general gas exchange

• Metabolic demand - how much ATP/aerobic respiration an organism needs
• The metabolic demand increases if the organism:

  • is endotherm - warm blooded - because aerobic respiration produces heat as a waste product
  • is larger/ has a greater volume because there are more cells
  • has increased energy levels
• Surface area - the number of cells on the outside of the organism in contact with the environment. The area over which diffusion of gases could occur
• Volume - the number of cells in the internal area of the organism. It is an indication of metabolic demand of an organism
• If the surface area of an organism is going to be used as the gas exchange surface the SA:V ratio needs to be large. If not an adaption is required:

  • If the volume is too large (more than 2-3 cells thick) simple diffusion through the surface area is not sufficient

  Either gain a circulatory system (Earthworm) or gain a specialised gas exchange organ (insects). Humans have both!
• Animals, medium of gas exchange and gas exchange organ:

  Mammals - air - lungs

  Birds - air - lungs (most complex system)

  Reptiles - air - lungs

  Amphibians - air - lungs and moist skin Or - water - external gills and moist skin

  Fish - water - internal gills
• Muscles contract and relax

  Elastic fibres stretch and recoil
• When a muscle contracts it gets thicker. This makes the lumen smaller which restricts the amount of air going into the lungs. The only time this is a good thing is when the air is polluted.
• Common features of gas exchange surfaces:

  • large surface area relative to volume
  • Permeable
  • Thin - short diffusion pathway
  • Moist - gases can dissolve and diffuse
  • Mechanism to maintain diffusion/concentration gradients
• Adaptions to gas exchange on land:

  Terrestrial animals have internal gas exchange surfaces e.g. alveoli and tracheoles. The advantages of this are:

  • It reduces heat and water loss
  • they can be ventilated to help maintain a concentration gradient
• As soon as an organism has an anatomy which is more than 2 cells thick, diffusion is too slow therefore they will require a transport system.

  This is either single circulatory systems or double circulatory systems.
• A medical use of artificial surfactant is in premature babies because it lowers the surface tension of the fluid in the alveoli and prevents the alveoli from collapsing
• Amphibians - terrestrial and aquatic

  Gas exchange surface: larvae (tadpole) live in water and have gills. The adult uses moist skin as a respiratory surface and use lungs when active

  Features of surface: Undergo metamorphosis.
• Reptiles - terrestrial

  Gas exchange surface: lungs and an in-out bellows-like arrangement. They have in-growth of tissue to increase surface area

  Features of surface: Movement of ribs aid ventilation of the lungs
• Birds - terrestrial

  Gas exchange surface: lungs and air sacs

  Features of surface: attached to the lungs are air sacs that act as bellows. When a bird breathes in any air that remains in the lungs from the last breath gets sucked into the air sacs meaning that the lungs are always filled with fresh air.