For a large-scale organism, how would you set the starting conditions? It seems unrealistic to not include some external influences from, e.g. air and diet. Whe

For a large-scale organism, how would you set the starting conditions? It seems unrealistic to not include some external influences from, e.g. air and diet. When you simulate a bigger organism, not only do you face a geometric increase in the number of cells but you also must account for externalities, I guess. I have no idea how you may do that on a mouse, let alone something as large as a human...