Passive properties of the urinary bladder in the collection phase

A model is presented that describes the passive properties of the urinary bladder in the collection phase. A black-box approach is used. The system under investigation, which is defined in terms of a pressure-volume relationship, is divided into four subsystems or blocks, namely two geometry blocks, a block describing the time-dependent properties of the bladder wall, and a block describing its length-dependent properties. Models have been developed and tested for each block separately. With regard to geometry, the bladder is described as a thick-walled sphere of constant tissue volume. The time-dependence of the properties of the wall can be explained using a visco-elastic model, and the length dependence of the wall properties is shown to yield elastic moduli which depend biexponentially on strain. Estimates of the value of the parameters involved were obtained from experiments on strips of urinary bladder, obtained from the local slaughterhouse. Combination of the blocks yielded an overall model of the passive properties of the urinary bladder in the collection phase. The model contains 14 parameters. The classical way of investigating the urinary bladder, by filling it slowly and measuring the pressure produced, yields a pseudostatic pressure-volume relationship called a cystometrogram. The model predicts the form of the cystometrogram accurately. However, analysis of a classical cystometrogram enables us to determine only three parameters of our model. A better measurement method is based on stepwise (or almost stepwise) straining of the urinary bladder. One stepwise straining yields eight parameters, provided the initial volume of the bladder is known, and several measurements on one bladder at different strains enable us to determine ten parameters. The results obtained with stepwise straining are compatible with the model.