Oxygen and Carbon Dioxide
Oxygen in the aqueous humor is derived from the blood supply to the ciliary body and iris, as the atmospheric oxygen flux across the cornea is negligible. Indeed, the corneal endothelium depends critically on the aqueous oxygen supply for the active fluid-transport mechanism that maintains corneal transparency. The lens and the endothelial lining of the trabecular meshwork also derive their oxygen supply from the aqueous. Oxygen is present in the aqueous humor at a partial pressure lower than that in arterial blood.
The carbon dioxide content of the aqueous ranges from about 40 to 60 mm Hg, contributing approximately 3% of the total bicarbonate. The relative proportions of carbon dioxide and bicarbonate determine the pH of the aqueous, which in most species ranges between 7.5 and 7.6. Carbon dioxide is continuously lost from the aqueous by diffusion across the cornea into the tear film and atmosphere.
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.