Follicular fluid rheology and the duration of the ovulatory process.

The fluid dynamics of ovulation were investigated to understand the mechanical role of follicular fluid in oocyte release. A set of equations describing the flow of fluid from an evacuating follicle was derived from basic principles. These equations demonstrate that, subject to assumptions about the available pressure differential and the source of the expulsive force, the size and shape of the ovulatory orifice have the largest influences on the rate of fluid loss, although the viscosity of the fluid is also an important variable. A thorough rheological examination of pig, bovine and human follicular fluids, performed using a cone-plate viscometer, demonstrated that these fluids have complex, non-Newtonian characteristics. The fluids also undergo time-dependent and spontaneous changes in viscosity at constant shear rates; some fluids were subject to coagulation-like events. Viscosity characteristics were unrelated to broad parameters of follicle development. The models used representative viscosity values to demonstrate that variations in the rate and duration of follicle evacuation, as observed by ultrasonography, could be explained largely by variations in fluid viscosity and the characteristics of the ovulatory orifice.