Gas Exchange in Fish - A-Level Biology AQA

Introduction

• Large, active, bony fish have high oxygen requirements.
• Simple diffusion is insufficient due to scaly skin preventing gas exchange.
• Bony fish have evolved specialized respiratory systems to extract oxygen from water.

Challenges of Aquatic Respiration

1. Water is denser and more viscous than air, leading to slower oxygen diffusion.
2. Water contains less oxygen compared to air.
3. High oxygen demands due to the active nature of bony fish.

Role of Gills

• Gills enable efficient oxygen uptake from water and carbon dioxide removal.

Structure of the Gills

1. Operculum Flap: Covers the gills.
2. Gill Filaments: Stacked structures containing gill lamellae.
3. Gill Lamellae: Surrounded by extensive blood vessels.

Adaptations for Efficient Gas Exchange

1. Large Surface Area: Provided by the lamellae.
2. Thin Membranes: Minimize diffusion distance.
3. Rich Blood Supply: Maintains steep diffusion gradients.
4. Countercurrent Flow: Enhances concentration gradients.
5. Overlapping Filament Tips: Increase resistance, slowing water flow for better gas exchange.

Countercurrent Flow System

• Blood and water flow in opposite directions across the lamellae.
• Efficiency Mechanism:
  1. Oxygen-rich blood meets water with high oxygen content, maximizing oxygen diffusion.
  2. Oxygen-poor blood meets oxygen-reduced water, allowing continued oxygen diffusion.
  3. Maintains a steep concentration gradient across the gill.

Comparison with Parallel Flow

• Countercurrent exchange is more efficient than parallel flow.
• Parallel flow reduces the concentration gradient, limiting oxygen absorption.

Conclusion

• Bony fish utilize countercurrent exchange for efficient gas exchange, meeting their high oxygen needs by maintaining steep diffusion gradients through specialized gill structures.