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272A > L10 Aqueous Humour > Flashcards

L10 Aqueous Humour Flashcards

1
Q

What is the aqueous humour?

A

It is a transparent, colourless medium that bathes the avascular tissues in the front of the eye

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2
Q

Where is AH secreted?

A

It is secreted into the posterior chamber by the epithelial cells that line the ciliary body

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3
Q

Where does AH leave the eye?

A

Leaves the eye by passive bulk flow through the trabecular meshwork

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4
Q

What are the roles of the aqueous humor?

A
  • Imposing an intra-ocular pressure (IOP) that inflates the globe, important for normal optical function
  • Supplying nutrition to the avascular ocular tissues.
  • Removal of metabolic wastes excreted by the avascular tissues
  • Accumulation of ascorbic acid in the anterior segment that act as an antioxidative agent by scavenging free radicals
  • Facilitates local immune responses during inflammation.
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5
Q

Which avascular tissues does the aqueous humor bathe?

A

Cornea, trabecular meshwork, lens and anterior vitreous

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6
Q

What is the IOP determined by?

A

1) Rate of formation of AH by ciliary epithelium
2) Rate of drainage of AH through trabecular meshwork
3) Pressure of episcleral veins into which canal of Schlemm drains.

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7
Q

What is an ideal IOP range to maintain correct geometrical relationship between cornea, lens, and retina?

A

10-20mmHg

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8
Q

What happens if the IOP is too high or low?

A

High - leads to glaucoma
Low - Hypotony - eyeball collapses

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9
Q

What is the rate of aqueous humor formation? What is the anterior chamber and posterior chamber volume? How quick is the turnover of aqueous?

A

Rate: 2-3 ul/min
Volume in anterior chamber: 250ul
Volume in posterior chamber: 60ul
Turnover: 1.5-2 hrs

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10
Q

Aqueous humor formation decreases when:

A
  • With sleep - 1.08 ul/min during night, compared to 2.51ul/min during day
  • Advancing age (-2% per decade)
  • Disease
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11
Q

Relative to the plasma, aqueous humour is:

A
  • Slightly hypertonic (ph 7.2)
  • 15-20x higher conc of ascorbate
  • Deficit of protein (0.02% in aqueous vs 7% in plasma)
  • Slightly higher in Cl and lactic acid
  • Slightly deficit of Na, HCO3-, CO2, glucose.
    Refractive index of AH = 1.33336, plasma = 1.376
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12
Q

Diseases like uveitis cause AH protein concentrations to ____ due to breakdown of blood-aqueous barrier

A

increase

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13
Q

Blood-aqueous barrier:

A
  • barrier to movement of substances from plasma to aqueous humour
  • formed by endothelium of capillaries of the iris and ciliary epithelium
  • barrier stops many substances including drugs to travel across it
  • Inflammation can break down this barrier, allowing drugs and large molecules to penetrate the eye
  • As the inflammation subsides, the barrier usually returns.
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14
Q

There is exchange between AH and which other tissues?

A

Vitreous humor, cornea, lens

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15
Q

What does the vitreous humour get from AH?

A

Glucose, amino acids, and inorganic ions

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16
Q

Exchange between AH and cornea:

A

Cornea receives glucose and amino acids
Cornea releases metabolic products such as lactic acid via endothelium pumps

17
Q

Exchange between AH and lens:

A

Lens receives glucose and amino acids
Lens releases metabolic products such as lactic acid
Lens acts as a reservoir of glutathione (antioxidant)

18
Q

Which three processes contribute to AH formation?

A

1) Diffusion of solutes across all cell membranes (passive)
2) Ultrafiltration or the bulk flow of blood plasma across ciliary capillary into ciliary body stroma (passive)
3) Active secretion across ciliary epithelium - accounts for 80-90% of AH formation

19
Q

The ciliary body consists of:

A

Surface ciliary epithelium
Underlying stroma (vascularised)
Ciliary muscle
Blood and nerves supplies

20
Q

What are the 3 principal functions of the ciliary body?

A

1) Production of lens zonules
2) Accommodation of lens
3) Secretion of aqueous humour

21
Q

Ciliary body: blood supply

A
  • High rate of blood flow
  • Supplied by anterior ciliary and long posterior ciliary arteries
  • These arteries fuse together to form major arterial circle located at the base of the iris and ciliary body.
  • Each ciliary processes is supplied by one or two small branches from the arterial circle.
22
Q

CB extends from Iris to ora seratta posteriorly. Smooth muscle is arranged in 3 bundles;

A

Longitudinal, radial, and circular

23
Q

Contraction of the longitudinal smooth muscle of the ciliary body causes:

A

Opens up the trabecular meshwork and scleral spur

24
Q

Contraction of the radial smooth muscle of the ciliary body causes:

A

Widens the uveal trabecular spaces

25
Q

Contraction of the circular smooth muscle of the ciliary body causes:

A

Lens zonules to relax and change shape of the lens

26
Q

Ciliary epithelium is classifed as:

A

Pars plana: smooth surface epithelium approx 4mm wide and joins with ora seratta.
Pars plicata: consists of 70 radially arranged folds called ciliary processes. Double layer of epithelium

27
Q

Coupling of __ and __ forms a functional unit for AH secretion

A

PE-NPE

28
Q

____ cells are joined together by tight junctions to form the blood-aqueous barrier

A

NPE

29
Q

Destabalisation of blood-aqueous barrier by _____ or ____ results in leakage of plasma proteins into the anterior chamber which results in “flare”

A

acute aveitis or trauma

30
Q

AH formation is driven by __ secretion mediated by NP-NPE cell pair. 3 steps:

A

Cl-

1) Stromal Cl- entry into PE
2) Diffusion through gap junction
3) NPE cell secretion of Cl-
4) To maintain electroneutrality, Cl- attracts Na+, thereby creating an osmotic gradient for water.

31
Q

Uptake of Cl- and Na+ ions in PE cells is linked to their exhange with H+ and HCO3-. HCO3- and H+ production is in turned linked to the enzyme ____

A

Carbonic anhydrase

32
Q

Formula for HCO3-…

A

CO2 + H2O –carbonic anhydrase–> H2CO3 –> H+ + HCO3-

33
Q

______ is a potent inhibitor of CA, and it decreases the rate of H+ and HCO3- production, and therefore ciliary body secretion.

A

Acetazolamide

34
Q

The ciliary body receives innervation from:

A

Parasympathetic nerve fibres that release Ach
Sympathetic nerve fibres that relase adrenaline

35
Q

Anti-glaucoma medications use ____ and ____ mechanisms to regulate AH secretion

A

cholinergic and adrenergic

36
Q

Stimulation of a-receptors ____ IOP by reducing AH formoation through inhibition of adenylate cyclase

A

lowers

37
Q

Stimulation of b-receptors ____ IOP by increased AH formation through activation of Adenylate cyclase. As a result, b-receptor blockers are used to lower IOP.

A

increase

272A (13 decks)

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