The eye pressure (Intraocular Pressure=IOP) is intricately related to the production and the drainage of Aqueous humour and it is essential to normal functioning of the eye. Aqueous humour is a clear fluid and mainly composed of water (98%), amino acids, electrolytes, ascorbic acid, glucose, immunoglobulins and glutathione.
The main functions of Aqueous humour involves in maintaining eye pressure, inflating the eye ball and maintaining the shape and size of the eye, providing nutrition (e.g. amino acids and glucose) for the eye and to defend against pathogens.
Production and drainage of Aqueous Humour
Aqueous humour is produced by the ciliary processes (about 70 in total per eye) of the ciliary body specifically the non-pigmented epithelium of the ciliary body (pars plicata)) and is secreted into the posterior chamber of the eye situated behind the iris (the coloured part of the eye). It then flows forward through the pupil and into the anterior chamber of the eye (the space in front of the iris & behind the cornea). It then drains out of the eye via the trabecular meshwork and into the Schlemm’s canal and finally via 25-30 collector channels (aqueous veins) to the episcleral veins.
In normal circumstances there is equilibrium where Aqueous humour is continually produced by the ciliary processes and this rate of production is balanced by an equal rate of aqueous humour drainage. Small variations in the production or outflow of aqueous humour may have a large influence on the eye pressure.
The greatest resistance to aqueous flow is provided by the trabecular meshwork, and the inner wall of the Schlemm’s canal (diameter of 190-350 Micrometers) and this is where most of the aqueous outflow occurs. The pressure of Aqueous Humour is normally 15 mmHg (0.6 in Hg) above atmospheric pressure, so if a needle is introduced surgically into the eye the fluid flows out easily.
The normal eye pressure is in the range of 10-21mm Mercury and vast majority of people will have their eye pressure in the middle of this range. Very high eye pressure as well as very low eye pressure (under 6mm Mercury and called Hypotony) are detrimental to the normal functioning of the eye and can impair vision. Higher is the level of eye pressure greater is the risk of Glaucoma. The raised pressure in the eye is caused by the breakdown of the equilibrium between the production of and the drainage of the aqueous humour. It may be as a result of either increased production or decreased outflow of aqueous humour. The peripheral part of the anterior chamber, between the cornea and the iris region is referred to as the filtration angle (at the junction of the white and black of the eye but inside the eye). A delicate connective tissue web work, the trabecular meshwork through which the aqueous humour drain out of the eye. The production of the aqueous humour is a constant process; and its removal is vitally important: the balance between production and drainage determines the eye pressure.
Aqueous humour is produced at an average rate of 2.0–3.0 µL/min, and its composition is altered as it flows from the posterior chamber, through the pupil, and into the anterior chamber. The volume of the Anterior Chamber is 250 microliters (µL) and the volume of the Posterior Chamber is 60 microliters (µL), therefore the turnover of aqueous humour is 1.5 - 2 hours.
Aqueous humour formation is known to decrease during sleep (suppression of 45±20%), advancing age (decrease of 2% per decade), uveitis, Retinal detachment, and ciliochoroidal detachment. The rate of aqueous humour formation is relatively pressure-insensitive.
Please feel free to discuss any of the above points with Mr Raj if you would like further information.
For private patients & referrals
Mr Akash Raj
Consultant Ophthalmologist, Glaucoma specialist & Cataract surgeon
(Alternate Thursday AM)
01384 632 640
Private Secretary: Liz Carter : 01384 632 636
Fax: 01384 632702
FAO Liz Carter
Mr Akash Raj
Consultant Ophthalmologist, Glaucoma & Cataract specialist
For NHS Referrals through GP/Opticians
Mr Akash Raj
Consultant Ophthalmologist (Glaucoma Lead)
Extn. 5815 (NHS Secretary: Lyn Eaton)
FAO Lyn Eaton