Swim bladder fish is an outgrowth of the esophagus.
The swim bladder helps the fish to be at a certain depth, where the weight of the displaced fish water equal to the weight of the fish. Thanks to the swimming bladder of the fish does not spend additional energy to maintain the body at this depth.
Fish deprived of the possibility to arbitrarily inflate or shrink swim bladder. If the fish is immersed, increasing the pressure of the water on her body, squeezed it, and swim bladder is compressed. The lower falls fish, the greater the water pressure, the more compressed the body of the fish and the more rapidly continues its decline. And when the fish rises to the upper layers, the water pressure on it decreases, the expansion of the bladder. The closer to the surface of the water is the fish, the more time expanding the gas in the swimming bladder, which reduces the weight of the fish. This further pushes the fish to the surface.
So, the fish can not adjust the volume of the bladder. But in the walls of the bladder there are nerve endings that send signals to the brain when it is compressing and expanding. The brain on the basis of this information, sends commands to the Executive bodies - muscles, through which the fish carries out motion.
Thus, the swim bladder of the fish is its hydrostatic apparatus, providing its balance: it helps the fish to remain at a certain depth.
Some fish with swim bladder can produce sounds. Some fish he serves as a resonator and transducer sound waves.
By the way...
Swim bladder appears during embryonic development of fish as an outgrowth of the intestinal tube. Further, the channel that connects the swim bladder to the esophagus, may remain or overgrown. Depending on whether the fish this channel, all fish are divided into atkritumu and secretuser. Otkrytiye fish can swallow air and thus to control the volume of the bladder. To atkritumu include carp, herring, sturgeon. Secretuser fish gases are extracted and absorbed through a dense plexus of blood capillaries on the inner wall of the bladder - red body.See also:
Do fish have a swim bladder?
Cm. also the head of "Entertaining physics" J. Perelman Assignment"fish bladder"