Gas Exchange and Size

• As organisms get bigger, their surface area to volume ratio becomes smaller.
• A small, single celled organism will be able to get required substances to all parts of the cell more easily, and have relatively more surface area to uptake substances - they are more likely to survive than larger ones.
• Increase in size can be accomplished by:

  • an organism becomes multicellular
  • mass flow, (mass transport), mechanisms are developed
  • organs for exchange of substances are developed which are thin walled.
• Fick's Law:

  Rate of diffusion is proportional to (surface area x conc. gradient) over length of diffusion pathway.
• Gas Exchange in Insects:

  • insects have many, highly branched tracheoles for gaseous exchange with the muscle cell.
  • the conc. gradient is maintained by the muscle cells using up O2 in aerobic respiration.
  • Abdominal pumping also assists with the conc. gradient
  • the diffusion pathway between tracheoles and the muscle cells is short as the walls of them are thin and there are many and they're highly branched.
• Gas Exchange in Fish:

  • the fish have many lamellae, this increases the surface area
  • blood and water move in the opposite direction - counter current flow
  • this ensures that the concentration gradient is maintained along the whole length of the lamellae.
  • the blood in the lamellae is separated by a thin layer of cells in the water so oxygen and carbon dioxide have a short diffusion pathway.
• Reduction of water loss:

  • thick waxy cuticles to increase the length of the diffusion pathway
  • sunken gas exchange 'holes' so moisture is trapped and the water potential gradient is reduced. Hairs also trap moist air.
  • modified leaves to reduce the surface area for water loss
• Inspiration - breathing in

  • the diaphragm muscle contracts
  • external intercostal muscles contract and the internal intercostal muscles relax
  • the rib cage moves upwards and outwards
  • the thorax increases in volume and the pressure decreases below atmospheric pressure
  • therefore, air moves into the lungs down a pressure gradient
• Forced expiration - breathing out

  • the diaphragm muscle relaxes
  • internal intercostal muscles contract and the external intercostal muscles relax
  • the rib cage moves inwards and downwards
  • the thorax decreases in volume and the pressure rises above atmospheric pressure
  • so, the air moves out of the lungs down a pressure gradient.
  • in normal expiration the ribcage moves downwards and inwards passively