Glaucoma due to increased pressure arises in the front part of the eye. The space between the cornea and the lens contains aqueous humour. The iris divides this space into the anterior and posterior chambers. Behind the lens is the vitreous chamber lined by the retina and containing the jelly-like vitreous humour.

The humours help maintain the eyeball shape and IOP and provide oxygen and nutrients to tissues in the eye which lack their own blood supply.^8,12

Aqueous humour is constantly secreted in the posterior chamber by the ciliary body, a circular tissue behind the iris which is also involved in shaping the lens for focus. The aqueous humour moves forward through the pupil to the anterior chamber where it drains into the scleral venous sinus, also called the canal of Schlemm. This is through the spongy tissue of the trabecular meshwork in the drainage angle situated between where the iris meets the cornea under the conjunctiva.^{9,10,13,14,15}

The aqueous humour is then absorbed into the blood stream via epithelial cells, likened to a lymphatic process, helping convey immune response cells and remove waste products and cell debris.⁸

A much smaller volume of aqueous humour is removed by diffusing through ciliary muscle fibres, the uveoscleral pathway, and is the target for latanoprost, a prostaglandin F2α analogue.¹⁵

Glaucoma due to increased ocular pressure is usually due to blocked drainage but excessive aqueous humour production can also be a factor.⁸