Gas exchange in Fish

• Concurrent flow - Water and blood flow over and through the lamellae in the same direction
• Countercurrent flow - Water and blood flow over and through the lamellae in opposite directions to each other

  
• Countercurrent flow: (water and blood flow in opposite directions)

  1. When water flows over the gills in the opposite direction to the flow of blood in capillaries
  2. Ensures equilibrium is not reached
  3. This ensures that a diffusion gradient is maintained across the entire length of the gill lamellae