Aqueous JR 1 PDF

Aqueous Humour Dynamics Transparent, gelatinous fluid similar to plasma.   Normal conditions - anterior chamber filled with transparent aqueous humour. anterior chamber - bounded posteriorly by iris and lens, anteriorly by cornea and limbus.   anterior chamber depth - 3.5 mm. max width - about 1...

Aqueous Humour Dynamics Transparent, gelatinous fluid similar to plasma.   Normal conditions - anterior chamber filled with transparent aqueous humour. anterior chamber - bounded posteriorly by iris and lens, anteriorly by cornea and limbus.   anterior chamber depth - 3.5 mm. max width - about 12.5 mm.  Its volume can be estimated to 1/3 *  * 6.252 * 3.5 = 285 mm3 = 285 l.   Depth of the anterior chamber can vary implications for the angle that iris and limbus form at the iris root.  shallow anterior chamber implies - lens is placed forward and so is the pupillary margin.  A deep anterior chamber - associated with pupil that lies behind the plane of the iris root. variations - affect the angle of the anterior chamber, the angle formed by the intersection of the iris with the limbus. Most aqueous leaves the eye through the limbus, necessary that the fluid has unimpeded access to the drainage structures.  This is the case if angle of the anterior chamber is about 40o (in clinical terms an open angle).  Depth of the anterior chamber varies, the angle of the anterior chamber can be more acute, to a point when it is classified as closed angle.  Depending on the angle of the anterior chamber, glaucomas can be classified as open- or closed-angle glaucomas.   Aqueous - derived from blood flowing through the ciliary process and secreted into the posterior chamber behind the iris. Similar to blood plasma – slightly higher protein concentration  Aqueous flowing through posterior chamber - warmer than the aqueous in anterior chamber in contact with the relatively cool cornea.  Convection currents carry newly arrived aqueous upward and then down along the posterior surface of the cornea.  The capillaries in ciliary process - several endothelial cells joining to form a tubular structure. Adjoining cells - loosely attached with a few anchoring junctions.   Fenestrations - on both inner & outer surfaces, and the capillary is lined by the basement membrane of the endothelial cells. This structure is porous – fluid can flow.    Fluids & large molecules - easily flow out of highly permeable capillaries, where they begin transformation to aqueous humour. Blood flows in the capillaries of the cilary body- coarsely filtered by the capillaries' endothelial cells. Resulting plasma - then refiltered by pigmented and nonpigmented ciliary epithelial cells  Fluid moving out of the capillaries into the stroma of the ciliary processes is very similar to blood plasma. Three mechanisms are involved in aqueous formation: 1. Diffusion, 2. Ultrafiltration and 3. Metabolically Driven Transport.     Aqueous - continually produced and flows into anterior chamber must be removed at the same rate at which it is produced i.e. Balanced If rate of drainage lags rate of production intraocular pressure rises. Normal aqueous pressure - 15 mmHg https://www.youtube.com/watch? v=ybW6LsdcAx4    2 routes for aqueous drainage. uveoscleral flow. minor route (10 - 15% of outflow), fluid percolates into spaces in the connective tissue surrounding the muscle fibres in the ciliary body …  … and then moves into the space between the ciliary body and the sclera - removed by veins in the ciliary muscle and anterior choroid  Major route(85 - 90%) - through structures of the limbus, mainly the trabecular meshwork, lining one side of the chamber, and the canal of Schlemm a large modified blood vessel encircling the chamber angle.  From the canal of Schlemm, aqueous flows into the superficial episcleral vein through venous plexuses in the limbal stroma.  Under normal conditions 2-3 l of aqueous leave the anterior chamber every minute.  This is enough to empty the anterior chamber in 2 hours. Anatomy of the Drainage  In a histological section, the scleral spur projects like a thumb inward and behind the canal of Schlemm. Strands of the trabecular meshwork attach to its anterior surface, while some fibres of the meridional portion of the ciliary muscle fuse with its posterior surface.  The trabecular meshwork is a mesh formed by cords of collagen surrounded by endothelial cells with open spaces between the cords.  Slender cords form the uveal portion of the trabecular meshwork and lie closest to the chamber angle where they are joined by occasional extensions of tissue from the iris.  Cords in the corneoscleral portion are broader and flatter, while the open spaces are fewer and smaller.  The canal of Schlemm is separated by an assemblage of endothelial cells, fibroblasts and collagen (cribriform layer or juxtacanalicular tissue). The cribriform layer is the major source of outflow resistance.  