The urinary bladder usually holds 500-650 mL of urine (a pint). When the level of urine reaches about 25% of this amount, pressure of the accumulating fluid stimulates nervous impulses that relax the external sphincter, a muscle that forms a dense band around the urethra at the base of the bladder.
The detrusor muscle is a layer of the urinary bladder wall made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. The bladder is lined with a special epithelium composed of transitional cells. When the bladder is stretched, this signals the parasympathetic nervous system to contract the detrusor muscle. This encourages the bladder to expel urine through the urethra.