Aqueous Humor Biochemistry- Ocular Biochemistry

Optometry

1. Aqueous
HumorBiochemistry

2. AQUEOUS HUMOR
Aqueous humor is secreted by the nonpigmented ciliary epithelium (NPE)from a substrate of
blood plasma.
Aqueous humor is differentfrom plasma, as it has low protein content and a high
concentration of ascorbate. Ascorbate has antioxidant properties, and its high concentration
in the aqueous protects intraocular structures by blocking ultraviolet (UV) light.
The blood–aqueous barrier is composed of the tight junctions of the NPE, the iris vasculature,
and the inner wall endothelium of the Schlemm canal.Disruption of the blood–aqueous
barrier allows mixing of blood contents with ocular fluids, producing a plasmoidaqueous, as
occurs in anterior uveitis.

3. Water
Proteins: 5-16mg/100ml
Amino acids: aqueous/plasma concentration varies
from 0.08-3.14
Non colloidal constituents: conc. Of ascorbate,
pyruvate, lactate in higher amount while urea and
glucose are much less.
Insulin and steroid
Prostaglandins
Cyclic AMP (Adenosine monophosphate)
PHYSIOCHEMICAL
PROPERTIES

4. Volume of aqueous humouris about 0.31 ml.
Refractive index of aqueous humouris 1.33332. It is
slightly acidic with a phin the anterior chamber of 7.2
Normal aqueous production rate is 2.0-2.5µl/min.
Aqueous is slightly Hyperosmotic to plasma by 3-5
mOsml/L.
BIOCHEMICAL
COMPOSITION

5. Blood–Aqueous Barrier
Ocular fluids are separated from blood by barriers formed by the tight junctions of epithelial cells and those of
endothelial cells.
These barriers are called either blood–aqueous or blood–retina barriers, depending on their location in the eye.
Disturbationsof these blood–ocular barriers cause blood constituents to mix with ocular fluids; this mixing leads to
plasmoidaqueous, retinal exudates, or retinal edema.
The blood–aqueous barrier is composed of the tight junctions of the following: nonpigmented ciliary epithelium iris
vasculature inner wall endothelium of the Schlemm canal This barrier restricts plasma proteins from entering the
aqueous.
Consequently, aqueous is essentially protein-free, which gives it a refractive index of 1.336 and allows optical
clarity for transmission of light along the visual pathway.
The blood–aqueous barrier, along with active transport systems, also allows increased levels of ascorbate and
some amino acids in aqueous compared to levels in blood plasma

6. Blood–
Aqueous
Barrier

7. Aqueous Humor
Formation and
Secretion

8. Aqueous enters the posterior chamber from the ciliary
processes by means of active and passive physiologic
mechanisms:
active: energy-dependent secretion of certain ions and
substrates(70%)
passive: diffusion(10%)and ultrafiltration(20%)
Aqueoushumor is secreted
by the NPE from a
substrateof blood plasma. It
is secreted at a flow rate of
2–3μL/min, but this rate
varies according to our
circadianrhythm, dropping
to 1.0μL/min atnight.

9. The Active
transport
Mechanism
The active process of aqueous secretion involves
enzymes present in the NPE, such as sodium-
potassium adenosine triphosphatase (Na+ ,K+ -
ATPase) and carbonic anhydrase (CA).
Active secretion of sodium by Na+ ,K+ -ATPase and
accompanying anions creates high osmolarityon the
basolateral (aqueous) side of the NPE, and this in turn
promotes diffusion of water on the basolateral
(aqueous) side of the NPE, and this in turn promotes
diffusion of water.
In humans, CA is present in both PE and NPE

10. Cotransport is the coupled transport of 2 chemical substances
across a membrane, with one substance transported down
itsconcentration gradient, which drives movement of the other
substance against its concentrationgradient.
Symport and antiport are cotransportmechanisms.
Symportersaremembraneproteinsthatmediatethe
cotransportofmoleculesinthesamedirection
Antiporters mediate the cotransport of molecules in opposite
directions. The systems’ activities and cellular distributions
along themembranes of PE and NPE cells determine
unidirectional net secretion from the ciliary stroma to the
posterior chamber, a processthatinvolves 3steps :
1. uptake of solute and water at the stromal surface by PEcells
2. transfer of solute and water from PE to NPE cells through
gapjunctions
3. transfer of solute and water by NPE cells into the posterior
chamber

11. As a result of the primary active transport of Na+, other ions
(eg: Cl) and molecules (eg: ascorbic, some amino acids) are
transported over the epithelium by secondary active
transport. There is also a passive transport for HCo3 -.
To maintain electroneutrality, anions must accompany the
actively secreted Na+, Cl-can pass through Cl -channel in
basolateral membrane. HCO3 -can enter aqueous humour
via exchange with Cl -
The active transport of Na+ and the accompanying anions
create high osmolarity on the basolateral side of NIE cell,
which causes diffusion of water out of the cells.
The movement of water is facilitated by aquaporin(1 & 4).
Na+ and Cl must continuously enter the pigmented epithelial
cell for the continuous secretion of aqueous humour.
This is achieved by Na+/H+ and Cl/HCo3 -antiport and by
the Na+-K+-2Cl-co-transporter.

