Urine storage mechanisms

The urine storage process depends upon the coordinated control of a storage chamber, the bladder body, and its outlets, the bladder base and urethra. While urine storage disorders are of considerable clinical significance, the mechanisms underlying this process are poorly understood. Many species possess an ability to alter the duration of urine storage in the face of widely varying filling rates. The storage chamber appears largely responsible for this ability to alter capacity and compliance. However, there has been some controversy over the contribution of intrinsic smooth muscle and extrinsic neural systems to the storage process, which is partly related to the various methodologies used in experimental and clinical studies. Thus, the storage phase is greatly influenced by the filling regimes, anaesthesia, non-specific factors and infusates used in these studies. Further, the techniques used to examine and measure urine storage mechanisms often obscure the subtle nature of this process. There is little doubt that, under natural filling conditions, myogenic factors allow the bladder to store increasing volumes at low pressure. More recent studies indicate that, in addition to a quiescent parasympatho-excitatory drive, the extrinsic neural systems contribute to the storage phase with a precisely controlled sympatho-inhibitory drive. However, the sympatho-inhibitory drive does not increase capacity by promoting high compliance. Instead this drive partially reduces the level of bladder wall tension transduced by the bladder wall mechanoreceptors, and thus delays the time at which the micturition tension threshold is reached.