Gills

• Ventilation


  increasing flow of water over the surface
• Concurrent exchange


  water moves past the lamellae in the same direction that the blood is going inside the lamellae

  
• Why concurrent exchange is inefficient

  • water entering the gills has the highest level of oxygen while blood reaching the gills has the lowest
  • Diffusion will occur from high concentration in the water to low concentration in the lamellae until equilibrium is reached
  • As a result the blood has only gotten 50% of the available oxygen in the water
• Countercurrent exchange


  water flows past in one direction while the blood flows in the opposite direction

  
• Why countercurrent exchange is efficient

  • deoxygenated blood from the fish's body approaches the gills
  • it encounters water that has a lower oxygen concentration
  • diffusion occurs, due to the concentration difference
  • Blood continues to flow through the lamellae encountering water that has more and more oxygen in it, so it keeps picking up more and more oxygen
  • As a result 95% of the available oxygen is picked up