Bladder, in anatomy, organ that stores urine after its formation by the kidneys. It consists of three layers: a lining of mucous membrane, called epithelium; an intermediate layer of involuntary muscle, with fibers arranged in three layers, each running in a different direction; and an outer layer of connective tissue, covered by the peritoneum above and to the back and blending with the connective tissues of the body wall in front and below. Urine enters the bladder from the kidneys through two ureters and is discharged through the urethra. The openings of the ureters lie in the basal part of the bladder, each about 3.8 cm (about 1.5 in) from the opening of the urethra, which is located in the midline of the bladder at its lowest point. Average normal capacity of the adult bladder is about half a liter (about 1 pt). See also Urinary System.