alveoli and gas exchange

• The alveoli is where gas exchange takes place. Oxygen is added to the blood and carbon dioxide is removed from the blood.

  Adaptations of the alveoli to allow this to happen:

  1. Very large surface area (2 tennis courts) over which gas exchange can occur
  2. Very thin walls - 1 flattened cell thick (same as capillaries) so the gases do not have much of a barrier
  3. Inner walls are coated with a thin layer of water that allows oxygen to dissolve before travelling through the walls of the alveoli and capillaries
• Diffusion gradient :

  Oxygen is in a higher concentration in the air we inhale than in the blood. Oxygen diffuses into the blood and combines with haemoglobin in the erythrocytes (RBC) to form oxyhemoglobin.

  Carbon dioxide in the blood diffuses from the capillaries into the alveoli as it is in higher concentration in the blood.

  Exhaled air has a higher level of carbon dioxide than inhaled air and a lower level of oxygen.

  Carbon dioxide is transported in the blood plasma.
• Gaseous exchange


  Gaseous exchange is a process that involves the exchange of oxygen and carbon dioxide between capillaries and alveoli.
• Alveoli


  Microscopic structures at the end of the tiniest bronchioles in the lungs, arranged in clusters
• Gas exchange in the alveoli

  1. Oxygen moves out of the alveoli to the capillaries and into the bloodstream

  2. Carbon dioxide moves out of the capillaries into the alveoli and to the lungs to be exhaled
• Alveoli walls


  • Very thin (one cell thick)
  • Each alveoli is surrounded by capillaries through which gases are exchanged
• Alveoli structure


  Like bunches of grapes, which increases their surface area to allow the maximum crossover, or diffusion, of the two gases back and forth to make the process very efficient
• Diffusion


  Allows the exchange of oxygen and carbon dioxide between the alveoli and the capillaries
• Diffusion gradients


  Diffusion refers to the movement of molecules from an area of high concentration to an area of low concentration. This is the case with gases and for particles dissolved in solutions. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration.

  • The capillaries have a lower concentration of oxygen than the alveoli.

  • This results in diffusion of oxygen - from an area of higher concentration (the alveoli) to an area of lower concentration in the red blood cells (in capillaries).