The word you are looking for is "micturition". The micturition reflex is the proper term for the act of emptying one's bladder. Once the brain has made the decision to void the bladder, nerves activate the pontine micturition center in the wall of the bladder. This causes the muscles of the bladder wall to contract, while simultaneously relaxing the urethral sphincter. This combination of motions causes urine to be ejected from the bladder. In females gravity does the rest; in males, the bulbospongiosus muscle also contracts, pushing the urine out of the urethra.