The bladder is a muscle, shaped like a balloon, which expands as it fills up. Urine is made in the kidneys and passes down tubes called ureters, to be stored in the bladder.
There are two muscles that ensure your bladder stores and releases urine appropriately and prevent any leaks: the pelvic floor muscles, which provide support and hold the bladder in place, and the sphincter, which seals a tube called the urethra until you are ready to pass urine.
In between visits to the toilet, the bladder relaxes and fills up. When you go to the toilet, the pelvic floor muscles relax, the bladder squeezes, the sphincter opens and urine comes out down the urethra. Your brain controls your bladder, automatically telling it when to hold on and when to empty.
A normal bladder can hold up to a pint (between 400 and 600ml) of urine, and empties completely four to seven times a day.