Representative illustration of the MTM of ‘Model 1’ that consists of three reactors in series representing the stomach, duodenum and jejunum, respectively. Ca0 stands for the initial concentration of phenol red present in the glass of water; Q0 stands for the input flow (namely 250 mL in 2 min); RwD and RwJ stands for the secretion rate constants in the duodenum and jejunum, respectively; Kaw represents the water absorption rate constants, present in the duodenum and jejunum; KtD and KtJ are the first-order transit rate constants in the duodenum and jejunum, respectively.