Aqueous Humor Dynamics Flashcards
how many diopters does the aqueous humor contribute to refraction
1.33 D
why does the eye need aqueous humor
shape, optical properties, shock absorber, and carries antioxidants, oxygen and nourishment to cornea and lens
what are the avascular structures of the anterior segment
lens and cornea
why does the aqueous humor need constant turnover
to deliver nutrients and wash out metabolic waste products
what are 4 parts to the ciliary body
muscle, vessels, epithelia lining ciliary processes and autonomic nerve terminals
what are the 3 muscles of the ciliary body
longitudinal, circular, and radial
what are the 3 vessel circles of the ciliary body
episcleral circle, intramuscular circle, and MAC
what is the primary blood supply to the iris and ciliary processes
Major arterial circle
what are the 2 branches of the MAC that supply the ciliary processes
anterior and posterior ciliary process arterioles
what is the functional unit responsible for production of aqueous humor secretion
the ciliary process
what 3 things make up the ciliary process
capillaries, stroma, and epithelia
what occupies the center of each ciliary process
capillaries
why does aqueous humor carefully control filtrate of blood
otherwise certain things will enter the anterior chamber and inhibit light from reaching the retina or scattering light
what component is in higher concentration in aqueous humor compared to blood
ascorbate (vitamin C)
why does aqueous humor have a high concentration of vitamin C
it is an antioxidant
what components are in blood that you don’t want to have present in aqueous humor
proteins, lipids, cholesterol, hemaglobin
what is the main/maximum source of aqueous humor formation
secretion (80-90%)
when is aqueous humor production the lowest
at nighttime
when is aqueous humor production the highest
in the mornings
is aqueous formation an active or passive process
an active process, requires energy
what mediates the active process of aqueous formation
the transport of certain ions and substances against the concentration gradient
what are the 2 enzymes in the aqueous formation process
Na, K-ATPase and carbonic anhydrase
where is Na, K-ATPase predominantly bound
to plasma membrane of non-pigmented epithelium
which ions are pushed into the anterior chamber that cause water to follow
sodium ions
how would you stop water from entering the anterior chamber and decrease aqueous production
use a carbonic anhydrase inhibitor
what do bicarbonate and hydrogen ions form
carbonic acid (H2CO3)
what acts on carbonic acid to produce CO2 and H2O
carbonic anhydrase
which part of the anterior chamber, anterior or posterior, has a higher pressure
the posterior has a higher pressure
what causes the pressure gradient to move the aqueous humor to the anterior chamber
having a higher pressure in the posterior chamber helps move the fluid into the anterior chamber through the pupil
why does the vitreous act as a barrier for aqueous humor
the GAGs in the vitreous give it a gel consistency and help push the aqueous into the anterior chamber
what are the two pathways aqueous humor can drain out of the eye
trabecular meshwork and uveoscleral
describe the steps of the trabecular meshwork pathway
after entering the TM it goes to the inner wall of Schlemm’s canal and into its lumen, then into collector channels, aqueous veins, and episceral venous circulation
what are the steps to the uveoscleral route
across iris root, uveal meshwork and the anterior face of ciliary muscle, through connective tissue, between muscle bundles, through suprachoriodal space and out through sclera
what causes both total and trabecular outflow to decline
age
what makes a blood aqueous barrier in the ciliary body
the pigmented and non-pigmented epithelial layers of ciliary processes are joined by tight junctions
what does the tight junction of the PE and NPE block from entering the posterior chamber
intermediate and high molecular weight substances
where is the blood aqueous barrier in the trabecular meshwork route
the endothelia of the inner wall of schlemms canal are slightly joined to prevent movement of fluid and solutes from the canal into the trabecular meshwork
what are 2 other parts of the eye that have nonfenestrated capillaries and are not permeable and contribute to the blood aqueous barrier
the iris and retina
what is the IOP an outcome of
inflow and outflow of aqueous
what happens to the IOP if plasma components increase in anterior chamber
the IOP should increase
what is a pseudofacility
something that removes aqueous humor or helps decrease the IOP
what happens to the IOP if a disease, trauma or drug induces the breakdown of the blood aqueous barrier
the IOP will increase
what might decrease secretion of aqueous if the blood aqueous barrier has a breakdown
pseudofacility and the inflammation process reduces the rate of active secretion (interfering with active transport mechanisms)
in a general population, what is the mean IOP
15.8
what is the magic number IOP in a general population
21
what are 3 factors that affect IOP in the long term
genetics, age, and gender
what contributes the most to long term IOP
genetics
how does age affect long term IOP
it increases with age
how does gender affect long term IOP
males and females are the same IOP until age 40, then females have a greater increase in IOP
does refractive error affect long term IOP
no, the myotic eye has a thinner wall and can be easily deformed making the IOP appear to be lower than it really is
how can seasonal variation affect IOP
IOP is great in winter than summer, weigh gain in the winter and venous flow is decreased by excess weight
what are 5 examples of factors that increase IOP short term
supine position, blinking and forceful closure, head upside down, moving eyes back and forth, activities causing valsalver maneuver (exhale with mouth closed)
what are 4 examples of factors that will decrease IOP short term
exercise, alcohol, heroin and marijuana
what drugs may cause a short term increase in IOP
caffeine and tobacco (new smokers)
if alcohol decreases IOP short term, why would excessive amounts lead to an increase in IOP
the alcohol will dehydrate the body to decrease the IOP but the amount of water taken with the alcohol will increase the IOP
what was the original type of tonometer used
indentation-schiotz
what is the most commonly used tonometer
the applanation- Goldmann
what is Imbert-Fick’s law
used to explain applanation tonometry- W=PxA (w=external force, P=pressure, A=area)
what are the 3 conditions for Imbert-Fick’s law
perfect sphere, dry surface, and infinitely thin
what is the modified Imbert-Fick law
W+S=PxA+B (S=surface tension and B=force required to bed the cornea)
using the modified Imbert-Fick law, which to factors cancel each other out
the corneal elasticity and surface tension of the tears balance and cancel each other out (S and B)
what is a perkins tonometer
a goldmann on a handheld device
what are 5 factors that can affect IOP measurements
central corneal thickness, corneal curvature, age related changes to cornea, IOP effects on cornea, and biomedical properties (rigidity, hydration, and elasticity)
If a patient has keratoconus, how would their IOP measurements be affected
their cornea has more curvature and has a thinner central thickness- it would under estimate the IOP
how would age related changes affect IOP measurements
as we age the cornea becomes dehydrated and stiffer (harder to push in) and IOP will be over estimated
how do IOP effects on the cornea affect the IOP measurements
if IOP is high the cornea will compress (it doesn’t have anywhere else to go) and will be more difficult to push in- over estimate the IOP
what is hysteresis
a visco-elastic material (the cornea)
what is a pneumotonometer
an IOP measuring device that uses air and takes very rapid measurements, measures IOP continuously
what is the rate of the pneumotonometer
200/sec, up to 20 sec (measures 7 pulses and selects 5 best to calc IOP)
what are the two flow rates (IOP values) depending on which stage (systolic or diastolic) the heart is in
pulsatile or steady flow
what is the net flow of the eye
how much blood is reaching the eye at a given time, can be useful in disease diagnosis (subtract the pulsatile and steady flow = net flow)
what is pulsatile ocular blood flow
represents choroidal circulation
why would you need to measure 24 hour IOP
uncertain cases of NTG (normal tension glaucoma), new drugs or modalities
