Ocular Physiology and Biochemistry
Ocular physiology and biochemistry
This domain tests aqueous dynamics, corneal transparency, tear film, lens metabolism, and retinal homeostasis. Exam items often combine a formula with a clinical scenario such as glaucoma or dry eye.
Core rules
- Aqueous humor is secreted by the ciliary epithelium using Na+/K+-ATPase, carbonic anhydrase, and bicarbonate transport. Flow is from posterior to anterior chamber.
- Goldmann equation: IOP = (F/C) + P_e, where F is aqueous formation, C is outflow facility, and P_e is episcleral venous pressure.
- Corneal transparency depends on stromal collagen order and relative dehydration, maintained by endothelial Na+/K+-ATPase.
- Lens metabolism is mainly anaerobic; the lens uses glucose via glycolysis and the pentose phosphate pathway to manage oxidative stress.
- Tears have a lipid, aqueous, and mucin layer; hyperosmolarity and inflammation are central in dry eye.
Worked example
If aqueous formation is 2.5 µL/min, outflow facility is 0.25 µL/min/mmHg, and episcleral venous pressure is 9 mmHg, estimate IOP. IOP = 2.5/0.25 + 9 = 10 + 9 = 19 mmHg. The calculation shows that either increased formation or reduced facility raises IOP; in open-angle glaucoma, reduced facility is the dominant problem. Check units: facility must be in the same volume/time units as formation.
Traps
- Using mm Hg instead of µL/min/mmHg for facility.
- Forgetting that episcleral venous pressure sets the outflow back-pressure.
- Assuming corneal dehydration is passive; it is metabolically driven.
- Confusing tear hyperosmolarity with simple aqueous deficiency.