Gas Exchange System in Bony Fish

Introduction

• Focus on gas exchange in bony fish, which have a skeleton made of bone.
• Examples include tuna, salmon, cod, and trout.
• Bony fish face challenges in gas exchange due to:
  • High oxygen requirement due to size and activity.
  • Low surface area to volume ratio.
  • Scaly surface not allowing gas passage.
• Lower oxygen concentration in water compared to air.
• Evolution of specialized systems to maximize oxygen extraction.

Structure of the Gas Exchange System

• Operculum: Flap of tissue behind the head, covering the gills.
• Opercular Cavity: Space behind the operculum containing the gills.
• Gills:
  • Oxygen-rich water enters through the mouth.
  • Water passes over gills where gas exchange occurs:
    • Oxygen diffuses into blood.
    • Carbon dioxide diffuses into water.
  • Water exits through the opercular opening.

Detailed Structure of Gills

• Gill Arches: Bony structures from which gill filaments extend.
• Gill Filaments: Numerous filaments extend from each gill arch.
• Gill Lamellae (Gill Plates):
  • Cover the gill filaments and site of gas exchange.
  • Water flows between lamellae allowing gas diffusion.
  • Adaptations:
    • Large surface area for diffusion.
    • Short diffusion distance.
    • Extensive capillary network to transport oxygen.

Countercurrent Exchange System

• Blood flows opposite to water flow in gill lamellae, creating a countercurrent system.
• Advantages:
  • Maintains a steep concentration gradient for oxygen across the lamellae.
  • Prevents equilibrium, allowing continuous diffusion.
  • Enables up to 80% oxygen extraction.
• Comparison with Parallel Flow:
  • Parallel flow results in equilibrium and only 50% oxygen extraction.

Water Flow Mechanism in Bony Fish

• Unlike sharks, bony fish can maintain water flow without swimming.
• Mechanism:
  • Mouth Opening: Water enters the buccal cavity (mouth space).
  • Buccal and Opercular Cavity Dynamics:
    • Buccal cavity enlarges, increasing volume.
    • Operculum is shut, and opercular cavity volume increases, reducing pressure.
    • Buccal cavity floor lifts, increasing pressure and driving water over gills.
  • Mouth Closing and Operculum Opening:
    • Sides of opercular cavity squeeze, increasing pressure.
    • Water is forced out through the operculum.

Conclusion

• Understanding of the gas exchange system in bony fish covers the operculum and gill structure, the efficiency-enhancing countercurrent system, and the specialized water flow mechanism.