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Physiology of the Eye > Aqueous Humor and Intraocular Pressure > Flashcards

Aqueous Humor and Intraocular Pressure Flashcards

1
Q

What is the major drug detoxification site in the eye?

A

ciliary body

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2
Q

What are some key functions of the ciliary body?

A
  • formation of aqueous humor
  • generation of IOP
  • maintenance of IOP through formation of aqueous humor, modification of the trabecular meshwork through muscular contracture, and anatomic opening of the non-conventional outflow pathway
  • accommodation
  • blood and nerve supply to the anterior segment
  • formation of blood-aqueous barrier
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3
Q

What are the two distinct portions of the avian ciliary body?

A

anterior and posterior

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4
Q

What are the three main mechanisms of aqueous humor formation?

A
  • active secretion
  • ultrafiltration
  • simple diffusion
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5
Q

Why does plasmoid aqueous in domestic species clot easily?

A

because it possesses high levels of fibrinogen

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6
Q

What is the refractive index of aqueous humor?

A

1.355

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7
Q

What account for the difference between the anterior chamber and posterior chamber concentrations of some compounds?

A

The metabolic needs of the cornea and lens will lead to differences.

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8
Q

What are the chemical components of aqueous humor, and are the concentrations of these compounds generally higher or lower in the aqueous humor compared to plasma?

A

-proteins
-immunoglobulins
-enzymes
-lipids
Generally much lower in in the aqueous compared to plasma.

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9
Q

What is the relative concentration of carbohydrates found in the aqueous humor compared to the plasma?

A

Generally 80% of what is found in plasma, which is most limey a reflection of their use by the lens and cornea rather than by the blood aqueous barrier.

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10
Q

Characterize urea in the aqueous humor compared to plasma.

A

Generally 80% of what is found in plasma, likely due to slow penetration through the blood aqueous barrier.

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11
Q

In general, what is the difference in concentration of amino acids between the aqueous and plasma?

A

Amino acids are typically found in greater concentrations in the aqueous, which suggest an active transport mechanism.

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12
Q

For which species are amino acids lower in the aqueous comported to the plasma, and why might this be?

A

The dog, where the vitreous may act as a sink for amino acids that enter the anterior chamber.

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13
Q

What are the major cations in the aqueous humor?

A

sodium (makes up 95%), potassium, calcium and magnesium

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14
Q

What are the major anions in the aqueous humor?

A

chloride, bicarbonate, phosphate, ascorbate, and lactate.

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15
Q

Lactate in the aqueous humor is found in higher or lower concentration relative to plasma?

A

Much higher

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16
Q

What is the relationship between sodium, chloride and bicarbonate in the aqueous humor?

A

the combined concentration of bicarb and chloride must equal that of NaCl, because the anion and cations concentrations much balance each other out. Bicarb and chloride also follow sodium into the aqueous humor during active secretion.

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17
Q

What is the concentration of ascorbate in the aqueous humor compared to the plasma, and what is the proposed mechanism behind the difference?

A

Ascorbate is found in higher concentrations in the aqueous humor due to an active transport mechanism.

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18
Q

What are the proposed functions of ascorbate in the aqueous humor?

A
  • storage of chatecholamines in the iris
  • cofactor in electron transfer reactions
  • reducing agent in hydroxylation reactions
  • partially absorbs ultraviolet radiation
  • antioxidant
  • helps regulate production of GAG’s in trabecular meshwork
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19
Q

Are aqueous humor levels of ascorbate higher in diurnal or nocturnal animals, and what is the possible significance?

A

Higher in diurnal animals suggesting protection against light induced damage.

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20
Q

What is the most important factor in aqueous humor production?

A

active transport of certain solutes by the ciliary epithelium

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21
Q

What is the rate of aqueous humor production most dependent on, and what enzyme is chiefly responsive for this?

A
  • The rate of solute secreation/transfer into the aqueous humor.
  • carbonic anhydrase
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22
Q

What likely accounts for the majority of outflow resistance in the conventional outflow pathway?

A

Trabecular Meshwork ( also called the juxtacanalicular zone)

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23
Q

The back pressure created by venous system (either scleral venous plexus or Schlemm’s canal) accounts for approximately how much of resistance.

A

50-75%

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24
Q

Venous pressure in all species studies is determined to be between what?

A

8-12mmHg

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25
Q

AV anastomosis in the episclera has been demonstrated in what species?

A
  • rabbit
  • dog
  • owl monkey
  • cynomoglus monkey
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26
Q

Briefly describe the proposed importance of AV anastamosis in the episclera of those species that possess it.

