The bladder is a hollow, muscular, balloon-like organ that collects and stores urine. It is in the lower part of the abdomen (the pelvis).

The inside of the bladder is covered with a urine-proof lining (called the urothelium) which stops urine being absorbed back into the body. The cells of this lining are called transitional cells or urothelial cells.

Our kidneys (most people have two) produce urine, which is carried to the bladder by thin tubes called ureters. The bladder stores the urine which is made up of water and waste products that the body does not need. When the bladder is full enough, nerve signals are sent to the brain. To get rid of the urine, the muscle of the bladder contracts, forcing the urine out of the body through another fine tube called the urethra.

In women the urethra is a short tube which lies in front of the vagina. In men the urethra is longer as it passes through the prostate gland and to the tip of the penis.