Aquatic Respiration - Gills

Gills

Large aquatic animals have developed gills for respiration which are specifically adapted to their function, for example, they have:

• A large surface area to allow as much oxygen to enter the gills as possible due to the fact that more of the gas comes into contact with the membrane
• Good blood supply to maintain the concentration gradient needed
• Thin membrane to allow for a short diffusion pathway
• each gill arch has two rows (hemibranchs) of gill filaments
• each gill filament has many lamellae

In osteichthyes, the gills contain 4 gill arches on each side of the head, two on each side for chondrichthyes or 7 gill baskets on each side of the fish's head in Lampreys In fish, the long bony cover for the gill (the operculum) can be used for pushing water. Some fish pump water using the operculum. Without an operculum, other methods, such as ventilation, are required. 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 countercurrent flow to maximize the intake of oxygen that can diffuse through the gill. Countercurrent flow occurs when deoxygenated blood moves through the gill in one direction while oxygenated water moves through the gill in the opposite direction. This mechanism maintains the concentration gradient thus increasing the efficiency of the respiration process as well and prevents the oxygen levels from reaching an equilibrium. Cartilaginous fish do not have a countercurrent flow system as they lack bones which are needed to have the opened out gill that bony fish have.