Mammalian gaseous exchange system

• Large surface area to provide more space for molecules to pass through

  • the alveoli are lined with thin moisture, which evaporates and is lost when breathed out

  The barrier to exchange is comprised of the wall of the alveolus and wall of capillary - the cells and their plasma membranes readily allow diffusion of oxygen and carbon dioxide, as molecules are small and non-polar
• Thin barrier to reduce the diffusion distance :

  • alveolus wall is one cell thick
  • capillary wall is one cell thick
  • capillary wall is in close contact to the alveolus wall
  • both consist of squamous cells - flattened or very thin
  • capillaries are narrow so red blood cells can squeeze against the capillary wall - makes them closer to the air in the alveoli and reducing rate of flow

  total barrier of diffusion is 1µm, 2 squamous cells
• A good blood supply

  • helps maintain a steep concentration gradient , so gases can continue to diffuse
  • the blood transport system transports carbon dioxide from tissues to lungs. This ensure a high concentration of carbon dioxide in blood than that in air of alveolus
  • this blood also transports oxygen from away from lungs. this ensure a high cocnentration in air in alveolus than that in blood
• Ventilation

  ensures :

  • concentration of oxygen in air of alveolus is higher than in blood
  • conentration of carbon dixide in air of alveolus kept lower than in blood
  • therefore the concentration gradient necessary for diffusion is maintained
• Inspiration

  • diaphragm contracts to move down and become flatter - this displaces the digestive organs downwards
  • the external intercostal muscles contract to raise the ribs
  • the volume of the chest cavity is increased
  • the pressure in chest cavity drops below the atmospheric pressure
  • air is moved into lungs
• Exhalation

  • diaphragm relaxes and its pushed up by the displaced organs underneath
  • the external intercostal muscles relax and the ribs fall; the internal intercostal muscles can contact to help push air out more forcefully - this usually only happens during exercise or coughing and sneezing
  • the volume of the chest cavity is decreased
  • the pressure in the lungs increases and rises above the pressure in the surrounding atmosphere
  • air is moved out lungs