QS Study

The internal structures of the eye are bathed and supported by the aqueous humour, a crystal clear fluid. It is secreted from the ciliary epithelium, a structure supporting the lens. This fluid nourishes the cornea and the lens and gives the eye its shape.

Formation:

It is a thin, transparent fluid similar to plasma. It’s made up of 99.9% water; the other 0.1% consists of sugars, vitamins, proteins, and other nutrients. Two mechanisms are involved. These are – Ultrafiltration and Secretion.

(1) Ultrafiltration: Capillary wall in the ciliary processes act as a semipermeable membrane. It allows only small molecules to leave the bloodstream to be collected into its stroma as aqueous humour.

(2) Secretion: Active secretion of the aqueous humour occurs by the ciliary processes by two mechanisms –

  1. Bicarbonate system mediated by carbonic anhydrase.
  2. Na+ K+ ATPase.

After formation, it is modified by the cornea and lens. It results in a fall in glucose and bicarbonate concentration but a rise in lactate levels.

Aqueous Humour 1

Fig: Aqueous Humour

Circulation and drainage – It drains out of the eye through the trabecular meshwork. The drainage angle is located between the iris and the clear covering of the eye (cornea), where the iris meets the white outer covering (sclera) of the eye.

  • After formation in the ciliary region,
  • Flows from the posterior chamber in the anterior chamber through the pupil,
  • An angle of the anterior chamber,
  • Canal of Sehlemm,
  • Aqueous veins,
  • Episcleral venous plexuses.

The normal flow rate is more than .2µl/min.

Function

  • 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.
  • Provides inflation for expansion of the cornea and thus increased protection against dust, wind, pollen grains, and some pathogens.
  • Transport vitamin C in the front segment to act as an antioxidant agent.

Clinical conditions associated with aqueous humour –

When there is an imbalance between the synthesis and drainage of the aqueous humour then there is a rise of intraocular pressure leading to glaucoma. It may be an open angle or closed angle.

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