Composition of the Aqueous Humor
Table 9-1 summarizes the composition of the aqueous humor compared with that of plasma and vitreous. Aqueous secretion is not an ultrafiltrate of plasma (as was once speculated), because it is produced by energy-dependent processes in the epithelial layer of the ciliary body. This mode of production allows precise control to be maintained over composition of the fluid that bathes the structures essential for normal vision.
The ionic composition of the aqueous humor is determined by selective active transport systems (eg, Na+,K+-2Cl− symport, Cl−- and Na+,H+ antiports, cation channels, water channels [aquaporin], Na+,K+-ATPase, K+ channels, Cl– channels, H+-ATPase) that participate in secretion of aqueous humor by the NPE. Active secretion of ions and molecules leads to higher levels of ascorbate and some amino acids in aqueous than in plasma.
Molecular studies have shown that the secretory properties of the ciliary epithelium are not limited to ions and electrolytes but extend to a wide range of molecules with different molecular masses. Common features of many of these molecules are their local synthesis in the ciliary epithelium and their secretion by the NPE cells through the regulatory pathway into the aqueous humor. Among the proteins whose messenger RNA expression has been demonstrated are
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plasma proteins (eg, complement component C4, α2-macroglobulin, selenoprotein P, apolipoprotein D, plasma glutathione peroxidases, angiotensinogen)
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proteinases (eg, cathepsin D, cathepsin O)
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cellular retinaldehyde–binding protein (CRALBP) and other components of the visual cycle
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neurotrophic factor (eg, PE-derived factor)
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neuropeptide-processing enzymes (eg, carboxypeptidase E, peptidylglycine-α-amidating monoxygenase)
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neuroendocrine peptides (eg, secretogranin II, neurotensin, galanin)
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bioactive peptides and hormones (eg, atrial natriuretic peptide, brain natriuretic peptide)
Table 9-1 Comparison of Components of Plasma, Aqueous Humor, and Vitreous
These findings support the view that the ciliary epithelium exhibits neuroendocrine properties that are directly related to the makeup of the aqueous humor and its regulation. The aqueous humor composition is in dynamic equilibrium, determined both by its rate of production and outflow and by continuous exchanges with the tissues of the anterior segment. The aqueous contains the following:
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inorganic ions and organic anions
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carbohydrates
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glutathione and urea
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proteins
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growth-modulatory factors
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oxygen and carbon dioxide
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Yang W, Bradley JC, Reid TW, McCartney DL. Growth factors in aqueous humor. Ophthalmology. 2011;118(5):1003.
Inorganic Ions
The concentrations of sodium, potassium, and magnesium in the aqueous are similar to those in plasma, but the level of calcium in aqueous is only half that in plasma. The 2 major anions are chloride and bicarbonate. Phosphate is also present in the aqueous (aqueous-to-plasma ratio, ≈0.5 or lower), but its concentration is too low to have significant buffering capacity. Iron, copper, and zinc are all found in the aqueous humor at essentially the same levels as in plasma: approximately 1 mg/mL.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.