Mathematical relationships for simple models of the filling and evacuation of the urinary bladder have been found and analyzed. These make the determination of the concentration of microbes in the urinary bladder at a given moment possible in relation to different parameters such as the rate of urine flow from the ureters, the microbe concentration in urine, the reproduction rate of microorganisms, the capacity of the urinary bladder and the size of the residue which remains in the bladder after miction.