Ventilation and gas exchange - Fish

• Gills - Organs of gaseous exchange:

  • Large surface area
  • Good blood supply
  • Thin layers
• Operculum (Bony flap):

  • Cover gill cavity
  • Maintains a flow of water over gills
• Gill filaments:

  • In large stacks - gill plates
  • Need a flow of water to be kept apart - expose large surface area for gaseous exchange
• Gill lamellae - Main site of gaseous exchange in fish:

  • Rich blood supply
  • Large surface area
• Operculum - Flap that covers gills
• Structure of the gaseous exchange system in fish

  A) operculum

  B) gill lamellae

  C) gill filaments

  
• When the mouth is open and buccal cavity is lowered:

  1. Volume of buccal cavity increased - Pressure drops - water moves in
  2. Opercular valve is shut and opercular cavity expands
  3. Pressure in opercular cavity lowered
  4. Buccal cavity moves up - Pressure increased
  5. Water moves from buccal cavity into gills
• When mouth closes:

  1. Operculum opens - Sides of opercular cavity move inwards
  2. Pressure of the opercular cavity increases
  3. Water forced over the gills and out of the operculum
  4. Floor of buccal cavity is moved up - Flow of water maintained over the gills
• The counter current system:

  • Blood and water flows in opposite directions - An oxygen concentration gradient between water and blood maintained along the gill
  • Oxygen continues to diffuse down the concentration gradient - Higher level of oxygen saturation of blood is reached