How do fish move? Swim bladder function.

Abstract

Many fish use a gas-filled swimbladder as a buoyancy organ. The swimbladder originates as an unpaired dorsal outgrowth of the posterior foregut, the pharynx. While in physostomous fish the embryonic connection to the pharynx persists, in physoclistous fish it is lost during early development. In most fish larvae, initial inflation of the swimbladder is achieved by gulping air shortly after hatching, but some species have been reported to inflate the swimbladder without surfacing. Failure to inflate the swimbladder reduces viability. Hydrostatic pressure increases with increasing water depth. Accordingly, the volume of the flexible-walled swimbladder is reduced during descent and increases during ascent. In order to keep the swimbladder volume and thus the buoyancy status constant, fish must secrete gas into the swimbladder during descent and gas must be resorbed from the swimbladder during ascent. Gas secretion into the swimbladder requires the activity of gas gland cells, which acidify the blood and thus induce a decrease in its gas-carrying capacity. As a consequence, gas partial pressures in the blood increase, providing a pressure head for the diffusive transport of gas from the blood into the swimbladder. In a second step, the initial increase in gas partial pressure in the blood is multiplied by back-diffusion and countercurrent concentration of gas molecules in a countercurrent system, the rete mirabile. Thus, very high gas partial pressures can be achieved in swimbladder blood vessels, high enough to explain the presence of gas-filled swimbladders at a water depth of several thousand meters.