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 $$\frac{\partial c}{\partial t}+U.\mathrm{\nabla }c=\nu {\mathrm{\nabla }}^{2}c.$$ Here $$U.\mathrm{\nabla }c=u\frac{\partial c}{\partial x}+v\frac{\partial c}{\partial y}+w\frac{\partial c}{\partial z}.$$ It represents the motion of the drug due to the convection in the chyme. The term $\nu {\mathrm{\nabla }}^{2}c$ is defined as $$\nu {\mathrm{\nabla }}^{2}c=\nu (\frac{{\partial }^{2}c}{\partial x^2}+\frac{{\partial }^{2}c}{\partial y^2}+\frac{{\partial }^{2}c}{\partial z^2}).$$ It represents the motion of the drug due to diffusion through the chyme. In this expression $\nu$ is a constant called the coefficient of molecular diffusion.