MECHANICAL PROPERTIES AND FUNCTIONS OF THE URINARY BLADDER

I. Stretch Behavior of the Bladder In Vivo

Abstract

A stretch test in vivo for living material was newly developed and applied to the dog bladder. The stress-strain curve of the bladder was speed-dependent and with hysteresis. The mechanical properties of the bladder in vivo was essentially viscoelastic.
The stress-strain curve of a normal living bladder was downward convex and rose very slowly. On the other hand, that of an extirpated bladder or neurogenic bladder after denervation of the pelvic nerves demonstrated a rapid elevation.
A computer analysis based on a visco-elastic simulation model proposed by Glantz was carried out to evaluate changes of the curve under various conditions, which successfully demonstrated the mechanical properties of the bladder. The model proposed by Glantz explained the mechanical properties of the bladder wall.
The following formula represented the stress-strain curve by the model:
φ=-β(φ+α)²/γ+β(φ+α)C+α²βe^βct/γ
φ: force, α & β: elastic constant, γ: viscous constant.
According to their properties, α was designated as proportional stiffness and β as exponential stiffness.
The three constants, α, β and γ for the stress-strain curve obtained experimentally from the bladder under different conditions were calculated by computer analysis. Both proportional and exponential stiffness elevated in the extirpated bladder. In the denervated bladder, changes of the constants were not significant immediately after cutting the pelvic nerves. However, the proportional stiffness (α) elevated remarkably two weeks after the cutting. Compared with these abnormal bladders, the elastic constants in the normal living bladder were every low. The viscous constant did not change under any conditions that were examined in this study.
The visco-elastic properties of the bladder and its low elastic constants seemed to provide the normal bladder function in reservoir of urine.