Aquatic respiration

Aquatic respiration is the process whereby an aquatic animal obtains oxygen from water.

Earth's natural bodies of water have a low oxygen concentration--much lower than the level of oxygen in air at the earth's surface. Smaller organisms can obtain sufficient oxygen through the skin (e.g. flatworms), but larger organisms require special structures to collect enough oxygen to sustain life.

Fish have developed gills for respiration which have:
 * large surface area
 * high blood flow
 * small/short diffusion distances
 * contain four gill arches (bony fishes)or two gill arches (cartilaginous fish) on each side of the fish's head
 * each gill arch has 2 rows (hemibranchs) of gill filaments
 * each gill filament has many lamellae

The operculum in fish is a long bony cover for the gill that can be used for pushing water. Some fishes pump water using the operculum. Without an operculum, other methods are required, such as ventilation. Some species of sharks use this system. When they swim, water flows into the mouth and across the gills. Because these sharks rely on this technique, they must keep swimming in order to respire.

Bony fish use a type of countercurrent flow to maximize the intake of oxygen that diffuse through the gill. Countercurrent flow is when deoxygenated blood moves through gill in one direction while oxygenated water moves in the gill in the opposite direction. This mechanism maintains the concentration gradient and increasing the efficiency of the respiration process as well. Cartilaginous fish do not have a countercurrent flow system as they lack bones which are needed to have the opened out gill that a bony fishes have.

This oxygen comes from molecules of oxygen gas (O2) dissolved in the water. The oxygen atom present in the water molecule (H2O) is not suitable for respiration.