Aqueous Humor Flashcards
Aqueous humor comes from
Circulation from the major circle of the iris by an ultrafiltration mechanism
(Similar to CSF)
Aqueous humor is produced by
Ciliary processes
Aqueous humor is necessary for
Maintaining the proper shape of the eye and its optical properties
What part of ciliary body is AH facing?
Non pigmented epithelium (NPE) faces the posterior chamber (it crosses from the pigmented epithelium that is facing circulation)
Fenestrated capillaries allow for transportation of AH because
Capillaries are lined by endothelium with PORES and NO TIGHT JUNCTIONS, so they are very permeable to macromolecules. Allows for the passing of nutrients
Blood flows from ____ to _____ and drains through _____.
Anterior to posterior and drains through the choroidal veins
3 physiological processes contribute to the formation and composition of AH:
Which is more relied on?
1.Diffusion
2. Ultrafiltration
3. Active secretion
Mostly relying on active secretion
Ultrafiltration needs
Pressure
Diffusion and ultrafiltration are ___ processes requiring______.
Passive
No active cellular participation
How does diffusion across cell membranes happen? What is more likely to go through?
It moves down the concentration gradient.
Substances with high lipid solubility move readily through membranes in this way.
What does ultrafiltration depend on
Pressure
Ultrafiltration is due to bulk flow of blood plasma across fenestrated ciliary capillary endothelium into the ciliary stroma, down
A pressure gradient across a semipermeable membrane (hydrostatic pressure)
If you have a pressure gradient and your IOP increases what will happen to the ultrafiltration
It will decrease because the gradient is made smaller
Relationship between IOP and ultrafiltration?
Lower IOP, higher ultrafiltration into the eye because of pressure gradient
Relation between hydrostatic force and ultrafiltration?
Increasing hydrostatic force will increase the ultrafiltration
Active secretion requires ____ provided by ____.
Requires energy normally provided by hydrolysis of ATP.
AGAINST THE CONCENTRATION GRADIENT
What does active secretion transport?
Na+ is actively transported into the posterior chamber by NPE of the ciliary body. Results in H2O following
Major mechanism for making the AH is
Secretion (not pressure)
Active secretion is pressure ____ at near physiologic IOP (normal IOP)
Insensitive
Ultrafiltration of AH formation is ____ to changes in IOP
Sensitive
As IOP increases _____ decreases
Ultrafiltration
Pseudofacility
When IOP decrease = more filtration
When IOP increases= grading smaller, less filtration
Pressure induced decrease in inflow appears as an increase in outflow when measuring outflow by tonography and constant pressure perfusion
In healthy eyes the area of AH formation is
2.5 ul/min
The rate of AH formation and drainage is 1% per min.
Drainage can go through
Trabecular meshwork or uveal scleral pathway (most through TM)
Enzymes needed for active process
Na+/K+ ATPase
Carbonic anhydride
Can be targeted for modification to the formation of AH
Where is Na+/K+ ATPase located? Where is Na+ and K+ transported to?
Basolateral infoldings of the NPE
Na+ towards aqueous
K+ toward stroma
Na+/K+ ATPase inhibitors like ouabain, vanadate are
Cardiac glycosides
When you you block Na+/K+ ATPase in the ciliary processes
It will significantly decrease the rate of AH formation and IOP
But not usually used to block production of AH because of effects
Inhibition of Na+/K+ ATPase topically will cause
No effect on IOP and may cause corneal edema by interfering with epithelial Na+/K+ ATPase pump
Na+/K+ ATPase inhibiter administered systemically will cause
So many side effects seen in eyes and cardiovascular
Carbonic anhydrase found in the eye
II (aqueous) & IV
CA inhibitor administered systemically will cause
Decrease in AH secretion by as much of 50%
CA inhibition will block
HCo3- and H+
Both affect Na+ transport
Blocking CA will decrease HCO3
transport of Na+ from the cytosol of the NPE to the aqueous wont happen
-decreases intracellular pH, inhibiting Na+/K+ ATPase
Altering formation
Inhibiting CA will decrease H+ formation
Resulting in a decrease in the Na+ & H+ exchange
Inhibition of CA systemically will cause
Systemic acidosis -> decreasing AH formation
CAI in low doses (methazolamide)
Will inhibit ocular enzyme without affecting erythrocytes and renal enzymes and avoiding systemic acidosis.
Will suppress AH formation
To achieve adequate suppression AH secretion using CAI it is required to have
More than 99% of ciliary CA suppressed
Rate of production of AH in a glaucoma person
Is the same as in a normal person
How do we measure the rate of AH formation
Fluorophotometry
Flourescein is given to the pt. A subsequent decline in anterior chamber concentration is used to calc aqueous flow
How does AH production vary during the day
Drops during sleep
But IOP increases during sleep (supine position)
Affect of Age on AH
Production decreases
Decreased outflow facility
What are some anti glaucoma drugs that will increase drainage?
Alpha 2 adrenergic agonist
Prostaglandin analogs
What are some glaucoma drugs that will affect AH production
Alpha 2 agonist
Beta antagonist
Cholinergic (muscarinic) agonist
CAI
Antoglaucoma medication that will reduce formation of HCO3- which in turn reduces AH secretion?
CAI
What causes AH to be different from plasma (2)
BAB
Active transport of substances by the ciliary epithelium
Protein:
AH vs plasma
200x «< protein in AH
CL- ions and A.A.:
AH vs plasma
More in the AH than plasma
Ascorbic acid in AH
20X more in AH than plasma
ANTIOXIDANT
It has to stop the oxidation that occurs when UV light goes across
Lactate
AH vs plasma
More in AH
Metabolic products accumulated way more
Increases when getting closer to the lens
How does uveitis affect the AH?
How will it look in the slit lamp?
Uveitis = inflammation
Inflammation will increase permeability and leak protein
Tyndall effect will be seen in slit lamp exam: flare/light scatter in anterior chamber
Classic Model of BAB
Tight junctions are present between non pigmented ciliary epithelial cells.
And in between endothelial cells if iris vascularture
The proteins present ate actively transported from ciliary epithelium into the AH
