Tear production and evaporation in the normal eye.

The volume of tears in the eye depends on the rate of production and the rate of elimination by drainage and evaporation. A mechanism for the regulation of tear production mediated through changes in tear tonicity has been suggested. High evaporation of tears with a patent drainage system has been thought to be related to reduced tear volume, increased tonicity and an eventual stimulus to production. In this study no significant correlation was found between tear evaporation, measured by a modified evaporimeter system, and tear production, as measured by the turnover rate with a scanning fluorophotometer. This result suggests that regulation of tear production through the mechanism of tonicity change may not be the only, or principal, mechanism in the normal eye.