A

May function in the rabbit and dog (species which do not have capillary beds in the episclera) as an emergency system to elevate IOP after anterior chamber paracentesis

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27
Q

What does the term pseudofacility refer to?

A

pressure-dependent ultrafiltration formation of aqueous humor (could be altered during measurement of IOP, so this is factored into the equation)

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28
Q

To what degree can pseudofacility account for error in the measurement of aqueous humor flow

A

up to 20% error

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29
Q

In the cat, a center in what part of the brain has been shown to cause changes in IOP when stimulated?

A

diencephalon

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30
Q

What factors that have regulatory roles in the renal epithelium may also effect IOP.

A
  • antidiuretic hormone
  • cortisol
  • prolactin
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31
Q

What factors affect unconventional (uveoscleral) outflow?

A
  • the state of the ciliary body

- hydrostatic pressure difference between the anterior chamber and suprachoroidal space.

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32
Q

Contraction of the ciliary musculature does what to uveoscleral outflow?

A

Decreases it (likely due to decreased extracellular space)

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33
Q

Uveoscleral outflow accounts for what percentage of outflow in the dog?

A

15%

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34
Q

Uveoscleral outflow accounts for what percentage of outflow in the cat?

A

3%

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35
Q

Uveoscleral outflow accounts for what percentage of outflow in the rabbit?

A

13-25%

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36
Q

Uveoscleral outflow accounts for what percentage of outflow in the primate?

A

5-40%

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37
Q

What is episcleral venous pressure in the dog?

A

10-12mmHg

38
Q

What is episcleral venous pressure in the cat?

A

8mmHg

39
Q

What is episcleral venous pressure in the rabbit?

A

9mmHg

40
Q

What is episcleral venous pressure in the primate?

A

10-11mmHg

41
Q

What is aqueous humor formation in the dog?

A

5.22 uL/min

42
Q

What is aqueous humor formation in the cat?

A

6-7 uL/min

43
Q

What is aqueous humor formation in the rabbit?

A

1.84 uL/min

44
Q

What is aqueous humor formation in the primate?

A

2.75 uL/min

45
Q

The volume of the anterior chamber relates to aqueous outflow in what way?

A

Animals with larger eyes have faster outflow rates per minute.

46
Q

What aspect of the angular aqueous plexus is though to be the main site of resistance for aqueous outflow?

A 
endothelial lining
(extracellular matrix also adds to resistance)
47
Q

What are the three methods in which aqueous can pass directly into the angular aqueous plexus?

A

transcellular pores, large vacuoles, pinocytic vesicles

48
Q

A malfunction of endothelial phagocytosis and reduced numbers of trabecular endothelial cells may contribute to the development of what disease process?

A

primary open-angle glaucoma

49
Q

Briefly, what are some of the functions of GAG’s in the trabecular meshwork?

A

they form highly viscous and elastic gel solutions that function as a biologic gel filtration system and account for a significant amount of the resistance to outflow.

50
Q

What is Darcy’s Law

A

Formula for measuring flow in a gel filtration system

51
Q

What are three procedures used to investigate the formation of aqueous humor?

A
  • cannulation of the anterior chamber: constant rate/constant pressure perfusion
  • Direct view/measurement of recently formed aqueous
  • use of aqueous humor markers (radioactive fluorsecien, paraminophippuric acid): Measures the decay rate of intracamerally injected isotopes
52
Q

What are three procedures used to investigate the exit of aqueous humor?

A
  • ocular perfusion to lower IOP
  • perilimbic suction cup
  • tonography (conventional outflow pressure sensitive)
  • use of markers
53
Q

What are some common markers used in investigating exit of aqueous humor?

A

-fluorescein
-nitrotetrazolin
-latex spheres
-radioactive tracers
(can be used in both conventional and uveoscleral outflow routes)

54
Q

What are some common methods used to measure episcleral venous pressure?

A
  • partial to complete collapse of the episcleral veins
  • torsion balance
  • pressure chamber (filled with air or saline)
  • air jet
  • ocular compression
  • direct cannulation and measurement by transducer
55
Q

What is the most common method for measuring aqueous humor formation?

A

constant pressure perfusion technique

56
Q

Briefly describe the two-step Barany and Scotchbrook procedure.

A

-constant perfusion of the anterior chamber at two different levels of IOP (`2-3 and 8-10mmHg above baseline) for several minutes. Total facility is determined by dividing the difference in perfusion rates between the two levels of IOP, by the pressure difference between the two levels. (pseudofacility can account for up to 0.2uL/min/mmHg)

57
Q

What does topical pilocarpine do to uveoscleral outflow?

