Fish/How fish function

How fish function
Specialized functions in fish help them to survivein water. Among these are gills, swim bladders, and the "lateral line system".

Breathing
Fish breath by drawing air in through the mouth and passing it over the gills. Oxygen in the water is absorbed by filaments in the gills and passed on to the blood. At the same time, carbon dioxide and other wastes are being expelled. Some species, such as the anabantid family have developed extra breathing organs for collecting oxygen in stagnating waters. For instance, members of the anabantid family have developed a labyrinthine auxiliary organ near the gills that holds atmospheric air gulped from the surface and extracts oxygen from it. Some catfishes also gulp air and extract oxygen in a capillary-rich offshoot of the gut.

Swim bladder
Most fish have a gas filled bladder that acts as a buoyancy compensation device, enabling the fish to maintain position anywhere in the water. The bladder automatically inflates or deflates to compensate for pressure, equalizing the weight of the fish with that of the surrounding water.

Lateral line system
A fish's nervous system is linked to the outside world by a series of perforations in a single row of scales known as the lateral line. The line runs horizontally along the length of the fish. Vibrations from the fish's own movements are reflected back from obstacles and detected by nerve endings deep inside the lateral line, similar to ecolocation used by bats.

Osmosis
A fish's skin acts as a semipermeable membrane, or a one-way transfer system for water. Osmosis causes fluid to diffuse through this membrane until there is an equal concentration on both sides of the membrane. In freshwater fish, the fluid is more concentrated than the liquid in which it lives. Thus, water constantly passes into the fish. To avoid bursting, freshwater fishes excrete as much water as possible and drink little. Conversely, marine fishes lose water to the more concentrated sea water outside, and must drink constantly and excrete little. Few fish can pass from one type of water to the other without at least mild distress.