Urine then leaves the kidneys and travels down the tu- bular ureters. The muscles in the ureter walls constantly tighten and relax to bring urine into the bladder. Small amounts of urine enter the bladder approximately every 10-15 seconds. The bladder is a hollow muscular organ. The bladder stretches until it is full with urine. Healthy adults can hold up to 16 oz(2 cups) of urine in their bladders for 2-5 hours. Sphincter muscles prevent the urine from leaking out of the bladder. When the bladder is full. nerves send a message to the brain, which then causes the individual to feel the urge to urinate. During urination. the brain signals the muscles in the bladder to contract and the sphincter muscles to relax. This causes urine to empty out of the bladder.