Ocular Anatomy, Physiology, and Pathology
This domain links structure to function: how corneal transparency, aqueous turnover and retinal nutrition actually work, and which pathological process follows when each fails.
Anterior segment
- Cornea: epithelium (regenerates), Bowman's (does not), stroma (~90% of thickness, lamellar collagen), Descemet's, endothelium (non-regenerating; roughly 2,500 cells/mm² at birth, declining slowly with age).
- Transparency depends on stromal deturgescence maintained by endothelial Na⁺/K⁺-ATPase pumps and tight junctions; oedema increases light scatter.
- Aqueous is secreted by the ciliary epithelium at about 2.5 µL/min; exits through the trabecular meshwork and Schlemm's canal (~90%) and the uveoscleral route.
- Tear film: mucin (goblet cells), aqueous (lacrimal gland), lipid (meibomian glands).
Posterior segment
The neurosensory retina has ten layers. Photoreceptor outer segments are phagocytosed daily by the retinal pigment epithelium; the outer retina is nourished by the choriocapillaris, the inner retina by the central retinal artery. The optic nerve carries roughly one million axons, with macular fibres temporal and papillomacular bundle fibres vulnerable early in toxic and nutritional optic neuropathies.
Worked example
Using the Goldmann equation, IOP = F/C + P_ev. With aqueous inflow F = 2.5 µL/min, outflow facility C = 0.25 µL/min/mmHg and episcleral venous pressure 8 mmHg, IOP = 10 + 8 = 18 mmHg. Halving inflow to 1.25 µL/min gives 5 + 8 = 13 mmHg, which is why inflow-suppressing drugs have a limited ceiling compared with agents that raise facility.
Traps
Assuming endothelium regenerates; blaming corneal oedema on the endothelium when the epithelial barrier is intact and the stroma is simply hydrated; confusing aqueous production with tear secretion; overlooking the crystalline lens's continuous growth throughout life.