Aqueous Humor Production
As shown in Figure 1-2 in Chapter 1, aqueous humor is produced by the ciliary processes in the posterior chamber and flows through the pupil into the anterior chamber. The average rate of aqueous humor production is 2–3 μL/min while awake, decreasing by about 50% during sleep. Because the anterior segment volume is approximately 200–300 μL, the eye’s total volume of aqueous humor is turned over about every 100 minutes. The ciliary body contains approximately 80 ciliary processes, each of which is composed of a double layer of epithelium over a core of stroma and a rich supply of fenestrated capillaries (Fig 1-2). These capillaries are supplied mainly by branches of the major arterial circle of the iris. The apical surfaces of both the outer pigmented and the inner nonpigmented layers of epithelium face each other and are joined by tight junctions, which are an important component of the blood–aqueous barrier. The inner nonpigmented epithelial cells, which protrude into the posterior chamber, contain numerous mitochondria and microvilli; these cells are thought to be the site of aqueous production. The ciliary processes provide a large surface area for secretion.
Aqueous humor enters the posterior chamber by the following physiologic mechanisms:
Active secretion refers to transport that requires energy to move sodium, chloride, bicarbonate, and other ions (currently unknown) against an electrochemical gradient. Active secretion is independent of pressure and accounts for the majority of aqueous humor production. It involves, at least in part, activity of the enzyme carbonic anhydrase II. Ultrafiltration refers to a pressure-dependent movement along a pressure gradient. In the ciliary processes, the hydrostatic pressure difference between capillary pressure and IOP favors fluid movement into the eye, whereas the oncotic gradient between the two resists fluid movement. The relationship between secretion and ultrafiltration is not known. Diffusion involves the passive movement of ions, based on charge and concentration, across a membrane.
In humans, aqueous humor has a higher concentration of hydrogen and chloride ions, a higher concentration of ascorbate, and a lower concentration of bicarbonate compared with plasma. In the normal eye, the blood–aqueous barrier keeps the aqueous humor essentially protein-free (1/200–1/500 of the protein found in plasma), allowing for optical clarity. Albumin accounts for approximately half of the total protein. Other components of aqueous humor include growth factors; several enzymes such as carbonic anhydrase, lysozyme, diamine oxidase, plasminogen activator, dopamine β-hydroxylase, and phospholipase A2; and prostaglandins, cyclic adenosine monophosphate (cAMP), catecholamines, steroid hormones, and hyaluronic acid. The composition of the aqueous humor is altered as it flows from the posterior chamber, through the pupil, and into the anterior chamber. This alteration occurs across the hyaloid face of the vitreous, the surface of the lens, the blood vessels of the iris, and the corneal endothelium; and it is secondary to other dilutional exchanges and active processes. See BCSC Section 2, Fundamentals and Principles of Ophthalmology, for further discussion of aqueous humor composition and production.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.