gas exchange in fish

• Gas exchange surface in fish

  • Large surface area to volume ratio
  • Short diffusion distance
  • Mechanism to maintain concentration gradients
• Fish are waterproof because of their scales and they have a relatively small surface area to volume ratio, so they can't simply diffuse oxygen across their surface
• Gills


  The gas exchange surface in fish
• Water contains 30 times less oxygen compared to air, so fish have adaptations to help maintain concentration gradients and overcome this challenge
• Diffusion rate calculation


  Diffusion is proportional to surface area x concentration difference / diffusion path length
• Structure of fish gills

  • Four layers of gills on both sides of the head
  • Gill filaments aligned in stacks
  • Gill lamellae positioned at right angles to gill filaments
• Gill filaments


  The long parts sticking out from the gills
• Gill lamellae


  The very thin structures that run horizontally across the gill filaments
• Water flows over the gills

  Enters through the mouth and exits through a gap in the side of the head
• Adaptations of fish gills

  • Large surface area to volume ratio
  • Short diffusion distance
  • Mechanism to maintain concentration gradients
• Capillary network


  Inside each gill lamellae, provides a short diffusion distance
• Counter-current flow


  Water flows in the opposite direction to the blood flow in the capillaries
• Counter-current flow


  Ensures equilibrium is not reached, maintaining a diffusion gradient across the entire length of the lamellae
• In an exam question, key points about the counter-current exchange principle would be: 1) It ensures equilibrium is not reached, 2) The water flows in the opposite direction to the blood in the capillaries