12. The another enzyme carbonic anhydrase is
abundantly present in the basolateral membrane of
PE & NPE.
It converts water and Co2 to carbonic acid and
subsequently dissociate into H+ & HCo3-.
This HC03 is essential for active secretion of aqueous
humour.
*Note -Carbonic anhydrase inhibitors and β-blockers
are used systemically and topically in the treatment of
glaucoma to reduce the rate of aqueous humor
formation.

13. ThePassive
transport
Mechanism
ULTRAFILTRATION
•Also known as relative dialysis.
• The process by which the fluid and solutes cross
through the semipermiable membrane.
• Capillary blood flow-150 ml/min 4% through
fenestrations Favoured by hydrostatic pressure
difference between capillary and interstitial pressure.
Enough to move fluid to stroma but further requiered
active transport. Leads to form stromal pool

14. ThePassive
transport
Mechanism
DIFFUSION
• Due to active transport of the substance from the
stromal filtrate into posterior chamber, there occur
osmotic and electrical gradient
•Thereforewater,chlorideandothersmallplasma
constituentsmoveintotheP/Ctoequalizethe
osmoticandelectricalgradientbytheprocess

15. FACTORS
AFFECTING
AH
FORMATION
Aqueous formation (F), facility of outflow (C), and
episcleral venous pressure (PO) are the major
intraocular determinants of IOP.
These factors are related to one another by the
Goldmann equation:
F = C (Po -Pv)
Where PO is IOP in undisturbed eye
With the discovery of pressure independent mech the
equation is modified
F = C (Po -Pv)+U
U is the sum of pr independent pathways

16. FACTORS
AFFECTING
AH
FORMATION
Blood flow to ciliary body: profound vasoconstriction
decreases formation.
Sympathetic system: stimulation by β2 & inhibition via
α2 receptors.
Parasympathetic system: decreases via M3 receptors.
Intracelluarregulators: cyclic AMP increases aqueous
formation

17. AQUEOUS HUMOR OUTFLOW
Theaqueoushumorleavestheeyeattheanteriorchamber
anglethroughtrabecularmeshwork,theSchlemm'scanal,
intrascleralchannels,andepiscleralandconjunctival
veins.
Thispathwayisreferredtoastheconventionalor
trabecularoutflow.
Intheunconventionaloruveoscleraloutflow,aqueous
humorexitsthroughtherootofiris,betweentheciliary
musclebundles,thenthroughthesuprachoroidalscleral
tissues.
Trabecularoutflowaccountsfor70%to95%ofthe
aqueousoutflow.
Andremaining5%to30%byuveoscleraloutflow.

18. CELLULAR ORGANIZATION OF THE
TRABECULAR OUTFLOW PATHWAY
The trabecular meshwork: (trabeculam) is a sieve like structure at the angel of the anterior chamber
(AC) through which 90% of aqueous humourleaves the eye.
It has three component. The uveal meshwork is the innermost portion, consisting of cord-like
endothelial cell-covered strands arising from the iris and ciliary body stroma.
The intertrabecular space are relatively large and offer little resistance to the passage of aqueous.
The corneoscleral meshwork lies external to the uveal meshwork to form the thickest portion of the
trabeculum. It is composed of layers of connective tissue stands with overlying endothelial-like cells. it
confers greater resistance to flow than uveal meshwork.
The juxtacanalicular (cribiform) meshwork is the outer part of the trabeculam, and links the
corneoscleral meshwork with the endothelium of the inner wall of the canal of sclemm.
It consists of cells embedded in a dense extracellular matrix with narrow intracellular spaces, and
offers the majors portion of normal resistance to aqueous outflow.

19. The schlemmcanal is a circumferential channel within the perilimbal sclera.
The inner wall is lined by irregular spindle-shaped endothelial cells containing infoldings (gaint
vacuoles) that are thought to convey aqueous via the formation of trabecular pores.
The outer wall is lined by smooth flat cells and contains the opening of collector channels.
COLLECTOR CHANNELS
25-30 intrascleral aqueous vessels Valveless, wide at origin Direct system Indirect system
EPISCLERAL VEINS Drain ultimately in to cavernous sinus via ant ciliary and sup ophthalmic
veins

20. VACUOLATION THEORY OF AQUEOUSTRANSPORT
ACROSS SCHLEMM'S CANAL
Vacuolation theory of
aqueous transport
across the inner wall
of the schlemm's
canal:
1. Non-vacuolated
stage.
2. Stage of early
infolding of basal
surface of the
endothelial cell.
3. Stage of
macrovacuolar
structure formation.
4. Stage of vacuolar
transcellular channel
formation.
5. Stage of occlusion
of the basal infolding

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