Exchange surfaces and breathing

• Features of a specialised exchange surface:

  • Good blood supply - steeper gradient = faster rate of diffusion
  • good ventilation - maintains diffusion gradient
  • large surface area - provides area for exchange limitations of sa:v ratio
  • thin layers - short diffusion distance
• Inspiration

  • Diaphragm contracts and flattens
  • external intercostal muscles move upwards and outwards
  • volume of thorax decreases and pressure increases to greater than atmospheric air
  • air begins to move into lungs through nasal passage until pressure equalises
• Expiration

  • Diaphragm relaxes and moves upwards back into dome shape
  • external intercostal muscles move inwards and downwards
  • volume of thorax increases and pressure decreases below atmospheric air
  • air is removed from lungs until pressure equalises
• transport in insects

  • air enters through spiracles which can be opened and closed by sphincters to reduce water loss ( mainly closed when O2 demands are low)
  • leading away from spiracles are tracheae which carry air hrough the body
  • tracheae is lined with chitin which keeps tubes open when pressed or bent, it also causes them to be impermeable to most gases
  • the tracheae branches into smaller tubes called tracheoles which are freely permeable to gases and where most gaseous exchange takes place
  • tracheal fluid is found at the end of tracheoles which limits penetration of air for diffusion
• transport in fish

  • mouth opens and buccal cavity is lowered
  • volume of buccal cavity increases, and pressure decreases so water enters
  • simultaneously, the opercular valve is shut and the opercular cavity containing gills expands
  • pressure in the opercular cavity decreases
  • floor of the buccal cavity move upwards, increasing pressure - water then moves from buccal cavity over the gills
  • mouth closes and operculum opens, sides of opercular cavity move inwards, increasing pressure, forcing the water over the gills and out the operculum
  • buccal cavity moves up slowly to maintain the flow of water
• Describe a countercurrent system

  Blood and water flow in opposite directions so the oxygen gradient between blood and water is maintained
• Tidal volume

  Volume of air that moves in and out of lungs with each resting breath
• vital capacity 


  volume of air that can be inhaled when the strongest possible exhalation is followed by the deepest inhalation
• inspiratory reserve volume

  maximum volume of air you can breathe in over and above a normal inhalation
• expiratory reserve volume

  extra amount of air you can force out of your lungs above the normal tidal volume of air you breathe out
• residual volume

  volume of air that is left in your lungs when you’ve exhaled as hard as possible
• total lung capacity

  the sum of the vital capacity and residual volume
• what is the breathing rate?

  number of breaths taken per minute
• how to calculate ventilation rate

  Ventilation rate = tidal volume x breathing rate (per minute)