This article concerns the so-called irrigation system in the Timaeus' biology (77a–81e), which replenishes our body’s tissues with resources from food delivered as blood. I argue that this system functions mainly by the natural like-to-like motion of the elements and that the circulation of blood is an important case study of Plato’s physics. We are forced to revise the view that the elements attract their like. Instead, similar elements merely tend to coalesce with each other in virtue of their tactile (...) features as the atomists describe. The notion of attraction is replaced with this notion of mere coalescence. I begin by outlining how blood is made from food. I then argue that an understanding of health and disease compels us to read Plato as if he were an atomist and to abandon the popular scholarly interpretations according to which the elements attract each other. (shrink)