Aqueous Humor Formation

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  • Topic: Ciliary body, Eye, Capillary
  • Pages : 7 (1791 words )
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  • Published : January 27, 2013
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MECHANISM OF AQUEOUS HUMOR FORMATION

There is a constant flow of aqueous humor through the anterior segment of the eye. The aqueous is formed by the ciliary process and flows from the posterior chamber to anterior chamber through the pupil and exits the eye at the angle. The secretion of aqueous humor generates the intraocular pressure required for an optically efficient globe. The flow of aqueous provides nutrition for the avascular ocular tissues that it bathes, the posterior surface of the cornea, trabecular meshwork, crystalline lens and anterior vitreous.The volume of the aqueous in the anterior chamber turns over approximately once every 100 minutes.This constant flow of aqueous replenishes nutrients that have been taken up by the avasculat tissues and carries away their metabolic wastes.The factors that determine the the IOP is given by the Goldmann equation. P0 =F/C+Pv

Where P0 is the IOP, F is the rate of aqueous humor formation in (l/min, C is the facility of outflow in (l/min/mmHg and Pv is the episcleral venous pressure in mmHg

SITE OF FORMATION OF AQUEOUS HUMOR

The precise location of aqueous humor production appears to be predominantly in the anterior portion of the pars plicata along the tips or crests of the ciliary processes, since this region has been shown to have

a).increased basal and lateral interdigitations, mitochondria and rough endoplasmic reticulum in the non-pigmented ciliary epithelium b).more numerous fenestrations in the capillary endothelium
c).thinner layer of capillary stroma and
d).an increase in all organelles and gap junctions between pigmented and non-pigmented epithelia

When Sodium fluroscein is administered systemically and the ciliary body is absorbed with a special gonioprism, fluroscein stained aqueous is seen primarily at the tips of the ciliary process.

The site of active transport is considered to be the non-pigmented epithelial cells, especially in the cell membrane of the lateral interdigitations. Since this area has

a) abundant Na+ K+ activated AT Pase and Carbonic anhydrase b) higher specific activity for glycolytic enzymes

Blood Aqueous Barrier
The blood aqueous barrier consists of all of the barriers to the movement of substances from the plasma to the aqueous humor.In ciliary body the barrier include: vascular endothelium, basementmembrane, stroma, Pigmented epithelium, Tight junction between nonpigmented epithelial cells. In iris the nonfenestrated iris vessels contribute to blood aqueous barrier. Destabilization of the blood aqueous barrier with leakage of plasma proteins into the anterior chamber has important clinical consequences.

Breakdown of the blood aqueous barrier occurs after paracentesis of the anterior chamber with subsequent leakage of plasma protein into the aqueous. This inflow of plasma like or “plasmoid” aqueous is also called as “Secondary Aqueous”.

MECHANISM OF AQUEOUS HUMOR FORMATION

Diffusion:

It is the movement of a substance across a membrane along its concentration gradient. As the aqueous humor passes from the posterior chamber to Schlemm’s canal, it is in contact with the ciliary body, iris, lens, vitreous, cornea and trabecular meshwork. There is sufficient diffusional exchange with the surrounding tissues that the anterior chamber aqueous humor resembles plasma more closely than does the posterior chamber aqueous humor. Aqueous humor provides glucose, amino acids, oxygen and potassium to surrounding tissues and removes carbondioxide, lactate and pyruvate.

Ultrafiltration:

It refers to the movement of substances along a pressure gradient that is dialysis under applied pressure. In the case of the ciliary body, fluid movement is favored by the hydrostatic pressure difference between the capillary pressure and IOP and resisted by the difference between the oncotic pressure of plasma and the aqueous humor. The ciliary...
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