Aqueous Humor Dynamics Flashcards
Why do we need aqueous humor?
Provide structure of the globe, nutrition, and refraction of 1.33.
What parts of the eye depend on aqueous humor for nutrition? Why?
Cornea and lens. They are both avascular, need nutrition from somewhere.
Why does aqueous humor have a constant turnover of fluid in the eye?
There are waste products accumulating in the eye. They need to be removed and replaced with fresh nutrients.
What are four things present in the ciliary body?
Muscles, vessels, autonomic nerve terminals, epithelia lining ciliary processes.
What three parts does the ciliary body muscle consist of?
Longitudinal, radial, circular.
Which part of the ciliary body muscle is the transitional muscle?
Radial.
Which part of the ciliary body muscle attaches ciliary body to the limbus?
Longitudinal.
Which part of the ciliary body muscle form anterior and inner portions of the ciliary body?
Circular.
What is the primary blood supply to the iris and ciliary processes?
Major Arterial Circle located in the ciliary body.
How many branches does the MAC supply to the ciliary processes? Which is fenestrated and non-fenestrated?
Two branches.
Posterior:non-fenestrated, anterior:fenestrated.
What is the functional unit responsible for the production and secretion of aqueous humor?
Ciliary process.
What three things does the ciliary process consist of?
Stroma, capillaries, and epithelium.
The two layers of ciliary epithelium surrounding the stroma are?
Pigmented and non-pigmented epithelium.
What is the main source of aqueous fluid in the eye?
Blood through carefully controlled filtration processes.
Aqueous fluid provides a source of what to the corneal endothelium and lens?
Antioxidants.
Why is important that aqueous humor serves as a shock absorber?
If not for aqueous humor, lens would be hitting iris all the time.
Why do the lens require antioxidant?
Lens requires antioxidants to remove free radical in order to stop cataracts developing way to faster than we would like.
The antioxidant in the aqueous humor we speak of is what?
Vitamin C or ascorbate.
Why is calcium removed?
Too much will cause clearly to be decreased and because calcium can cause more cataracts to be generated.
Name 4 things removed in aqueous fluid that can potentially cause problems if not removed?
Calcium, cholesterol, hemoglobin, globulin.
Other than ascorbate, name one other component that is present in the aqueous fluid?
Bicarbonate.
Is it normal to find red blood cells in your aqueous?
No! That’s a problem. Things should not be floating around.
What is the true meaning of “secretion”.
Process by which aqueous humor formation is at its maximum.
What are three processes by which the ciliary process releases its contents?
Diffusion, ultrafiltration, and secretion (80-90%).
During which time is the rate of aqueous humor production largest in a normal individual?
Morning
During which time is the rate of aqueous humor production smallest in a normal individual?
Night time.
Can glaucoma patients have a different circadian rhythm when it comes to aqueous humor production?
Yes.
What two enzymes are responsible for the production of aqueous humor?
Na-K-ATPase and carbonic anhydrase.
Is the aqueous humor production an active process or passive process?
Active. Needs energy to work against its concentration gradient.
Where is Na-K-ATPase located and what is its function?
Predominantly bound to plasma membrane of non-pigmented epithelium. This enzyme helps push the Na+ ions into posterior chamber to which water follows.
Can aqueous humor be altered? As in the amount produced? If so, how?
Yes aqueous humor can be altered depending on the concentration of Na-K-ATPase.
What is the purpose of carbonic anhydrase?
Carbonic anhdrase acts on carbonic acid to produce CO2 and H2O.
If you stop carbonic anhydrase with an inhibitor, what happens to the production of aqueous humor?
No conversation of carbonic acid to water occurs, so no water produced which causes a decrease production of aqueous humor.
What two things are going to help remove aqueous humor from posterior to anterior part of anterior chamber through pupil?
Pressure and anatomical restrictions.
What role does the vitreous humor play for the flow of aqueous humor?
Vitreous humor has glycosaminoglycans that allow it to be a gel like substance. This acts as a barrier and pushes aqueous humor to flow through the pupil.
How many mechanism are involved in removing aqueous humor from the posterior part of the anterior chamber? What are they?
- Conventional route via trabecular meshwork
and unconventional route via uveoscleral pathways.
If episcleral venous pressure is high, can the fluid still flow out?
Not very well. If IOP is lower than episcleral pressure, fluid cant flow out the eye.
What can cause episcleral venous pressure to increase?
Tumor rarely but more commonly, coughing or sleeping.
As fluid passes through the plates of trabecular meshwork, how does this fluid get into schlemm’s canal?
Vacuoles are formed that take fluid into schlemm’s canal.
What happens to the aqueous fluid in the uveoscleral pathway?
Fluid passes through connective tissue between ciliary body muscles into suprachoriodal space and out the sclera.
What happens to the aqueous fluid in the trabecular pathway?
Fluid passes through trabecular meshwork, into schlemm’s canal to collector channels, and out to the episceral venous circulation.
What can cause increase in the spaces between muscles?
Medications we give that increases these spaces between muscle by restructuring features and allowing more fluid to go out.
