2.46 explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries


  1. Waste CO2 is excreted out of the blood stream
  2. Oxygen  is diffused into the bloodstream, replacing the CO2

Features of alveoli 

  • Large surface area (more surface area for the oxygen to diffuse into)
  • Richly supplied with blood as the distance between the alveoli and blood capillaries is very short (important, so there is less diffusion distance)
  • Most lining for dissolving gases (moist with mucus)
  • Very thin walls (only one cell thick – less diffusion distance)
  • Ventilation of the lungs during breathing helps maintain the concentration gradient (so that fresh oxygen is brought into the lungs and waste CO2 is removed pronto)
  • Concentration gradient helps move the blood through the capillaries, freshly absorbed oxygen is transported away and unwanted CO2 is continually being brought to the alveoli surface

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