Features of Exchange Surfaces

• Effective exchange surfaces in organisms

  • Large surface area
  • Short diffusion distance (thin)
  • Good blood supply
  • Ventilation mechanism
• Oxygen diffusion across the cell wall and cell surface membrane of Chlamydomonas

  1. Oxygen molecules diffuse to the centre of Chlamydomonas

  2. Takes 100 milliseconds
• Diffusion is an efficient exchange mechanism for Chlamydomonas
• Root hair cells


  • Increase surface area
  • Increase rate of water uptake by osmosis
• Alveoli


  • Walls are only one cell thick and flattened
  • Gases have a very short diffusion distance
  • Large number of alveoli
  • Extensive capillary network
• The average human adult has around 480-500 million alveoli in their lungs, equaling a surface area of 40-75 m2
• Concentration gradient is maintained

  Adequate blood supply brings substances that have just entered the blood away from the exchange site
• Counter-current system in fish gills

  1. Water flow is in the opposite direction to blood flow

  2. Ensures concentration gradient is maintained along the whole length of the capillary

  3. Water with highest oxygen content is next to oxygenated blood

  4. Water continues to supply the blood with oxygen along the whole gill arch
• Ventilation mechanism in mammalian lungs

  1. Breathing removes air with low oxygen and high carbon dioxide and replaces it with air that has high oxygen and low carbon dioxide

  2. Maintains a higher concentration of oxygen in the alveoli than in the blood