swim bladder, large, thin-walled sac in some fishes that may function in several ways, e.g., as a buoyant float, a sound producer and receptor, and a respiratory organ. The swim bladder, or air bladder, is located in the dorsal portion of the body cavity and is filled with gases. When gas is added to the swim bladder, by diffusion through the blood vessels in the bladder walls, the fish becomes less dense overall; when gas is removed the fish becomes more dense. The addition and removal of gases is a mechanism by which the density of the fish can be made equal to that of the surrounding water at a given depth. The swim bladder produces sound by vibrating; these sounds are probably used in courtship. The organ also amplifies water-borne sounds and thus is an aid to hearing. In most fish the swim bladder has no connection to the digestive tract, but in some, such as the lungfish, there is a connecting tube leading to the pharynx, indicating that the organ may aid in respiration.

gall bladder, small pear-shaped sac that stores and concentrates bile. It is connected to the liver (which produces the bile) by the hepatic duct. When food containing fat reaches the small intestine, the hormone cholecystokinin is produced by cells in the intestinal wall and carried to the gall bladder via the bloodstream. The hormone causes the gall bladder to contract, forcing bile into the common bile duct. A valve, which opens only when food is present in the intestine, allows bile to flow from the common bile duct into the duodenum where it functions in the process of fat digestion. Sometimes the substances contained in bile crystallize in the gall bladder, forming gallstones. These small, hard concretions are more common in persons over 40, especially in women and the obese. They can cause inflammation of the gall bladder, a disorder that produces symptoms similar to those of indigestion, especially after a fatty meal is consumed. If a stone becomes lodged in the bile duct, it produces severe pain. Gallstones may pass out of the body spontaneously; however, serious blockage is treated by removing the gall bladder surgically.

bladder, urinary, muscular sac located in the pelvis that stores urine and contracts to expel it from the body. Urine enters the bladder from the kidneys through the ureters and is discharged from the body via the urethra. The bladder of the adult human can hold over a pint (0.6 liters) of urine. When the level of urine reaches about half this amount, pressure of the accumulating fluid stimulates nervous impulses that relax the external sphincter, a muscle that forms a dense band around the urethra at the base of the bladder. This muscle can be controlled voluntarily in most mammals. The muscles in the wall of the bladder also contract, forcing urine out through the urethra. The bladder is subject to infection (commonly called cystitis) and the formation of stones. Its normal function may also be affected by nervous disorders or by external pressure, as from prostatic enlargement or pregnancy. See urinary system.

air bladder, in fish: see swim bladder.

Muscular membranous sac under the liver that stores and concentrates bile. Pear-shaped and expandable, it holds about 1.7 fluid oz (50 ml). Its inner surface absorbs water and inorganic salts from bile, which becomes 5–18 times more concentrated than when it leaves the liver. The gallbladder contracts to discharge bile through the bile duct into the duodenum. Disorders include gallstones and inflammation (cholecystitis). Surgical removal of the gallbladder (cholecystectomy) has no serious side effects.

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Malignant tumour of the bladder. The most significant risk factor associated with bladder cancer is smoking. Exposure to chemicals called arylamines, which are used in the leather, rubber, printing, and textiles industries, is another risk factor. Most bladder cancers are diagnosed after the age of 60; men are affected more than women. Symptoms include blood in the urine, difficulty urinating, excessive urination, or, more rarely, painful urination. Bladder cancer can be treated with surgery, radiation therapy, or chemotherapy.

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