Aqueous humor formation is a biological process that is subject to circadian rhythms. Aqueous humor is formed by the 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 2-1). Each of the 80 or so processes contains a large number of capillaries, which 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 actual site of aqueous production. The ciliary processes provide a large surface area for secretion.
Aqueous humor formation and secretion into the posterior chamber result from
active secretion, which takes place in the double-layered ciliary epithelium
Active secretion, or transport, consumes energy to move substances against an electrochemical gradient and is independent of pressure. The identity of the precise ion or ions transported is not known, but sodium, chloride, and bicarbonate are involved. Active secretion accounts for the majority of aqueous production and 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 is the passive movement of ions across a membrane related to charge and concentration.
In humans, aqueous humor has an excess of hydrogen and chloride ions, an excess of ascorbate, and a deficit of bicarbonate relative to plasma. Aqueous humor is essentially protein free (1/200–1/500 of the protein found in plasma), which allows for optical clarity and reflects the integrity of the blood–aqueous barrier of the normal eye. Albumin accounts for about half of the total protein. Other components 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. Aqueous humor is produced at an average rate of 2.0–2.5 µL/min, and its composition 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 is secondary to other dilutional exchanges and active processes. BCSC Section 2, Fundamentals and Principles of Ophthalmology, discusses aqueous humor composition and production in detail in Part IV, Biochemistry and Metabolism.