Gas Exchange in Fish

Structure of Gills

• Gills are found behind the head of the fish and are composed of gill filaments
• At right angles to the gill filaments are gill lamllae which increase the surface area for gas exchange
• gill plates line the gill lamellae sides and are the surface area for gas exchange
• Capilliaries carry deoxygenated blood to the gill plates and pumps it through the plates in the opposite direction to the flow of water
  • this countercurrent flow ensures the concentration of oxygen is always greatest in water and smallest in the gill plates
  • this constant concentration gradient allows for the maximum amount of oxygen to be absorbed from the water
  • no diffusion equilibrium is reached so the maximum volume of oxygen is absorbed
  • visa versa for carbon dioxide
• Fish can't use their body surface for gas exchange as they are waterproof and have a small SA:V