Write $c$ for the concentration of the chyme. The partial derivative $\frac{\partial c}{\partial t}$ measures the rate of change in the concentration with respect to time. As before, write $U=(u,v,w)$ for the velocity of the chyme (with its eddies). The equation modelling enhanced diffusion is $$\frac{\partial c}{\partial t}+U.\nabla c = \nu \nabla^2 c.$$ Here $$U.\nabla c = u \frac{\partial c}{\partial x}+ v \frac{\partial c}{\partial y} + w \frac{\partial c}{\partial z}.$$ It represents the motion of the drug due to the convection in the chyme. The term $\nu \nabla^2 c$ is defined as $$\nu \nabla^2 c = \nu \left(\frac{\partial^2 c}{\partial x^2} + \frac{\partial^2 c}{\partial y^2}+\frac{\partial^2 c}{\partial z^2}\right).$$ It represents the motion of the drug due to diffusion through the chyme. In this expression $\nu$ is a constant called the \emph{coefficient of molecular diffusion}.