aqueous humour

humour, aqueous

Clear, colourless fluid that fills the anterior and posterior chambers of the eye. It is a carrier of nutrients for the lens and to a larger extent the cornea, especially of glucose and essential amino acids. It contributes to the maintenance of the intraocular pressure. It is formed in the ciliary processes, flows into the posterior chamber, then through the pupil into the anterior chamber and leaves the eye through the trabecular meshwork passing to the canal of Schlemm and then to veins in the intrascleral venous plexus (Fig. H3). A small amount (10% to 15%) also flows out of the eye via the uveoscleral pathway. The aqueous in the anterior chamber is a component of the optical system of the eye. It has an index of refraction of 1.336, slightly lower than that of the cornea, so that the cornea/aqueous surface acts as a diverging lens of low power. It is a fluid very similar to blood plasma but with a much lower concentration of protein and a higher concentration of ascorbate.The rate of aqueous humour outflow varies between 2.0 μl/min and 3.0 μl/min via both the conventional (trabecular meshwork and Schlemm's canal) and the unconventional (uveoscleral) pathways. This rate is normally equal to the rate of aqueous secretion. If the rate of outflow is lower than the rate of secretion intraocular pressure increases. See aqueous flare; ultrafiltration.

Fig. H3 Outflow of aqueous humour