Eye Physiology: Circulation

Aqueous Humour

This is a specialised fluid that bathes the structures within the eye.

• It provides oxygen and metabolites and contains bicarbonate ions.
  • The latter functions to buffer the H+ produced in the cornea and lens by anaerobic glycolysis
• Also contains ascorbate- a powerful antioxidant.

• The humour is not just an ultrafiltrate of plasma:
  • It is produced by an energy dependent process in the epithelial layer of the ciliary body

Orgnanisation of the ciliary epithelium

• Consists of pigmented and non-pigmented epithelial cells
  • The inner pigmented layer is a continuation of the retinal pigmented epithelium.  This is overlain by non-pigmented epithelium in the ciliary body and the iris

Flow

• Aqueous humour, produced by the ciliary body into the posterior chamber of the eye, flows into the anterior chamber (between the lens and the iris)
• It then is drained into the scleral venous sinus through the trabecular meshwork and the canal of Schlemm, situated at the angle between the ires and the cornea  (iridocorneal angle)
  • NB a small amount diffuses through the vitreous and is absorbed across the retinal pigment epithelium

Ion Movement (briefly)

• The epithelial cells produce bicarbonate and H(+) from CO2 (via carbonic anhydrase), which is transported in exchange for chloride (Cl-) and Na(+) out into the PE (i.e. not in the aqueous chamber)
• Na and Cl then are transported and diffuse into the aqueous humour (water follows)
  • NB Carbonic anhydrase inhibitors block this process (e.g. acetazolamide is used for treatment in closed angle glaucoma)

Volumes/Production

• 1-3μl secreted/min- most flows into the anterior chamber (60μl in post vs 250μl in ant)
  • Turnover in the posterior chamber is around 30 mins and turnover in the anterior chamber is around 120 mins (this is quick compared to CSF of 400mins)
  • This maintains IOP at around 17mmHg (normal is classed as between 10 and 20 mmHg)