Aqueous Humour: Anatomy & Physiology

The thin, watery fluid that fills the space between the cornea and the iris (anterior chamber) .produced specifically and continually by the non-pigmented epithelium of the ciliary body .the  rate of production must be balanced by an equal rate of aqueous humour drainage. Small variations in the production or outflow of aqueous humour will have a large influence on the intraocular pressure

It flows through the narrow cleft between the front of the lens and the back of the iris, to escape through the pupil into the anterior chamber, and then to drain out of the eye via the trabecular meshwork.

Intraocular pressure is a property of aqueous humor . The fluid is normally at 15 mm (0.6 inch) Hg above atmospheric pressure. If the fluid is leaking, due to collapse and wilting of cornea, the hardness of the normal eye is therefore corroborated.

Composition of Aqueous Humor :

Relative to plasma, aqueous humor has a:

  • Slight hypertonicity and acidity (pH 7.2 in AC)
  • Marked excess of ascorbate (15 times greater than arterial plasma)
  • Marked deficit of protein (0.02% in aqueous vs. 7% in plasma)
  • Sight excess of chloride and lactic acid
  • Slight deficit of sodium, bicarbonate, carbon dioxide, and glucose
  • Protein and antibodies in aqueous equilibrate with those in serum when a plasmoid aqueous occurs with an anterior uveitis
  • Albumin/globulin ratio is similar to plasma, although there is less gamma globulin

Formation of aqueous humor:

Aqueous is derived from plasma within the capillary network of the ciliary processes by three mechanisms:

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  1. Diffusion – lipid-soluble substances are transported through the lipid portions of the cell membrane proportional to a concentration gradient across the membrane
  2. Ultrafiltration – water and water-soluble substances, limited by size and charge, flow through theoretical micropores in the cell membrane in response to an osmotic gradient or hydrostatic pressure; influenced by intraocular pressure, blood pressure in the ciliary capillaries, and plasma oncotic pressure

Note: Diffusion and ultrafiltration are both passive mechanisms, with lipid- and water-soluble substances from the capillary core traversing the stroma and passing between pigmented epithelial cells and limited by the tight junctions of the non-pigmented epithelial cells

  1. Active transport (secretion) – water-soluble substances of larger size or greater charge are actively transported across the cell membrane, requiring the expenditure of energy; Na-K ATPase and glycolytic enzymes are present in nonpigmented epithelial cells. Active transport is decreased by hypoxia, hypothermia, and any inhibitor of active metabolism.  Active transport accounts for the majority of aqueous production.
  • Rate of aqueous Humor Formation :  2.0 –  3.0  microliters /  min .
  • Volume of Anterior Chamber 250 microliters
  • Volume of Posterior Chamber 60 microliters
  • Turnover of aqueous: 1.5 – 2 hours

Drainage

Aqueous humour is continuously produced by the ciliary body and hence has to be drained out .

The drainage route for aqueous humour flow: the posterior chamber, then the narrow space between the posterior iris and the anterior lens , through the pupil to enter the anterior chamber, the trabecular meshwork. From here, it drains into Schlemm’s canal by one of two ways:

  • Directly:  via aqueous vein to the episcleral vein
  • Indirectly: through collector channels(25-30) to the episcleral vein by intrascleral plexus and eventually into the veins of the orbit.

Uveoscleral outflow : second major route of aqueous drainage is through the face of the ciliary body and iris root to the ciliary muscle and suprachoroidal space to either veins in the ciliary body, choroid, and sclera or through scleral pores to episcleral tissue( accounts for an estimated 20% of total outflow)

ANATOMY

LIMBUS

Transition zone between the cornea and sclera Internal indentation, or scleral sulcus, houses meshwork structures and serves as the principal site of aqueous outflow

Trabecular Meshwork

Three dimensional set of diagonally-crossing collagen fibers Contained within the scleral sulcus, converting the latter into a circular channel, Schlemm’s canal Consists of a connective tissue core surrounded by endothelium and may be subdivided into three portions:

  • Uveal meshwork
  • Corneoscleral meshwork
  • Juxtacanalicular connective tissue

Schlemm’s canal

An endothelial-lined channel averaging 190-370 microns in diameter. Pressure-dependent changes in aqueous outflow such as increased resistance to aqueous outflow with elevation of intraocular pressure may be secondary to a collapse of Schlemm’s canal.

Ciliary Body

Extends 6 mm from the scleral spur and posterior pigment epithelium of the iris anteriorly to the retina and choroid at the ora serrata posteriorly.  Composed of muscle, blood vessels and epithelia.

Ciliary processes (approximately 70)

  • Pigmented – comprises outer layer, adjacent to stroma, and composed of low cuboidal cells with multiple basal infoldings, a large nucleus, and melanosomes
  • Nonpigmented – inner layer, adjacent to aqueous in the posterior chamber, consisting of columnar cells. Intercellular tight junctions form major element of blood-aqueous barrier.  Tips or crests of nonpigmented ciliary epithelia are site of active secretion, hence numerous mitochondria, rough endoplasmic reticulum, pinocytic vesicles, Na-K activated ATPase and carbonic anhydrase layers
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