• The urinary bladder fills with urine by a series of peristaltic contractions at a rate of 0.5-5 ml/min. This is accomplished without a significant increase in intra-vesical pressure and without any involuntary bladder contractions. In other words, the normal bladder is highly compliant over the usual volume range. Compliance is a function of the components of the bladder wall, including elastic tissue and smooth muscle. Intra-vesical pressure depends on the hydrostatic pressure at the bladder neck, bladder wall tension and on the transmission of intra-abdominal pressure.
• The urethra remains closed during the filling and storage phases, and the intra-urethral pressure always exceeds the intra-vesical pressure. Closure is maintained by a number of factors, including:
Passive and active effects of striated and smooth muscle components. Elasticity of the urethra.
Transmission of intra-abdominal pressure to the proximal urethra by a flutter-
valve effect. Surface tension of the urethra. Submucosal vascularity of the urethra.
• The normal bladder stores 350-500 ml.
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