Fig. 1 |. Mammalian metabolic fluxes.

a, Organismal-level metabolism. Mammals convert oxygen and dietary nutrients into CO₂, urea and other waste products. In healthy adults, these processes precisely balance at the whole-body level. b, Metabolic tasks are divided among tissues. For example, in the Cori cycle, the liver uses circulating lactate, made by muscle, to produce circulating glucose. c, At the tissue level, metabolic inputs and outputs are also subject to mass balance constraints, with net chemical transformations within the tissue measurable by sampling incoming and outgoing blood.