The passive behavior of the urinary bladder in the collection phase is described by a number of independent input–output relations characterizing the function of the bladder wall and the geometry of the bladder. Assembly of these relations yields a model characterized by twelve parameters. Two of these parameters can be determined from a static cystometrogram. Stepwise filling of the bladder gives a pressure-decrease curve which can be described by three exponential terms and a constant and yields nine of the desired parameters, or ten if two or more fillings are done on the same bladde