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The bladder is a flexible sack formed from layers of muscles. As the bladder fills up, the walls expand. Although the bladder can hold about a pint of urine, we normally feel a need to go to the bathroom when it is about half full.
To empty the bladder, the muscular walls contract. Because of the bladder's many layers, it can contract even while holding the neck of the bladder open to empty.
When it is empty, the bladder is about the size and shape of a pear. It is located in the lower part of the belly. It is attached in several places to the walls of the abdomen by ligaments. Its precise location is affected by whether it is full or empty and pressure from the surrounding organs.
The bladder receives urine from the kidneys through the ureters. It passes the urine out of the body through the urethra.
The urethra is a tube of muscle that contracts to keep the bladder closed until the body is ready to release urine. In women, the urethra is about an inch and a half long. It opens outside the body just above the opening to the vagina. In men, the urethra is about eight inches long. It passes through the penis to an opening at its tip. The prostate gland is immediately below the bladder and completely surrounds the urethra.