Topic 6: Aqueous Humour & Intra-ocular pressure Flashcards
describe AQH and its properties
transparent, colourless fluid contained in the anterior and posterior chambers of the eye
volume: 0.25ml (anterior), 0.06ml (posterior)
RI: 1.336
describe location of AQH
bounded anteriorly by corneal endothelium except at the far periphery (reaching the trabecular meshwork
bounded posteriorly by crystalline lens within pupil aperture, anterior surface of iris, and peripherally by the anterior face of the ciliary body
state the functions of AQH
(1) provide positive intro-ocular pressure, giving shape to the globe
(2) provide nutrients and remove waste products from the cornea, iris and crystalline lens
(volume of aqueous in anterior chamber turns over once every 100 minutes. the constant flow of AQH replenishes nutrients and carries away waste)
state the site of AQH production
inner non pigmented ciliary epithelium (NPE) in the pars plicata region
state the source of aqueous humour
ciliary arterial system (blood supply to CB is MAC), derived from blood plasma (water + proteins) flowing in the ciliary arteries that will need to be processed (AQH is low level of protein)
explain the 3 processes in AQH formation
(1) diffusion - movement of molecules (LIPID SOLUBLE PARTICLES) along concentration gradient
(2) ultra-filtration - movement of Smaller molecules (SMALLER WATER MOLECULES) across a semi-permeable membrane with hydrostatic force
(3) active transport - movement of Larger molecules (LARGER WATER MOLECULES) against concentration gradient using cellular energy
explain circulation of AQH
the 3 processes happen in the (1) NPE ciliary processes.
the molecules are transported out of the cell membranes into the (2) posterior chamber
water gets added and aqueous humour moves from posterior to (4) anterior chamber via the (3) pupil
AQH circulates in the anterior chamber (due to convection currents) and provide nutrition and remove waste from cornea, iris and lens
explain anterior chamber angle
structures: Trabecular meshwork, Schlemm’s canal, scleral spur, ciliary body
closed angle: only TM seen partially or no structures seen at all
where drainage starts, AQH exits angle into TM
wide open angle: all structures up till scleral spur seen
explain conventional aqueous drainage pathway
80%-90% of AQH (from ciliary processes > posterior chamber > anterior chamber) gets drained via the anterior chamber angle, into the trabecular meshwork, schlemm’s canal, collector channels, aqueous veins, episcleral veins and anterior ciliary veins
explain the uveoscleral pathway/alternate AQH outflow
10%-20% AQH drains through the face of the ciliary body in the region just posterior to the scleral spur, in the apex of the anterior chamber.
AQH flows through the ciliary muscle into the suprachoriodal space
describe the blood-aqueous barrier
made up of ciliary NPE tight junctions (where 3 processes operate), iris junctions, iris blood vessels
the anatomical mechanism preventing exchange of materials between the chambers of the eye and blood vessels
explain the functions of blood-aqueous barrier
to maintain transparency of the aqueous humour
restricts the movement of sodium, large water-soluble ions, proteins & other large/medium molecules into aqueous humour
define intra-ocular pressure
the pressure within the eye maintained by aqueous humour
numbers regarding glaucoma suspects
IOP > 21 mmHg (normal 10-21)
Diurnal variation = 8-10 mmHg (normal 4-6)
Inter-ocular variation > 3mm Hg (normal < 3)
state some ocular situations where IOP rises above normal values
(1) closed angle - AQH does not get drained so the regulated outflow keeping IOP maintained is disrupted
(2) open angle with:
shrinkage/swelling/blockage of trabecular meshwork
narrowing of schlemm’s canal
increased episcleral venous pressure - pathway becomes more narrow
