Homogenization theory is used to derive macro-scale models of the mass transport equation. Seven cases are considered depending on what combinations of advection, diffusion, and metabolism dominate on the large scale. The order of the Péclet and Damköhler numbers determines the form of the averaged mass transport equation. It is found that for oxygen transport in the capillary beds in the brain, advection and metabolism dominate on the macro-scale. The equations derived are general, and hence can be used to determine the appropriate macro-scale mass transport equation for any solute.