Urine, made in your kidneys, is transported to your bladder via two narrow tubes known as ureters. As your bladder fills up with urine it stretches. An adult bladder can usually hold about a pint of fluid comfortably. It can hold more, but as it gets fuller it can be painful.
When your bladder stretches beyond a certain point, nerves in the bladder wall send a message to your brain telling it that your bladder is getting full and needs to be emptied.
Taking a leak
Urine leaves the body by flowing out of the bladder down a tube called the urethra. The junction between the bladder and urethra is opened and closed by a muscle known as a sphincter. When you decide you need to urinate your brain tells this sphincter to relax, opening the bladder-urethra junction. At this moment, the bladder contracts, forcing the urine down the urethra and out of the body.