Introduction to Compartmental Analysis

Abstract

In the context of compartmental analysis, a living organism can be described as an open biological system existing in a steady-state far from thermodynamic equilibrium. Thermodynamic equilibrium is a state in which no biological processes can occur because there are no potential gradients to drive them; no differences in mechanical potential to drive blood flow, in concentrations to drive diffusion, in chemical potentials to drive metabolism, in electrical potentials to drive ions, and in temperature to drive heat flow. Steady-state and thermodynamic equilibrium share the characteristic that they are invariant in time. Thermodynamic equilibrium is also invariant in space. The steady-state variance of constituent chemicals in space is the focus of compartmental analysis. Spatial variance is assigned to the interfaces between abstract compartments rather than to the living system as a whole. As the compartments by this definition are in thermodynamic equilibrium internally, they are incompatible with life but we choose to ignore this fundamental characteristic. Compartmental analysis uses the principles of biophysics and mathematics to determine the velocity of exchanges among the compartments (biochemical processes) and the relative size of the individual compartments (biochemical pools) in vivo, using tracer molecules, defined as markers that do not perturb the system. During a medical study or biological experiment, the tracer and its metabolites assume different states, each of which may be well defined but all of which change and interact as functions of time. Eventually, one or more of these states may reach the steady-state characteristic of the native system, though far from thermodynamic equilibrium. This steady-state can be maintained only in thermodynamically open systems. If energy is no longer provided or expended, potential