Tear film physiology.

Current understanding of tear film physiology is based on the anatomy of each of the three layers, as well as on the changes that occur with blinking and on the physicochemical properties of the tear fluid. Abnormalities in tear film may be due to deficiency of aqueous tears or mucin, changes in the lipid layer, inadequate blinking, or epithelial pathology. Patients who have marginal deficiencies tend to be symptom-free but will develop symptoms promptly if contact lenses are fitted.