How an Equation from Physiology Can Become a Model

Abstract

Students taking a first course in physiology or biochemistry encounter certain equations very early that typically evince a foreboding, if not outright terror. Memorable examples include the Nernst equation and the Henderson-Hasselbach equation; one remembers the encounter, not the content. And there were more to come: Poiseuille’s law of fluid flow and Michaelis-Menten kinetics, the Gibbs-Donnan equilibrium; one could go on endlessly. Let me confess something right now: I knew the feeling of terror just as much as the next student, because I was never sure how to take a logarithm of a sodium concentration, or whether the intracellular concentration of ion was divided by the extracellular concentration or vice-versa, or why the electrical potential was negative and the action potential was depolarizing.

I shall take the simple-minded view that a theory is just a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model to observations that we make.