The bladder is a hollow organ in the lower abdomen. It stores urine, the waste that is produced when the kidneys filter the blood. The bladder has a muscular wall that allows it to get larger and smaller as urine is stored or emptied.  Urine passes from the two kidneys into the bladder through two tubes called ureters. Urine leaves the bladder through another tube, the urethra.  In women the urethra is very short; in men it is longer since it passes through the prostate gland and the penis.

The wall of the bladder has several layers. Cancer begins in the lining layer and grows into the bladder wall. Doctors need to know how deeply the cancer has spread into these layers in order to decide on the best treatment.