Fish and Insect Gas Exchange

• Tracheal System of an Insect

  • All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases
  • Insects have evolved a breathing system that delivers oxygen directly to all the organs and tissues of their bodies
• Spiracle


  An opening in the exoskeleton of an insect which has valves
• Insect Respiration


  1. Air enters through spiracles

  2. Flows into the system of tracheae

  3. Tracheae lead to tracheoles (narrower tubes)

  4. Tracheoles run between cells and into muscle fibres - the site of gas exchange
• Tracheae


  • Tubes within the insect respiratory system
  • Rigid rings of chitin keep the tracheae open
• For smaller insects, the tracheal system provides sufficient oxygen via diffusion
• Ventilation Mechanism in Insects

  1. Closing the spiracles

  2. Using abdominal muscles to create a pumping movement for ventilation

  3. During flight, fluid at the narrow ends of the tracheoles is drawn into the respiring muscle so gas diffuses across quicker
• Gills of Fish


  • Oxygen dissolves less readily in water
  • A given volume of air contains 30 times more oxygen than the same volume of water
• Structure of Fish Gills

  • Series of gills on each side of the head
  • Each gill arch is attached to two stacks of filaments
  • On the surface of each filament, there are rows of lamellae
  • The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries
• Mechanism of Gas Exchange in Fish Gills

  1. The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system

  2. The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary

  3. The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood
• Ventilation Mechanism in Fish

  1. The fish opens its mouth, lowering the floor of the buccal cavity which causes the volume inside to increase and pressure to decrease

  2. The higher pressure outside causes water to flow into the buccal cavity

  3. The fish then raises the floor of the buccal cavity, increasing the pressure and forcing water over the gills

  4. The operculum (flap of tissue covering the gills) is forced open by the increased pressure, allowing water to exit