Ocular Physiology and Biochemistry
This domain tests fluid dynamics, corneal clarity, photoreceptor biochemistry, and metabolic support of ocular tissues. Questions often combine a formula with a clinical scenario.
Core rules
- Aqueous humor is secreted by ciliary epithelium, largely by active transport involving carbonic anhydrase. It exits mainly through trabecular meshwork and Schlemm canal, with a smaller uveoscleral route.
- Steady-state IOP is approximated by IOP = (F/C) + Pe, where F is aqueous formation, C is outflow facility, and Pe is episcleral venous pressure.
- Corneal transparency depends on stromal glycosaminoglycan order and endothelial pump function; endothelial failure causes stromal edema and haze.
- Phototransduction: light converts 11-cis retinal to all-trans retinal, activates rhodopsin and transducin, lowers cGMP, closes cation channels, and hyperpolarizes photoreceptors. The RPE regenerates 11-cis retinal.
- Accommodation: ciliary muscle contraction relaxes zonules, allowing the lens to become rounder and increase power.
Worked example
Aqueous formation is 2.5 microliters per minute, outflow facility is 0.3 microliters per minute per mmHg, and episcleral venous pressure is 8 mmHg. Reasoning: IOP = 2.5/0.3 + 8 = 8.3 + 8 = 16.3 mmHg. If facility falls to 0.15 with unchanged formation, IOP rises to about 24.7 mmHg, illustrating why outflow obstruction raises pressure.
Traps that cost marks
- Treating aqueous formation as pure ultrafiltration; secretion is active and energy dependent.
- Forgetting that tear film has lipid, aqueous, and mucin layers, with goblet cells supplying mucin.
- Assuming lens metabolism is aerobic; the lens relies heavily on anaerobic glycolysis.