  Understanding biological phenomena requires a systemic approach that
incorporates different mechanisms acting on different spatial and temporal
scales, since in organisms the workings of all components, such as organelles,
cells, and organs interrelate. This inherent interdependency between diverse
biological mechanisms, both on the same and on different scales, provides the
functioning of an organism capable of maintaining homeostasis and physiological
stability through numerous feedback loops. Thus, developing models of organisms
and their constituents should be done within the overall systemic context of
the studied phenomena. We introduce such a method for modeling growth and
regeneration of livers at the organ scale, considering it a part of the overall
multi-scale biochemical and biophysical processes of an organism. Our method is
based on the earlier discovered general growth law, postulating that any
biological growth process comprises a uniquely defined distribution of
nutritional resources between maintenance needs and biomass production. Based
on this law, we introduce a liver growth model that allows to accurately
predicting the growth of liver transplants in dogs and liver grafts in humans.
Using this model, we find quantitative growth characteristics, such as the time
point when the transition period after surgery is over and the liver resumes
normal growth, rates at which hepatocytes are involved in proliferation, etc.
We then use the model to determine and quantify otherwise unobservable
metabolic properties of livers.
