Noncompartmental models of whole-body clearance of tracers: a review.

Noncompartmental models are defined as models that allow for transport of material through regions of the body that are not necessarily well-mixed or of uniform concentration. The clearance of a substance of interest (metabolite or drug) from a noncompartmental system will not necessarily be governed by a sum of exponentials or even be describable by a set of ordinary differential equations. The model may involve diffusion or other random walk processes, leading to the solution in terms of the partial differential equation of diffusion or in terms of probability distributions. It may use the theory of linear systems to obviate the need for defining any precise anatomical structure. A number of the models reviewed deal with plasma clearance curves that are best described by power functions of time. Circulatory models are reviewed from their inception to the present. Recent studies on clearance as a fractal process are introduced.