Fishes belongs to Class Pisces. And when it comes to fishes they have to float under water to survive, and function of air bladder is to provide buoyancy to fish so that it can float.
Air bladder is filled with gas and inflated to increase buoyancy in fish and when it is deflated body become less buoyant and fish loss its physical balance. Gas is directly gulped through mouth in fishes whose oesophagus is directly connected to air bladder or else supply by the blood vessels leading to air bladder.
