Fish

• Gills are protected by the operculum
• Fish have specialised exchange surfaces called gills
• There is a continuous flow of water over the gills
• Water is taken in through the mouth and passed over the gills
• Gills are made of filaments which increase surface area for gas exchange
• Lamella sprout from the filaments to increase surface area
• Thin diffusion pathway and good blood supply help maintain concentration gradient
• Parallel flow

  • Water and blood flow in same direction
  • diffusion gradient only at first part so concentrations equilibriate at 50%
• Countercurrent flow

  • water and blood flow in opposite directions
  • at any point water has higher concentration of oxygen
  • maintains concentration gradient of oxygen along gill