The swim bladder is a hydrostatic buoyancy organ unique to fish. It is capable of self-regulating so that the fish can stay at any level in the water.

Bladder, one of the organs of fish, scleractinian fishes, most of which have a swim bladder. The swim bladder occupies about 5 percent of the body's volume. Its shape is ovoid, conical, heart-shaped, horseshoe-shaped, and so on. The swim bladder is filled with gases mainly oxygen, nitrogen and carbon dioxide, with oxygen being the most abundant. Therefore, in an oxygen-deficient environment, the swim bladder can be used as an auxiliary respiratory organ to provide oxygen for the fish.

Containing air inside, the contraction and expansion of the swim bladder regulated by the swim bladder muscle can enable the fish to regulate the density of its body and rise or sink in the water (Explanation from the Modern Chinese Dictionary.

Most bony fishes have a swim bladder, a long thin sac located at the back of the body cavity. The swim bladder is generally divided into two chambers and contains oxygen, nitrogen and carbon dioxide. The swim bladder is connected to the esophagus by a swim bladder tube called the laryngeal swim bladder, which belongs to the swim bladder of lower bony fishes, such as the carp. A swim bladder without a tube is called a closed swim bladder and is found in higher bony fish such as perch.

Functionally, except for a few fish (such as lungfish, finfish) swim bladder has a respiratory function, for most fish, the swim bladder is a body specific gravity regulator, through the swim bladder gas changes to help regulate the body's floating and sinking.