The gas bladder is a gas-filled sac located in the portion of the fish. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. In gas bladders, a connection is retained between the gas bladder and the gut, allowing the fish to fill up the gas bladder by "gulping" air and filling the gas bladder through the pneumatic duct. In more derived varieties of fish, the bladder has a gas gland that can introduce gases to the bladder to increase its volume and thus increase buoyancy. To reduce buoyancy, gases are released from the bladder into the blood stream and then expelled into the water via the gills.
In order to introduce gas into the bladder, the gas gland excretes lactic acid; the resulting acidity causes the hemoglobin of the blood to lose its oxygen, which then into the bladder while flowing through a complex structure known as the ''rete mirabile''. Elsewhere, at a similar structure known as the ''oval window'', the bladder is in contact with blood and the oxygen can diffuse back.
Physoclist gas bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. Physostomes can "burp" out gas, though this complicates the process of re-submergence.
Gas bladders are only found in ray-finned fish, but a few of these fish that do not need to change water depth have lost them. Many fish, including sharks, can control their depth only by swimming ; others store fats or oils for the purpose.
In some fish, mainly freshwater species, the gas bladder is connected to the of the inner ear by the Weberian apparatus, which provides a precise sense of water pressure , and may also improve hearing.
The combination of gases in the bladder varies; in shallow water fish, the ratios closely approximate that of the , while deep sea fish tend to have higher percentages of oxygen. For instance, the eel ''Synaphobranchus'' has been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and 0.4% argon in its gas bladder.
Gas bladders are evolutionarily closely related to lungs. It is believed that the first lungs, simple sacs that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and into the gas bladders of today's fish. In embryonal development, both lung and gas bladder originate as an outpocketing from the gut; in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more "primitive" teleosts, and is lost in the more derived orders. There are no animals which have both lungs and a gas bladder.
In some Asian cultures, fish maw of certain large sea fishes is considered a food delicacy . It is usually served braised or in stews. Fish maws are also used in the food industry as a source of collagen. Fish maw can also be made into a strong, water-resistant glue.
Gas bladders in other animals
The Portuguese Man o' War has a special gas bladder that allows its top to float along the surface while its tentacles trail below the water. This organ is unrelated to the one in fish.