Schematic representation of simplification steps. a (Step2), lumping GIT and spleen to splanchnic and lumping heart, kidney, skin, and muscle to visceral. b (Step 3), fast transfer from liver to lung. c (Step 4), lumping splanchnic and visceral to Peripheral. d (Step 5), fast transfer from lung to plasma, intra-tissue compartments are shown. e (Step 6), equilibrium between plasma and Vascular compartments forming a Central compartment. f (Step 7) final simplified model