I am having issues with mass balance (creating mass out of nowhere, Nobel prize here I come...) in my PBPK model. Eventually the system evens out, but the first steps (hours 0-10 out of 72) end up spitting out about a 1-10% error as a fraction of the dose applied. I have tried different solvers and this has not eliminated the issue. The mass balance term is sensitive to parameter values as well.
Any pointers would help.