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

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  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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