It contains air. By adjusting the contraction and expansion of swim bladder muscles, fish can adjust their body density and rise or sink in the water.

Most bony fish have swim bladder, which is a slender sac located at the back of the body cavity. The swim bladder is usually divided into two parts, which contain oxygen, nitrogen and carbon dioxide respectively. The swim bladder has a swim bladder tube communicating with the esophagus, called the throat swim bladder, which belongs to lower bony fish, such as carp. The swim bladder without swim bladder tube, called closed swim bladder, belongs to the swim bladder of higher bony fish, such as perch.