Gas Exchange

• Inhalation

  • the diaphragm muscles contract and flattens
  • External intercostal muscles contract and the ribcage is pulled up and out
  • This causes the volume to increase and pressure to decrease in the thorax to below atmospheric pressure
• Exhalation

  • the diaphragm muscles relax and moves up
  • External intercostal muscles relax and the ribcage moves down and in
  • This causes the volume to decrease and pressure to increase in the thorax to above atmospheric pressure
• Why can't fish use their bodies as an exchange surface?

  Fish have a waterproof, impermeable outer membrane and a small surface area:volume ratio
• Features of a fish's gas transport system

  • Gills
  • Lamellae
• Ways in which the structure of fish gills is adapted for efficient gas exchange

  • Gills have many flat filaments with lamellae folds to increase the surface area
  • Gills have a thin surface so short diffusion pathway
  • Gills have a good blood circulation which maintains the concentration gradient
• process of gas exchange in fish

  1. fish opens its mouth to enable water to flow in
  2. then closes its mouth to increase pressure
  3. the water passes over the lamellae, O2 diffuses into the bloodstream
  4. waste CO2 diffuses into the water and flows back out of the gills
• Describe the lamellae

  • At right angles to the gil filaments: this increases the surface area
  • Blood and water flow across them in opposite directions - this is called the countercurrent exchange system
• Describe the gills

  • supported by arches along which are multiple projections of gil filaments which are stacked up in piles
  • Gills give fish a higher metabolic rate so they can use more oxygen
• Define counter-current flow

  When two materials flow in opposite directions to maintain a concentration gradient
• How does the countercurrent exchange system maximise oxygen absorbed by the fish?

  • Water and blood flow in opposite directions
  • To maintain a concentration gradient of oxygen
  • This allows for diffusion along the length of the lamellae
• Why can't insects use simple diffusion

  Insects have high metabolisms since flying requires large amounts of energy
• Features of the insect's gas transport system

  • Spiracles
  • Tracheae
  • Tracheoles
• Explain the process of gas exchange in insects

  • gases move in and out of the tracheae through the spiracles
  • a diffusion gradient allows oxygen to diffuse into the body tissue while waste CO2 diffuses out
  • contraction of muscles in the tracheae allows mass movement of air and out
• How is water loss limited in an insect's gas exchange system?

  • spiracles which can be open and close using valves
  • hairs around the spiracles
  • exoskeleton is impermeable
• Insect's tracheal system is adapted for rapid gas exchange

  Tracheae provide high oxygen concentration so high concentration gradient and fast diffusion into tissues
• Tracheoles have thin walls


  this increase the efficiency of gas exchange because it means there is a short diffusion distance to cells
• Insect's tracheal system is adapted for rapid gas exchange

  Insects can use abdominal pumping through tracheae meaning a steep diffusion gradient is maintained for oxygen and carbon dioxide
• Insect's tracheal system is adapted for rapid gas exchange

  Insects have highly branched tracheoles so there is a short diffusion distance to cells and a large surface area for gas exchange
• Insect's tracheal system is adapted for rapid gas exchange

  Fluid in tracheoles moves out during exercise meaning the final diffusion pathway is gas rather than liquid so faster diffusion to the exchange surface and a larger surface area
• Tracheoles are lined with a single layer of cells so short diffusion distance to cells