The process by which the urine from the urinary bladder is excreted out is called micturition. As the urine accumulates, the muscular walls of the bladder expands and cause the stimulation of the sensory nerves in the bladder, setting up a reflex action. This reflex stimulates the urge to pass out urine. In order to discharge urine, the urethral sphincter relaxes and the smooth muscles of the bladder contract. This forces the urine out from the bladder. An adult human excretes about 1 — 1.5 litres of urine per day.