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Ocular Physiology > Aqueous Humor Dynamics > Flashcards

Aqueous Humor Dynamics Flashcards

1
Q

how many diopters does the aqueous humor contribute to refraction

A

1.33 D

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2
Q

why does the eye need aqueous humor

A

shape, optical properties, shock absorber, and carries antioxidants, oxygen and nourishment to cornea and lens

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3
Q

what are the avascular structures of the anterior segment

A

lens and cornea

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4
Q

why does the aqueous humor need constant turnover

A

to deliver nutrients and wash out metabolic waste products

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5
Q

what are 4 parts to the ciliary body

A

muscle, vessels, epithelia lining ciliary processes and autonomic nerve terminals

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6
Q

what are the 3 muscles of the ciliary body

A

longitudinal, circular, and radial

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7
Q

what are the 3 vessel circles of the ciliary body

A

episcleral circle, intramuscular circle, and MAC

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8
Q

what is the primary blood supply to the iris and ciliary processes

A

Major arterial circle

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9
Q

what are the 2 branches of the MAC that supply the ciliary processes

A

anterior and posterior ciliary process arterioles

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10
Q

what is the functional unit responsible for production of aqueous humor secretion

A

the ciliary process

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11
Q

what 3 things make up the ciliary process

A

capillaries, stroma, and epithelia

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12
Q

what occupies the center of each ciliary process

A

capillaries

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13
Q

why does aqueous humor carefully control filtrate of blood

A

otherwise certain things will enter the anterior chamber and inhibit light from reaching the retina or scattering light

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14
Q

what component is in higher concentration in aqueous humor compared to blood

A

ascorbate (vitamin C)

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15
Q

why does aqueous humor have a high concentration of vitamin C

A

it is an antioxidant

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16
Q

what components are in blood that you don’t want to have present in aqueous humor

A

proteins, lipids, cholesterol, hemaglobin

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17
Q

what is the main/maximum source of aqueous humor formation

A

secretion (80-90%)

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18
Q

when is aqueous humor production the lowest

A

at nighttime

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19
Q

when is aqueous humor production the highest

A

in the mornings

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20
Q

is aqueous formation an active or passive process

A

an active process, requires energy

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21
Q

what mediates the active process of aqueous formation

A

the transport of certain ions and substances against the concentration gradient

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22
Q

what are the 2 enzymes in the aqueous formation process

A

Na, K-ATPase and carbonic anhydrase

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23
Q

where is Na, K-ATPase predominantly bound

A

to plasma membrane of non-pigmented epithelium

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24
Q

which ions are pushed into the anterior chamber that cause water to follow

A

sodium ions

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25
Q

how would you stop water from entering the anterior chamber and decrease aqueous production

A

use a carbonic anhydrase inhibitor

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26
Q

what do bicarbonate and hydrogen ions form

A

carbonic acid (H2CO3)

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27
Q

what acts on carbonic acid to produce CO2 and H2O

A

carbonic anhydrase

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28
Q

which part of the anterior chamber, anterior or posterior, has a higher pressure

A

the posterior has a higher pressure

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29
Q

what causes the pressure gradient to move the aqueous humor to the anterior chamber

A

having a higher pressure in the posterior chamber helps move the fluid into the anterior chamber through the pupil

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30
Q

why does the vitreous act as a barrier for aqueous humor

A

the GAGs in the vitreous give it a gel consistency and help push the aqueous into the anterior chamber

31
Q

what are the two pathways aqueous humor can drain out of the eye

A

trabecular meshwork and uveoscleral

32
Q

describe the steps of the trabecular meshwork pathway

A

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

33
Q

what are the steps to the uveoscleral route

A

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

34
Q

what causes both total and trabecular outflow to decline

A

age

35
Q

what makes a blood aqueous barrier in the ciliary body

A

the pigmented and non-pigmented epithelial layers of ciliary processes are joined by tight junctions

36
Q

what does the tight junction of the PE and NPE block from entering the posterior chamber

A

intermediate and high molecular weight substances

37
Q

where is the blood aqueous barrier in the trabecular meshwork route

A

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

38
Q

what are 2 other parts of the eye that have nonfenestrated capillaries and are not permeable and contribute to the blood aqueous barrier