The canal of Schlemm is a large anular vessel encircling the angle of the anterior chamber. The inner side of the canal, which is closest to the anterior chamber, has small insertions, the internal collecting channels.  Larger collecting external channels extend from the outer surface of the canal, through which aqueous is drained into small veins into the limbal stroma and ultimately into the episcleral veins.  The aqueous veins is an external collecting channel that bypasses the limbal veins, carrying aqueous directly to the episcleral veins. Glaucoma Acute glaucoma –obstruction occurs at periphery of iris and rapid increase of IOP Chronic glaucoma – trabecular meshwork gradually blocked leading to slow increase of IOP Primary open-angle glaucoma Chronic open-angle glaucoma – accumulation of collagen in the cribriform layer and within trabeculae. Slow rise in IOP from about 20mmHg to 25-35mmHg Damage to optic nerve fibers Primary closed-angle glaucoma Anterior surface of lens pushes iris anteriorly reducing angle and blocking trabecular meshwork IOP rises to 40-80 mmHg Optic disk swells Age-related Secondary glaucoma Open-angle: angle obstructed by cells, haemorrhage, tumour cells, lens matter etc. Closed-angle: angle closed by anterior displacement of lens due to e.g. tumour. Uveitis (fibrin causes adhesion of iris to trabecular meshwork) Quiz Time 1 Corneal Epithelium Thickness? What cell types does it contain? 2  Where are the goblet cells ?  What do they produce ? 3  Computer simulations calculate that during the evolutionary process, the camera eye took how many years to develop from single sheet of receptor cells ? 4  To widen palpebral fissure the upper tarsal plate is pulled up by which two muscles ? 5  What are the two main types of compound eye? 6  Astronmical telescope  FE = +70D  Angular , Fo = + 2D Magnification?  Length of Telescope? 7  What are a Diaphysis & an Epiphysis? 8  What is Tangential coma ? 9  What specific these? designs of telescopes are 10  What is the function of the cranium?  Which bone articulates with all other cranial bones, holding them together? 11  What are the 7 main functions of the tear film? 12  How does a refractive index gradient in an eye lens arise?  What are the main advantages such a lens? 13  Which is the only animal with hyperspectral colour vision ?  The human fovea has 200,000 receptors per mm2. How many do birds of prey have? 14  What is the main function of alpha- crystallin?  What causes light scattering in cataract? 15  What is the f ratio of an optical system?  If an optical system has an f ratio of 3, is it classified as a fast medium or slow? 1 Corneal Epithelium  Thickness?  50m thick, 5-6 cells thick What cell types does it contain?  Squamous, Wing, Basal (3) (2) 2  Where are the goblet cells ?  What do they produce ?  Conjunctiva  Mucus (2) (2) 3  Computer simulations calculate that during the evolutionary process, the camera eye took how many years to develop from single sheet of receptor cells ?  Less than half a million years (2) 4  To widen palpebral fissure the upper tarsal plate is pulled up by which two muscles ?  levator aponeurosis (2)  superior tarsal muscle (Müller’s) (4) 5  What are the two main types of compound eye?  Apposition eye  Superposition eye (3) (3) 6  Astronmical telescope F = +70D , Fo = + 2D  Angular Magnification? E Angular M = / = - FE/Fo  -35x (4)  Length  51.4 of Telescope? cm d = fo’ + fE’ (4) 7  What are Diaphysis & Epiphysis? (4) 8  What is Tangential coma ? distance Tp’Tm’ (4) 9  What specific designs of telescopes are these?  Newtionan (3) Cassegrain (3) 10  What is the function of the cranium?  Houses the brain (3)  Which bone articulates with all other cranial bones, holding them together?  The sphenoid (3) 11  What are the 7 main functions of the tear film?   1. smooth refracting surface 2. Transports metabolic products to and from the epithelial cells of the cornea (O2 in and CO2 out).  3. Prevents desiccation of the corneal epithelium.  4. Lubricates eyelids.  5. Bactericidal properties.  6. Provides a pathway for white blood cells in case of injury.  7. Provides nutrients (minor role) 2 marks each    12 How does a refractive index gradient in an eye lens arise? Variation in protein concentration from nucleus to cortex (4) What are the main advantages such a lens? Minimises spherical & chromatic aberrations (8) 13  Which is the only animal with hyperspectral colour vision ?  Mantis shrimp (2)  The human fovea has 200,000 receptors per mm2. How many do birds of prey have?  1 000 000 receptors per mm2 (2) 14  What is the main function of alpha-crystallin?  Chaperone protein, protects target proteins (beta & gamma-crystallin) from unfolding (2,2,2)  What causes light scattering in cataract?  Unfolded proteins which form large aggregates. (4) If comparable with wavelength of light in size will scatter light (4) 15  What is the f ratio of an optical system?  f = focal length/aperture (5)  If an optical system has an f ratio of 3, is it classified as a fast medium or slow?  Fast (5)