AQA A Level Biology复习笔记3.2.5 The Alveolar Epithelium

The Alveolar Epithelium


  • The exchange of oxygen and carbon dioxide occurs between the alveoli and the capillaries in the lungs
  • Oxygen and carbon dioxide are exchanged in a process of simple diffusion; (passive movement from high to low concentration)
  • The air in the alveoli contains a high concentration of oxygen. The oxygen diffuses from the alveoli and into the blood capillaries, before being carried away to the rest of the body for aerobic respiration
  • The blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide. The carbon dioxide diffuses from the blood and into the alveoli and is then exhaled


The movement of blood through the capillaries surrounding the alveoli maintains a concentration gradient to drive diffusion of oxygen into the blood


Features of the alveoli

  • Large number of alveoli
    • The average human adult has around 480 - 500 million alveoli in their lungs. This equals a surface area of 40 - 75 m2
    • The large number of alveoli increases the surface area available for oxygen and carbon dioxide to diffuse across


  • Thin walls
    • The walls of the alveoli are only one cell thick and these cells are flattened
    • This means that gases have a very short diffusion distance so gas exchange is quick and efficient


  • Extensive capillary network
    • The walls of the capillaries are only one cell thick and these cells are flattened, keeping the diffusion distance for gases short
    • The constant flow of blood through the capillaries means that oxygenated blood is brought away from the alveoli and deoxygenated blood is brought to them
    • This maintains the concentration gradient necessary for gas exchange to occur


Exam Tip

Remember that a membrane that is permeable to oxygen is usually permeable to water. As a result, moist surfaces are an unavoidable consequence of gas exchange surfaces. This layer of moisture increases the diffusion distance.