ABSTRACT
The distinction between ‘physiological’ and ‘actuator’ variables is a pragmatic one: state variables belong to the former category when it is comparatively straightforward to find dynamic laws for them on the basis of conservation laws (energy, stoichiometry) and general physiological principles. Whereas this distinction clearly depends on available information, the ‘physiological’ variables generally describe the ‘controlled plant’ whereas the ‘actuator’ ones tend to reflect the inner workings of the organism's regulatory systems. A coarse-graining based on gradient dynamics is outlined in the present chapter, and we argue that a variational principle can underpin such dynamics, combining the Pontryagin principle with the so-called quasi-stationary co-state approximation.
