces in the ciliary muscle (Lutjen-Drecoll and Tamm 1988; Gabelt and Kaufman 1989).

Some investigators consider uveoscleral outflow analogous to lymphatic drainage of tissue fluid in other organs, since the fluid may be drawn osmotically into the veins and may mix with tissue fluid from the ciliary muscle, ciliary processes and choroid (Johnson and Erickson 2000). In non-human primates, 40-50% of aqueous humor exits the eye by the uveoscleral route. In human eyes, most data has been collected by indirect calculations, with results suggesting a similar fraction, at least in eyes from younger individuals. An age-dependent reduction in uveoscleral flow in human eyes may explain the initial difference seen between non-human primate and human eyes (Alm and Nilsson 2009). In eyes treated with atropine, uveoscleral flow accounts for 4 to 27% of the total outflow, but with pilocarpine it was only 0 to 3% (Bill and Phillips 1971).

The biomechanics of aqueous humor flow within the anterior chamber also need to be considered. IOP is the loading force to which the outflow system normally responds. Evidence of tissue and cellular deformation in response to an IOP-induced load places TM resistance to IOP at Schlemm's canal endothelium. The Schlemm's canal endothelium attaches tightly to the TM by extending cytoplasmic processes into the juxtacanalicular space (Johnstone and Grant 1973; Grierson and Lee 1975c; Johnstone 1984). Well-characterized desmosomes, capable of sustaining cellular stress, are present between cell process attachments (Grierson and Lee 1974; Grierson and Lee 1975a). The trabecular lamellae contain type I and III collagen, which provide structural support in tension, and elastin, and a recoverable response over large excursions. The organization and distribution of elastin in trabecular lamellae is similar to that found in tendons (Hernandez and Gong 1996) and enables a mechanism for reversible deformation in response to cyclic hydrodynamic loading (Johnstone and Grant 1973). Progressive deformation of Schlem's canal juxtacanalicular cells, and trabecular lamellae with increasing IOP occurs in concert with progressive enlargement of the juxtacanalicular space. This movement causes cellular elements and ECM material to become less compact and progressively reduce the ability of the juxtacanalicular space to participate as a resistance element (Johnstone and Grant 1973; Grierson and Lee 1975a; Grierson and Lee 1975b). However, as IOP increases, resistance to aqueous outflow also increases (Moses 1977; Kaufman 1996). This makes the juxtacanalicular region an unlikely source of hydraulic resistance (Van Buskirk 1982). Trabecular and vascular endothelial cells are mechanosensors (Ingber 2002) that direct vessel wall self-organization (Ingber 2002) in order to optimize wall and shear stress. Pressure and shear stress-mediated signals in endothelia initiate a series of responses at the cellular, molecular, and genetic levels, and induce both rapid responses and slow adaptive changes that regulate pressure and flow (Davies, Barbee et al. 1997; Ingber 2002). These processes are not linear but are part of a highly complex interactive network (Ingber 2002) in which an alteration in any component requires a contemporaneous adjustment of numerous other components in an interactive fashion resulting in long-term homeostasis (de Jong 2002; Ingber 2003).


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