Aqueous Humor Dynamics Flashcards
Three methods of aqueous humor secretion?
Diffusion
Ultrafiltration
Secretion
When is aqueous humor production greatest?
In the morning (2.97 +/- 0.77 microlitre per min)
(8 am - noon)
When is the lowest production of aqueous humor?
At night (1.28 +/- ul per min)
(midnight to 6:00 am)
Na+K- ATPase is predominatly bound to?
Plasma membrane of non-pigmented epithelium
Na+ ions from the Na+K-ATPase are pushed where?
Into the posterior part of anterior chamber to which the water follows
Aqueous Humor Outflow via Trabecular outflow
70-95%
Aqueous Humor outflow via uveoscleral outflow?
5-30%
True or False: Aqueous Humor Outflow and trabecular outflow increase with age?
False
Factors that affect IOP: long term
Genetics
Age (IOP increases with age)
Gender (M=F 20-40; F > inc. 40)
Seasonal (Winter > Summer) (Weight)
Factors affecting IOP: Short Term
Diurnal variations (time of day, higher in the morning)
Postural variations (supine can cause increase)
Exercise (decrease)
Blinking and forceful closure (increase)
Activities causing Valsalver maneuver (exhale with mouth closed)
Eye movements
Food and Drugs
What kind of fluid was Aqueous Humor considered to be in the early 20th century?
Stagnant
Why do we need aqueous humor?
Shape, Optical Properties
Nourishment to cornea and lens
What is the refraction of the aqueous humor?
1.33332
Is the aqueous humor vascular or avascular?
Avascular
How does the aqueous humor have nutrients within it?
Since it is avascular the amount of nutrients within the aqueous humor depends on it’s constant turnover.
What else does the constant turnover of aqueous humor do?
Wash out metabolic waste products
What do the longitudinal fibers of the ciliary body muscles do?
Attach ciliary body to limbus
What do the circular fibers of the ciliary body muscles do?
Attach to the anterior and inner portions of the ciliary body
What do the radial fibers of the ciliary body do?
Connect longitudinal and circular fibers
What are the 3 main vessels of the ciliary body?
Episcleral circle
Intramuscular Circle
Major Arterial Circle
Where is the Major Arterial Circle and what does it supply?
Lies near the iris root; Is the primary supply of the iris and ciliary body
(Paralimbal branch of the LPCA)
The Ciliary Process is supplied by which two branches of the MAC?
Anterior ciliary process arteriole
Posterior Ciliary Process Arteriole
What is a ciliary process?
The functional unit responsible for production of aqueous humor secretion
What are ciliary processes made of?
1) Capillaries
2) Stroma
3) Epithelia
What portion of the ciliary process do the capillaries occupy?
The center of each process
What are the two layers of ciliary epithelium surrounding the stroma?
Pigmented and Nonpigmented epithelium
What are some functions of Aqueous Fluid?
Source of antioxidants
Carries oxygen
Maintain globe shape
Shock absorption
What makes aqueous humor fluid?
Carefully controlled filtrate of blood produced by ciliary body
Which chamber of the anterior chamber does the aqueous humor flow into after secretion?
The posterior chamber
Where does it go after being secreted into the posterior chamber?
Aqueous humor flows around the crystalline lens and through the pupil to the anterior chamber of the anterior chamber?
Does the rate of aqueous humor production follow a pattern?
Yes, it shows a circadian rhythm
How much aqueous humor is produced in the morning as opposed to night time?
In the morning there is much more production.
(2.97 uL/min vs. 1.28uL/min)
Is aqueous humor an active or a passive process?
Active, It works against the concentration gradient
What two enzymes are involved in aqueous formation?
Na,K-ATPase (bound to NPE; pushes Na+ into posterior chamber so water follows)) and Carbonic Anhydrase
What happens if you inhibit either enzyme?
Decreases aqueous production
What are the two pathways in which aqueous humor exits the eye?
Trabecular
Conventional
Trace the trabecular pathway
Trabecular Meshwork
Inner wall of schlemm’s canal into its lumen
Collector channels
Aqueous Veins
Episcleral Venous Circulation
Trace the unconventional/uveoscleral/posterior route
Iris Root
Uveal Meshwork
Anterior face of ciliary muscle
Through the connective tissue between muscle bundles
Suprachoroidal space
Sclera
What percent of aqueous humor outflows through each pathway?
Trabecular: 70-95%
Uveoscleral: 5-30%
How does aging affect aqueous humor outflow?
Both total and trabecular outflow decline with age
Capillaries of the ciliary processes and choroid are ______.
Fenestrated
How are the RPE, NPE and ciliary processes joined together?
Via tight junctions which create a blood aqueous barrier
(Selective to intermediate and high molecular weight substances)
Does the endothelia of the inner wall of Schlemm’s canal also have tight junctions?
Yes, it prevents the movement of fluid and solutes from the Canal of Schlemm into the trabecular meshwork and anterior chamber
The iris and retina do not possess tight junctions/similar epithelia to the blood aqueous barrier but remain impermeable. How?
They have nonfenestrated capillaries
IOP is the outcome of what?
Inflow/outflow of aqueous humor
If plasma components increase in the anterior chamber IOP will increase or decrease?
Increase
How do diseases, trauma or drug induced breakdown of the blood aqueous barrier affect IOP?
Increases IOP
How does the eye combat breakdown of the blood aqueous barrier?
Inflow by ultrafiltration is suppressed and thus reduces the rise in IOP
This is kown as Pseudofacility
How else is IOP rise combatted in the breakdown of the blood aqueous barrier?
Inflammation; possibly interferes with the active transport mechanisms
How is IOP measured?
Using a tonometer
What is the mean IOP in people ages 10-69?
15.8 with a standard deviation of 2.57
What is the IOP magic number?
21 (Mean +2SD)
Does IOP increase or decrease with age?
Increase
When do females begin to see greater increases in IOP?
Post 40 years of age
Does refractive error have anything to do with the pressure of the eye?
NOPE!!!
Is IOP greater in the winter or summer?
Winter because people are less active and thus gain weight and increase their BMI
What short term factors can affect your IOP?
Diurnal Variations
Postural Variations (supine = increase)
Exercise (decrease)
Forceful closure of eyes (increase)
Valsalver Maneuver (exhale with mouth closed = increase)
Eye Movements
How does Alcohol affect IOP?
Decrease
How does caffeine affect IOP?
Slight increase
How does Tobacco affect IOP?
May cause transient increase
How do Heroine and Marijuana affect IOP?
Decrease
What does tonometry do?
Measure pressures
What are the conditions of the Imbert-Fick Law?
Perfect Sphere
Dry
Infinitely Thin
(Pretty much impossible)
Factors affecting IOP measurements.
Central corneal thickness
Corneal curvature
Aging
IOP effects on cornea
Biomedical Properties (Rigidity, Hydration, Elasticity)
What is Hysteresis and how is it applied to the eye?
These are visco-elastic materials (like foam matresses); this applies to the eye because the Cornea is also visco-elastic
How does corneal curvature affect pressure estimates?
Steeper = overestimate
Flatter = underestimate
(Not a huge factor)
How does Corneal thickness affect IOP estimates?
Thicker = overestimate
Thinner = underestimate
Not a huge factor
How does rigidity affect IOP estimates?
Explains over 17mm of variance
What does pulsatile blood flow represent?
It is a calculated value that represents choroidal circulation
What is intraocular pressure telemetry used for?
24-hour IOP measurement; helps identify spikes in IOP both short and logn term of various drugs
Evaluate clinical efficacy
Two types of telemetry
Non-invasive (temporary (contact lens))
Invasive (Permanent implant)
