> I saw 'medical physicist' and wondered what you do. Thank you for 'a bit more context', I care! Very interesting stuff. Did you attend medical school + a physics program?

That's a whole separate long answer. I'm not a qualified doctor (and nor would I claim to be), but after a masters' degree in particle physics I moved into an explicitly interdisciplinary training programme that led to a doctorate and at other places in the country I did it in, a separate MPhil. During that initial year I spent a fair amount of time in the dissection room, learning anatomy, as well as most of the first three years (the foundational, preclinical part) of a medical degree combined into one (which contained lots of molecular biology, frankly). My final doctorate was between the departments of condensed matter physics (nominally my awarding institution), biochemistry, radiation oncology, and "the department of physiology anatomy and genetics", which is basically preclinical medicine. The people I work with are 50/50 recovering engineers or physicists, and qualified clinical medics who are trying to learn things like perturbation theory in their time off…

>"it just works", completely by luck What does your validation function look like for this? Whenever stuff "just works" for me I get a little nervous until I determine why.

Ah. I do know why: the relevant Damköhler numbers [0] are either very small (chemistry is much quicker than flow) or large (flow is much quicker than chemistry). So the approximations I am building in are justified and an awkward middle region is excluded; we also are only interested in small concentrations in a carrier fluid (e.g. blood, lymph) where the presence or absence of the species in question does not change its rheology.

I am lucky because we have evolved this way. If our circulatory system and its approach to metabolism was more similar to e.g. a reacting polymer foam ("can of expanding foam") which completely consumes its reactants as it goes, this implicit Lagrangian approach would likely not work.

[0] https://en.wikipedia.org/wiki/Damk%C3%B6hler_numbers