Process of excreting urine from the bladder (see urinary system). Nerve centres in the spinal cord, brain stem, and cerebral cortex control it through involuntary and voluntary muscles. The need to void is felt when the bladder holds 3.5–5 oz (100–150 ml) of urine and becomes uncomfortable at a volume of 14–15 oz (350–400 ml). The detrusor contracts and the sphincter (muscular constriction) of the urethra relaxes to empty the bladder. Normally it empties completely, but bladder stones or prostatic disorders can block outflow; poor muscle tone (especially in older women) or certain nervous system disorders can cause loss of control of urination (incontinence).
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