A

the iris and retina

39
Q

what is the IOP an outcome of

A

inflow and outflow of aqueous

40
Q

what happens to the IOP if plasma components increase in anterior chamber

A

the IOP should increase

41
Q

what is a pseudofacility

A

something that removes aqueous humor or helps decrease the IOP

42
Q

what happens to the IOP if a disease, trauma or drug induces the breakdown of the blood aqueous barrier

A

the IOP will increase

43
Q

what might decrease secretion of aqueous if the blood aqueous barrier has a breakdown

A

pseudofacility and the inflammation process reduces the rate of active secretion (interfering with active transport mechanisms)

44
Q

in a general population, what is the mean IOP

A

15.8

45
Q

what is the magic number IOP in a general population

A

21

46
Q

what are 3 factors that affect IOP in the long term

A

genetics, age, and gender

47
Q

what contributes the most to long term IOP

A

genetics

48
Q

how does age affect long term IOP

A

it increases with age

49
Q

how does gender affect long term IOP

A

males and females are the same IOP until age 40, then females have a greater increase in IOP

50
Q

does refractive error affect long term IOP

A

no, the myotic eye has a thinner wall and can be easily deformed making the IOP appear to be lower than it really is

51
Q

how can seasonal variation affect IOP

A

IOP is great in winter than summer, weigh gain in the winter and venous flow is decreased by excess weight

52
Q

what are 5 examples of factors that increase IOP short term

A

supine position, blinking and forceful closure, head upside down, moving eyes back and forth, activities causing valsalver maneuver (exhale with mouth closed)

53
Q

what are 4 examples of factors that will decrease IOP short term

A

exercise, alcohol, heroin and marijuana

54
Q

what drugs may cause a short term increase in IOP

A

caffeine and tobacco (new smokers)

55
Q

if alcohol decreases IOP short term, why would excessive amounts lead to an increase in IOP

A

the alcohol will dehydrate the body to decrease the IOP but the amount of water taken with the alcohol will increase the IOP

56
Q

what was the original type of tonometer used

A

indentation-schiotz

57
Q

what is the most commonly used tonometer

A

the applanation- Goldmann

58
Q

what is Imbert-Fick’s law

A

used to explain applanation tonometry- W=PxA (w=external force, P=pressure, A=area)

59
Q

what are the 3 conditions for Imbert-Fick’s law

A

perfect sphere, dry surface, and infinitely thin

60
Q

what is the modified Imbert-Fick law

A

W+S=PxA+B (S=surface tension and B=force required to bed the cornea)

61
Q

using the modified Imbert-Fick law, which to factors cancel each other out

A

the corneal elasticity and surface tension of the tears balance and cancel each other out (S and B)

62
Q

what is a perkins tonometer

A

a goldmann on a handheld device

63
Q

what are 5 factors that can affect IOP measurements

A

central corneal thickness, corneal curvature, age related changes to cornea, IOP effects on cornea, and biomedical properties (rigidity, hydration, and elasticity)

64
Q

If a patient has keratoconus, how would their IOP measurements be affected

A

their cornea has more curvature and has a thinner central thickness- it would under estimate the IOP

65
Q

how would age related changes affect IOP measurements

A

as we age the cornea becomes dehydrated and stiffer (harder to push in) and IOP will be over estimated

66
Q

how do IOP effects on the cornea affect the IOP measurements

A

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

67
Q

what is hysteresis

A

a visco-elastic material (the cornea)

68
Q

what is a pneumotonometer

A

an IOP measuring device that uses air and takes very rapid measurements, measures IOP continuously

69
Q

what is the rate of the pneumotonometer

A

200/sec, up to 20 sec (measures 7 pulses and selects 5 best to calc IOP)

70
Q

what are the two flow rates (IOP values) depending on which stage (systolic or diastolic) the heart is in

A

pulsatile or steady flow

71
Q

what is the net flow of the eye

A

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)

72
Q

what is pulsatile ocular blood flow

A

represents choroidal circulation

73
Q

why would you need to measure 24 hour IOP

A

uncertain cases of NTG (normal tension glaucoma), new drugs or modalities

Ocular Physiology (6 decks)

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