Anatomy of the Bladder
The empty bladder is about the size and shape of a pear. It is located in the lower pelvic cavity. Urine drains from the kidneys into the bladder through the ureters. From the bladder, urine is excreted through a tube called the urethra. In women, this tube is about 1.5 inches long and exits the body at the upper aspect of the vaginal opening. In men the urethra is about 8 inches long and passes through the penis, opening at its tip.
The bladder has flexible muscular walls three layers thick. As urine fills the bladder, these walls expand; they contract to expel urine. Although the bladder can hold about a pint of urine, the urge to urinate usually starts when it is about half full.