Describe the formation and drainage of aqueous humour.

Formation

• The aqueous humour is a clear fluid filling the anterior and posterior chambers, with a total volume of about 0.31 mL (0.25 mL anterior, 0.06 mL posterior), formed at a rate of about 2 to 2.5 microlitres per minute, giving a turnover of 1% of the anterior chamber volume per minute
• Site of formation — the non-pigmented epithelium of the ciliary processes of the pars plicata
• Mechanisms, in order of importance: active secretion, accounting for 80 to 90%, energy-dependent and mediated principally by Na-K ATPase and carbonic anhydrase II — which is precisely why carbonic anhydrase inhibitors reduce production; ultrafiltration; and diffusion
• Composition — it is not a simple plasma filtrate. Compared with plasma it has: ascorbate some 15 to 25 times higher, an antioxidant protecting against ultraviolet damage; protein some 200 times lower (0.02%), which is what makes it optically clear and gives the normal absence of flare; higher lactate and chloride; and lower glucose, urea and bicarbonate
• The blood-aqueous barrier — formed by the tight junctions of the non-pigmented ciliary epithelium and of the iris vascular endothelium. Its breakdown in inflammation allows protein and cells into the aqueous, producing the flare and cells of uveitis, and the plasmoid aqueous of severe inflammation

Drainage

• Aqueous is secreted into the posterior chamber → It passes through the pupil into the anterior chamber — and obstruction here is pupil block, the mechanism of angle-closure glaucoma → It flows in a convection current, warmer at the iris and cooler at the cornea, which is why keratic precipitates deposit in a triangle (Arlt) on the lower cornea → It reaches the angle, and leaves the eye by two routes → 1.
• Conventional (trabecular) outflow — 80 to 90%: through the trabecular meshwork, into schlemm’S canal, then through collector channels into the aqueous veins of ASCHER and the episcleral veins.
• This route is pressure-dependent, and the juxtacanalicular meshwork is the site of most of the resistance — and of the obstruction in primary open-angle glaucoma → 2.
• Uveoscleral (unconventional) outflow — 10 to 20%: through the ciliary muscle bundles into the suprachoroidal space and out through the sclera.
• It is pressure-independent, and it is the route enhanced by prostaglandin analogues, which is why they are such effective ocular hypotensives