AQUEOUS HUMOR FORMATION

GUEST AUTHOR

MOHAMMAD TARIQ

Ajmal Khan Tibbiya College

Aligarh, India

INTRODUCTION:

The primary site of aqueous formation is the ciliary processes.

Aqueous humor is formed by the ciliary processes, each of which is composed of a double layer of epithelium over a core of stroma and a rich supply of fenestrated capillaries.

Aqueous humor is derived from plasma within the capillary network of ciliary processes.

The normal rate of production is 2.3 µl/min.

There are three processes involved in the production of aqueous humor. They are as follows:

• 1.       Diffusion.
• 2.       Ultra-filtration.
• 3.       Secretion (Active transport).

FORMATION PROCESSES:

The various constituents of aqueous humor have to traverse three tissue layers of the ciliary processes, namely:

• The capillary wall.
• The stroma.
• Two layers of epithelium.

The various constituents of aqueous humor pass through these layers by the following processes:

Diffusion:

In this process there occurs a net flux of particles from areas of high concentration to areas of low concentration, so that molecules distribute themselves equally throughout the space in which they are contained by a biophysical movement of particles.

Diffusion also occurs across a semi-permeable membrane. Quantitatively the net movement of a substance across a semi-permeable membrane by diffusion occurs according to Fick’s Law of Diffusion.

Mainly lipid soluble substances are transported by diffusion through the lipid portions of the cell membrane of the ciliary processes.

Ultrafiltration:

The process by which fluid and its solutes cross semi-permeable membranes under a pressure gradient is called Ultrafiltration.

In the ciliary processes ultrafiltration occurs due to flow of water and water-soluble substances across the fenestrated ciliary capillary endothelia into the ciliary stroma in response to an osmotic gradient or hydrostatic pressure.

As blood passes through capillaries of the ciliary processes, about 4% of the plasma filters, through capillary wall into the interstitial spaces between the capillaries and ciliary epithelium.

By ultrafiltration most of the plasma substances pass out from the capillary wall and loose connective tissue. Thus, the plasma filtrate (dialysate) accumulates behind the pigmented and nonpigmented epithelium of the ciliary processes in response to osmotic gradient or hydrostatic pressure.

Secretion (Active transport):

Secretion is an active process by which some substances cross the cell membrane.

Due to expenditure of energy substances can be moved across a concentration gradient in a direction opposite to what would be expected by passive mechanism alone.

Active transport of the substances across the non-pigmented ciliary epithelium results in an osmotic gradient leading to the movement of other plasma constituents into the posterior chamber by ultrafiltration.

Active transport of the substances across the nonpigmented ciliary epithelium results in an osmotic gradient leading to the movement of other plasma constituents into the posterior chamber by ultrafiltration and diffusion. Sodium is primarily responsible for the movement of water into the posterior chamber.

Globular proteins in the cell membrane and expenditure of energy mediate active secretion of aqueous humor.

First the dialysate from the plasma is transported into the pigment epithelium. Paired Na+/H+ and Cl+/HCO3- antiports actively transport Na+ and Cl- from the stroma into the cells.

The active transport brought about by Na+ K+ activated ATPase pump, calcium and voltage gated ion channels and carbonic anhydrase system.

Substances that are actively transported in the process of aqueous formation are water soluble substances of larger size or greater charge e.g. Na+, Cl-, K+, ascorbic acid, amino acids and bicarbonates.