True or False: We can measure the uveoscleral outflow.
False. Total outflow-trabecular outflow=uveoscleral outflow.
What is the percentage of trabecular outflow?
70-95%.
What is the percentage of uveoscleral outflow?
5-30%.
Both total and trabecular outflow decline with what?
Age. As we grow older, we are not able to remove fluid from the eye so well.
What two things are joined together by tight junctions that allow for a blood aqueous barrier to be present?
RPE and non-pigmented epithelium of the ciliary processes.
What other two things in the eye have a blood-aqueous barrier?
Schlemm’s canal endothelium and the capillaries of the iris and retina are non-fenestrated.
Internal ocular pressure if the outcome of what two factors?
Inflow and outflow.
If plasma components increase in anterior chamber due to a breakdown of blood aqueous barrier (proteins get released with the aqueous humor), should IOP increase?
Yes.
During diseases, trauma, or drug induced breakdown of the blood aqueous barrier, what helps the IOP to decrease?
Pseudofacility.
What is pseudofacility?
Aqueous inflow via ultrafiltration is suppressed, slowing down the rise of IOP.
Can glaucoma exist without a high IOP?
Yes.
Normal range of IOP is?
Mean IOP 15.8 SD 2.57, magic number 21. (Not actually mean this much though, keep in mine).
What are some factors that affect IOP?
Inheritance, age, gender, refractive error.
What is the maximum contributor factor to IOP?
Inheritance.
As an individual gets older, what happens to IOP?
Increases because aqueous outflow is decreased with age.
Do females or males have higher IOP after the age of 40?
Females have increase in IOP after the age of 40.
Do myopic or hyperopic patients have higher IOP?
Hyperopic because smaller eyeball causes greater pressure.
During winter, does IOP increase or decrease?
Increase because venous flow is obstructed with gain weight.
Moving eyes left to right, like saccades does what to the IOP?
Increase.
Appropriate exercise increase or decrease IOP?
Decrease.
Supine position increase or decrease IOP?
Increase.
Blinking or forcefully closing eye increase or decrease IOP?
Increase.
Exhaling with your mouth closed cause IOP to increase or decrease?
Increase.
Eye movement cause increase or decrease in IOP?
Increase.
Does alcohol cause an increase or decrease in IOP?
Decrease. It’s a de-hydrant that moves water from all cells in the body, including the eye.
Does a lot of water cause increase or decrease in IOP?
Increase.
Does caffeine cause increase or decrease in IOP?
Increase.
Does tobacco cause increase or decrease in IOP?
Increase. Especially in new smokers. (Increase BP and pulse rate to increase and causes more fluid to flow into eye and constriction so essentially increases IOP).
Does heroin and marijuana cause an increase or decrease in IOP?
Decrease. Not enough for therapeutic level.
The current standard tonometry to measure IOP is called what?
Goldmann.
What is the basic formula to measure IOP?
W= P x A
W is external force
P is pressure inside sphere
A is area.
What law is used for applanation tonometry?
Imbert-Fick Law.
What are three assumption of Imbert-Fick Law?
Eye is sphere, perfectly dry, and thin.
Due to the inaccurate assumption, what is the modified Imbert-Fick Law?
W + S = P x A +B
S is surface area
B is the force required to bend the cornea.
As you increase force, what do you see during a Goldmann’s tonometry?
You see A and as you increase the force you eventually get the two spheres to come closer and closer. Once the inner circles are perfectly alined, you read the value and times that by 10.
What factors increase the error when measuring IOP? (Not IOP itself)
Corneal thickness is an error in measurement (does not cause true difference in IOP), corneal curvature (curved cornea-need more force to push and make it flat and flat cornea-don’t need to apply a lot of force),
Will you under estimate IOP with a flat cornea or very curved cornea?
Flat cornea.
Does keratoconus lead to over or under estimation?
Under because even though curvature is increase, central cornea gets extremely thin and flexible causing under estimation due to the cornea’s decrease in strength.
Cornea thinner, over or under estimate?
Under because you can easily bend it, flatten it. You don’t have to push so much.
True or False: Anything that is easy to bend, pressure is underestimate.
True.
What three biomechanical properties are important factors that affect IOP measuring?
Rigidity, hydration, and elasticity.
As we grow older, cornea gets hydrated, cornea gets stiffer, which means over or underestimation of IOP?
Over estimation.
Is cornea elastic or visco-elastic?
Visco-elastic.
Due to corneal curvature, what amount of difference is accounted?
1.76 mmHg.
Due to corneal thickness, what amount of difference is accounted?
2.87 mmHg.
Due to corneal biomechanics, what amount of difference is accounted?
17.26 mmHg.
Pneumotonometer measures up to what amount of IOP for how long?
200/sec for 20 seconds.
Pneumotonometer measures __ pulses and selects __ best to calculate IOP and pulsatile ocular blood flow.
7; 5.
Pulsatile ocular blood flow represents what?
Choroidal circulation.
Why is a 24 hour IOP measurement needed?
Certain diseases are progressive and sometimes doctors want to know if drugs are doing their job. 24 hour measurement can help with accuracy.
