A healthy adult bladder can hold up to 16 ounces (2 cups) of urine comfortably, according to the National Institutes of Health. How frequently it fills depends on how much excess water your body is trying to get rid of.
Circular muscles called sphincters close tightly to keep urine from leaking. The involuntary leakage of urine is called incontinence.
Nerves in the bladder tell you when it is time to empty it. The sensation becomes stronger as the bladder continues to fill and reaches its limit.
When you urinate, the brain signals the sphincter muscles to relax. At the same time, the brain signals the bladder muscles to squeeze, thus allowing urine to flow through the urethra. When these signals occur in the correct order, normal urination occurs.
Problems in the urinary system can be caused by aging, illness, or injury. The muscles in and around your bladder tend to weaken with age. Weak bladder muscles may result in your not being able to empty your bladder completely, leaving you at a higher risk for urinary tract infections, according to the NIH.