Urination

urination

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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urination [‚yu̇r·ə′nā·shən]

(physiology)

The discharge of urine from the bladder. Also known as micturition.

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Urination 

(also called micturition), the reflex act of excreting urine from the body.

Uropoiesis—the production of urine—is a constant process, but urination occurs only intermittently, as the urinary bladder fills. Humans urinate four to six times daily. Urine is prevented from escaping from the bladder by two ring-shaped layers of muscle fibers: an internal sphincter, which consists of smooth muscle fibers, and an external one, which consists of striated fibers. When the pressure within the bladder reaches 2–2.66 kN/m² (kilonewtons per square meter), or 15–20 mm Hg (millimeters of mercury), stimulation of the mechanoreceptors (the nerve endings of sensory neurons) in the wall of the bladder elicits the urge to urinate. In man, this critical pressure is achieved when 300–350 ml of urine accumulate in the bladder. During urination, the external sphincter relaxes reflexively as the muscles of the bladder wall contract.

Urination is regulated by the nerve centers in the spinal cord and brain; these centers are controlled, in turn, by the cerebral cortex. The filling of the urinary bladder arouses the cerebral cortex to elicit the urge to urinate. Certain neurological diseases, as well as injuries to the muscles of the urinary bladder, impair urination.

M. IA. RATNER