The bladder lining is comprised of specialised cells called urothelial cells, and the large muscle within the bladder wall is called the detrusor muscle. The urothelial cells are very elastic enabling the bladder lining to stretch, as well as waterproof in order to keep the urine within the bladder. The detrusor muscle relaxes to allow urine to fill the bladder and contracts to allow urination (passing of urine). When the detrusor muscle contracts, the urine flows down a tube that connects the bladder to the outside of the body, called the urethra.

There are also two rings of muscle that lie along the urethra, called the internal and external urethral sphincter. The internal urethral sphincter is not under conscious control, but the external urethral sphincter allows voluntary control of urination.