The background of hysteretic properties of the human umbilical arterial wall. Smooth muscle contraction and hysteresis of the pressure-radius curves.
Incongruency between the upward and downward routes of pressure-radius curves, a phenomenon called hysteresis, was studied on 124 human umbilical arterial segments. Such characteristic phenomena were found which are unusual if one compares them to the viscotic hysteresis of the unliving material. After a stay at low distending pressure, the loop of the forthcoming pressure-radius cycle became exceptionally wide. The width of the loop was relatively independent of the rate of pressure change. Equilibrium points were not always inside the loop. Addition of a smooth muscle contracting agent increased the width of the loop. At low pressure the segments contracted spontaneously and the shortened elements kept the shortened length for some time even at higher distending pressure. All phenomena found in connection with hysteresis can be explained if we suppose that some of the contractile elements contract during cyclic stretching relatively easily at low distending pressures. Due to the "catchlike" properties of this contraction, the shortened elements could resist against higher distending forces on the upward route and this way a delay in the dilatation process occurred. This made upward and downward routes different.