Normal Micturition

Abstract

Understanding the act of micturition necessitates consideration of the structure of the bladder muscle and its nervous regulation as well as the hydrodynamic principles concerned with storage and emptying of the bladder contents. The important facts about bladder function are now generally accepted. Knowledge of the physiology of smooth muscle makes it apparent that the best starting point for forceful micturition is the mediumfull bladder, when the muscle fibers are stretched to about twice their normal length in the resting position (Anderson et al., Carpenter). Urgency is no longer thought to indicate that intravesical pressure has increased and it is now accepted that even massive pressure peaks do not lead to involuntary loss of urine in a healthy individual (Baumann). The law of Laplace, which correlates volume and wall tension, is applicable to urine storage. This explains why quite different quantities of urine can be stored under similar, generally low, pressure conditions (Lawson and Tomlinson). Bladder voiding through the urethra is known to conform to the laws of turbulent currents, so that even slight variations in diameter may be important (Smith).