The role of aqueous humour
The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens. Within the anterior segment are two fluid-filled spaces divided by the iris plane:
- the anterior chamber between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris.
- the posterior chamber between the iris and the front face of the vitreous.
Aqueous humour fills these spaces within the anterior segment to provide nutrients to the lens and corneal endothelium, and its pressure maintains the convex shape of the cornea.
In a healthy eye, the aqueous humour does not mix with the firm, gel-like vitreous humour because of the lens
and its Zonule of Zinn between the two. Maintains the intraocular pressure and inflates the globe of the eye. Provides nutrition for the avascular ocular tissues; posterior cornea, trabecular meshwork, lens, and anterior vitreous. Carries away waste products from metabolism of the above avascular ocular tissues.
May serve to transport ascorbate in the anterior segment to act as an anti-oxidant agent. Presence of immunoglobulins indicate a role in immune response to defend against pathogens. Its main function is to provide diopteric power to the cornea.
Ions: HCO3-, buffers metabolic acids; Cl-, preserves electric neutrality; Na+; K+; Ca2+; PO42-.
Proteins: albumin, ß-globulins. Very low density due to filtration.
Ascorbate: anti-oxidative, protects against UV.
Lactate: produced by metabolism of anaerobic structures of the eye.
Amino acids: transported by cilary epithelial cells.
Production and drainage:
Aqueous humour is secreted into the posterior chamber by the ciliary body, specifically the ciliary processes. It flows through the narrow cleft between the front of the lens and the back of the iris, to escape through the pupil into the anterior chamber, and then to drain out of the eye via the trabecular meshwork. From here, it drains into Schlemm’s canal by one of two ways: directly, via aqueous vein to the episcleral vein, or indirectly, via collector channels to the episcleral vein by intrascleral plexus and eventually into the veins of the orbit.