Gas exchange

Oxygen intake for respiration, also carbon dioxide and, generally, ammonia elimination takes place across gas-exchange surfaces, usually the gills in fish. Water flows across gills, separated by the pharyngeal gill clefts, and supported by gill arches, and which possess highly folded surfaces covered by a very thin epithelium. Blood flow and water flow are separated only by the epithelium with a ‘countercurrent’ gas exchange between the two. A respiratory centre in the hind-brain is a respiratory rhythm pacemaker for the oral and pharyngeal ventilation movements creating water flow across the gills, although ‘ram ventilation’ occurs without such movements. The oxygen and carbon dioxide-carrying capacity of blood is increased considerably by temporary attachment to haemoglobin pigment in the erythrocytes. Some fish are air breathing, using lungs, swim bladder, skin or lips for gaseous exchange. Hypoxia, hypercapnia, supersaturation and high water temperatures present problems for fish respiration, which are discussed.