The urinary bladder is a hollow, muscular organ located in the pelvic cavity. It has several layers including an outer serous membrane, adventitia connective tissue layer, muscularis layer of detrusor smooth muscle, submucosa layer, and inner transitional epithelial lining. As the bladder fills with urine, the lining folds flatten to allow increased storage capacity. The muscularis layer allows the bladder to expand and contract and forms the internal urethral sphincter to hold urine inside.