Glaucoma Week Series 2011- episode 2 (Physiology of the aqueous humor)

Optom
Its good to be back. I was researching on today's topic- The Physiology of the Aqueous Humor.
To fully grasp the effect of increased intra-ocular pressure, the physiology of the aqueous humor and constituent components of the aqueous need to be studied. For instance, it is known that a range of 9-21 mmHg has been the arbitrary intra-ocular pressure which is chiefly due to the aqueous humor; the aqueous humor has been known to carry pre-cursors like glutamate that lead to retinal ganglion cell apoptosis in the presence of cellular Nitric Oxide and the presence of plasma protein in the aqueous humor which increases the intra-ocular pressure.
The primary function of the aqueous humor includes the following:
* Maintaining the structural integrity of the eyeball
* Provides nutrition for avascular structures of the eyes- the crystalline lens,cornea & trabecular meshwork.
*It facilitates cellular & humoral immune responses.
* It provides a barrier against cataractogenic ultraviolet radiations.

Formation of aqueous humor

 There is no clear-cut explanation of the exact mechanism of aqueous humor formation. The theory of Laber on simple filtration had been the generally accepted mechanism of aqueous humor formation until recent studies showed it could not explain the many differences between blood plasma and the aqueous humor.

Leber's theory of simple filtration assumes that the walls of the cillary capillaries of the eyes acts like a sieve and the aqueous humor was filtered through the walls of the capillaries by the pressure of the blood.
Blood plasma & the aqueous humor are roughly composed of equal soluble solutes but contain roughly unequal amounts of proteins, non-electrolytes(e.g. Glucose, urea etc), electrolytes (e.g. Sodium ions etc), organic acids. This theory of simple filtration could not explain it.

Hence leading to the postulations of other mechanisms, viz
1) Ultra-filtration which assumes that the cilliary walls acts as a semi-permeable membrance which allows water and soluble solutes to easily pass through leaving behind the non-soluble macro-molecules behind. This mechanism could not explain the reason why, say, freely diffusable urea and glucose are less in the aqueous than in the blood plasma etc.

2)Secretion-diffusion theory assumes the cilliary body as a semi-permeable membrane but secretion is by active transport in the presence of carbonate ions as buffer system or actively involved in the active transport of H+ ion which determines the flow of aqueous humor. Remember the effect of Diamox on aqueous humor secretion? It depends on this mechanism.


3) Pinocytosis assumes that the vesicles on the epithelium of the cillary body is a vesicle undergoing endocytosis in the presence of ATP (Adenosine Triphosphate) which explains the mechanism of secretion of aqueous humor.

Rate of flow of Aqueous humor

 About 150uL/Hr with a range of 1.1%-1.9% of the volume of the anterior chamber per minute.

Physical characteristics of AH

a) It has a higher osmotic pressure than blood plasma.

b) It as a slightly higher specific gravity than water.

c) It has a slightly more alkalinic PH than the blood. PH is 7.53.

d) It has a refractive index of 1.336.

Factors that affect the formation of AH

1) Breakdown of blood/aqueous barrier.
2) Changes in the lumen of cilliary capillaries.
3) Changes in IOP.
4) Diurnal Variation.
5) Age etc

To be continued,
Dr Victor Ezebuiroh Okwudiri