A model of the mechanics of smooth muscle reservoirs applied to the intestinal bladder.

A model of the mechanics of smooth muscle reservoirs was derived from the stress-strain relation in smooth muscle strips. The model was applied to the intestinal bladder reservoir but may also be applied to natural reservoirs such as the bladder. The relation between reservoir volume and pressure was calculated at different values of smooth muscle tone and was found to accord with clinical results. The muscle tone of the bowel when measuring off the bowel to construct the reservoir was found to have a profound influence on its ultimate volume. Stiffening of a part of the reservoir wall as a result of edema or fibrosis will cause a substantial decrease in functional capacity, both directly as a result of a reduced expansion of the diseased part and indirectly by enhancing the stretching of the adjacent normal wall. The pressure rise produced by a slow peristaltic contraction was calculated to depend both upon the strength of contraction and on the degree of reservoir filling: Almost no pressure rise will occur in the near empty reservoir, while the highest pressure rise was calculated to occur at a volume of about 300 ml. If only a part of the reservoir contracts, more filling is needed before the pressure increases and the maximum pressure will be reached at a substantially higher volume. Localized contractions will stretch the resting reservoir wall and so activate stretch receptors, which may cause a desire to void, resulting in reduced functional capacity.