AQA A Level Biology

Revision Notes

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

Gas Exchange, downloadable AS & A Level Biology revision notes

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.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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