3.3.2 Gas exchange

• Why do humans need a specialised exchange system?

  Our small SA:V means that we have too many cells needing oxygen that our small surface area can't provide by diffusion alone
• Lung adaptations for efficient gas exchange

  Large surface area for diffusion due to millions of small alveoli

  Ventilation and blood flow maintains steep concentration gradient for diffusion

  Lungs have elastic recoil so alveoli return to their original shape when breathing out
• Gas exchange system in humans

  Gas exchange surface - Walls of alveoli made of epithelial cells

  Adaptations:

  • Epithelial cells are thin = short diffusion pathway
  • walls of alveoli one cell thick = short diffusion path
  • Walls of capillaries in contact with alveoli walls = short diffusion path
• Antagonistic interaction of intercostal muscles

  In the lungs for breathing

  External - contract to breathe in, relax to breathe out

  Internal - Only contract to breathe out forcibly
• Mechanisms of breathing in

  1. External intercostal muscles contract
  2. Ribcage moves up and out
  3. Diaphragm muscle contracts
  4. Diaphragm moves down
  5. Volume in lungs increases
  6. Pressure in lungs decreases
  7. Air moves into the lungs down a pressure gradient
• Mechanisms of breathing out

  1. External intercostal muscles relax
  2. Ribcage moves down and in
  3. Diaphragm muscle relaxes
  4. Diaphragm moves up
  5. Volume in lungs decreases
  6. Pressure in lungs increases
  7. Air is forced out of the lungs down a pressure gradient
• Forced breathing out

  Internal Intercostal muscles contract, lowering the ribcage even further. This decreases volume in the lungs more and increases pressure, more air is forced out
• Tidal volume


  Volume of air that is inhaled/exhaled during normal breathing
• Ventilation rate

  number of breaths per minute
• Forced expiratory rate

  maximum volume of air that can be exhaled in 1 second
• forced vital capacity

  maximum volume of air that can be forced out of the lungs after maximum inhalation
• Asthma

  Muscle walls in bronchi contract and more mucus is secreted

  Airways become narrower, causing airflow to alveoli to be reduced

  Less oxygen diffuses into the blood

  Forced expiratory volume decreases
• Fibrosis

  Scar tissue increases the wall thickness of the alveoli so the diffusion pathway is larger. Lungs are unable to expand as much so hold a lower volume of air.

  Forced vital capacity decreases
• Emphysema

  Elasticity reduced, reducing the volume of air entering and leaving the alveoli. Reduces surface area for diffusion
• Gas exchange in single celled organisms

  Gas exchange surface is membranes

  • Large surface area - large SA:V
  • Steep concentration gradient - respiration uses oxygen and produces carbon dioxide
  • Short diffusion pathway - thin and large membrane
• Gas exchange in fish

  Gas exchange surface is the walls of lamella

  • Ventilation of gills - Water enters the mouth and is forced over the gills
  • Many thin gill filaments - large surface area, short diffusion pathway
  • Counter current flow - Blood and water flow in opposite directions across the lamellae. This maintains the steep concentration gradient along the whole length of the gill
• Gas exchange in insects

  Gas exchange surface is walls of tracheoles

  • Many tracheoles - large surface area
  • Tracheoles are thin - short diffusion path
  • Ventilation - abdominal pumping helps move air in and out
  • Steep concentration gradient - Aerobic respiration in cells uses oxygen and produces carbon dioxide
  • Limits water loss by closing spiracles more
• Gas exchange in dicotyledonous plants

  Gas exchange surface is the walls of mesophyll cells

  Problem for plants - lose water via transpiration from their gas exchange surface
• Limiting water loss in xerophytic (in hot dry environments) plants:

  • Thick waxy cuticle - waterproof so reduces evaporation, increases diffusion pathway
  • Fewer stomata - less surface area for transpiration
  • Small leaves - Less surface area for evaporation
  • Sunken stomata, hairy leaves, rolled leaves - traps water vapour close to leaf and prevents it from being blown away