A

decreases it (likely due to reducing the potential spaces within the anterior ciliary body. (this has been studied in the nonhuman primate, cat and dog)

58
Q

What is the size range used for observable tracers in measuring uveosclearal outflow?

A

10nm to 1um

59
Q

What are some common tracers used in the measurement of uveoscleral outflow?

A
  • microspheres
  • fluorescein-labeled dextran
  • Evans blue
  • nitro blue tetrozolium
  • India ink
  • trypan blue
  • thorotrast
  • blue dextran
  • colored proteins
  • C-inulin
  • ferritin
  • saccharated iron
  • yeast cells
60
Q

What is the volume of the anterior chamber in the feline eye?

A

0.6mL

61
Q

What is the volume of the posterior chamber in the feline eye?

A

0.3mL

62
Q

What is the volume of the lens in the feline eye?

A

0.3mL

63
Q

What is the volume of the vitreous in the feline eye?

A

2.8mL

64
Q

What is the volume of the anterior chamber in the canine eye?

A

0.4mL

65
Q

What is the volume of the posterior chamber in the canine eye?

A

0.2mL

66
Q

What is the volume of the lens in the feline eye?

A

0.5mL

67
Q

What is the volume of the vitreous in the canine eye?

A

3.2mL

68
Q

What is the volume of the anterior chamber in the rabbit eye?

A

0.3mL

69
Q

What is the volume of the posterior chamber in the rabbit eye?

A

0.06mL

70
Q

What is the volume of the lens in the rabbit eye?

A

0.2mL

71
Q

What is the volume of the vitreous in the rabbit eye?

A

1.5mL

72
Q

What is the volume of the anterior chamber in the bovine eye?

A

1.7mL

73
Q

What is the volume of the posterior chamber in the bovine eye?

A

1.5mL

74
Q

What is the volume of the lens in the bovine eye?

A

2.2mL

75
Q

What is the volume of the vitreous in the bovine eye?

A

20.9mL

76
Q

What is the volume of the anterior chamber in the equine eye?

A

2.4mL

77
Q

What is the volume of the posterior chamber in the equine eye?

A

1.6mL

78
Q

what is the volume of the lens in the equine eye?

A

3.1mL

79
Q

What is the volume of the vitreous in the equine eye?

A

28.8mL

80
Q

What is the volume of the anterior chamber in the porcine eye?

A

0.3mL

81
Q

What is the volume of the vitreous in the porcine eye?

A

3mL

82
Q

What non-invasive technique was developed as both a research and clinical tool to evaluate aqueous humor flow dynamics?

A

Flurorphotometry

83
Q

Do the dog and cat eye have greater or lesser ocular rigidity than the human eye?

A

The eye is less rigid (more elastic)

84
Q

Define ocular rigidity.

A

resistance offered by the fibrous tunic of the eye to a change in intraocular pressure. (can also be defined as change in IOP per incremental change in intraocular volume).

85
Q

What method is used to determine ocular rigidity?

A

Schiotz tonometry

86
Q

Ocular rigidity depends on what two factors?

A

the individual eye and IOP

87
Q

Characterize the diurnal variation in IOP seen in dogs and humans.

A

Highest pressure in the AM, lowest pressure in the afternoon

88
Q

Characterize the diurnal variation in IOP seen in non-human primates, rabbits and cats.

A

Highest pressures during the day, lowest pressures at night.

89
Q

What are some factors that influence short-term changes in IOP?

A
  • diurnal changes
  • forced eyelid closure
  • contraction of retractor bulb
  • coughing/valsava maneuver
  • abrupt changes in blood pressure
  • pulse
  • struggline/electroshock
  • changes in body/head position
  • succinylcholine
  • acidosis
90
Q

What are some factors that influence long-term changes in IOP?

A
  • aging
  • race/breed
  • hormones/glucocorticoids
  • growth hormone
  • estorgen
  • progesterone
  • obesity
  • myopia
  • gender
  • season
91
Q

What happens to the valleys and peak in daily IOP measurements in glaucomatous beagle dogs?

A

They are accentuated, where a difference of 10-15mmHg between peaks and valleys may be seen. (this is perhaps due to abnormal IOP control mechanism)

92
Q

Are there reported seasonal variations in IOP, and if so characterize them and in what species do they occur.

A
  • Seasonal variations have been reported in the human and rabbit.
  • IOP’s are lower in the summer and higher during the winter months.

Physiology of the Eye (10 decks)

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