Fish face a harder gas exchange problem than land animals, because water is a poor source of oxygen:• water holds far less dissolved oxygen than the same volume of air (roughly 1/30 as much at ordinary temperatures), and less still when the water is warm
• oxygen diffuses about 10 000 times more slowly in water than in air
• water is dense and viscous, so moving it over a surface takes a lot of energyFish are also relatively large, active animals with a small surface area to volume ratio. Their body surface is covered by scales and is not permeable enough for gas exchange, and diffusion across it could not supply their inner cells anyway.Bony fish therefore use a specialised gas exchange surface, the gills, found in chambers on each side of the head behind the mouth and covered by a bony flap, the operculum. The gills have a very large surface area, a thin barrier between water and blood and a rich blood supply. Their structure is covered in the next lesson.Two further features make fish gas exchange efficient:• water flows over the gills in one direction only, entering through the mouth and leaving through the opercular openings. A one-way flow wastes less energy than moving dense water in and out of the same opening, as lungs do with air.
• blood in the gills flows in the opposite direction to the water (the countercurrent principle), which lets fish extract a high proportion of the oxygen from the water.
Gills only work in water: in air the thin gill surfaces collapse and stick together, and the fish suffocates.
• oxygen diffuses about 10 000 times more slowly in water than in air
• water is dense and viscous, so moving it over a surface takes a lot of energyFish are also relatively large, active animals with a small surface area to volume ratio. Their body surface is covered by scales and is not permeable enough for gas exchange, and diffusion across it could not supply their inner cells anyway.Bony fish therefore use a specialised gas exchange surface, the gills, found in chambers on each side of the head behind the mouth and covered by a bony flap, the operculum. The gills have a very large surface area, a thin barrier between water and blood and a rich blood supply. Their structure is covered in the next lesson.Two further features make fish gas exchange efficient:• water flows over the gills in one direction only, entering through the mouth and leaving through the opercular openings. A one-way flow wastes less energy than moving dense water in and out of the same opening, as lungs do with air.
• blood in the gills flows in the opposite direction to the water (the countercurrent principle), which lets fish extract a high proportion of the oxygen from the water.
Gills only work in water: in air the thin gill surfaces collapse and stick together, and the fish suffocates.