11.1 Gas Exchange in Humans

2026 Syllabus Objectives

  1. Describe the features of gas exchange surfaces in humans, limited to: large surface area, thin surface, good blood supply and good ventilation with air
  2. Identify in diagrams and images the following parts of the breathing system: lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries
  3. Investigate the differences in composition between inspired and expired air using limewater as a test for carbon dioxide
  4. Describe the differences in composition between inspired and expired air, limited to: oxygen, carbon dioxide and water vapour
  5. Investigate and describe the effects of physical activity on the rate and depth of breathing
  6. Identify in diagrams and images the internal and external intercostal muscles
  7. State the function of cartilage in the trachea
  8. Explain the role of the ribs, the internal and external intercostal muscles and the diaphragm in producing volume and pressure changes in the thorax leading to the ventilation of the lungs
  9. Explain the differences in composition between inspired and expired air
  10. Explain the link between physical activity and the rate and depth of breathing in terms of: an increased carbon dioxide concentration in the blood, which is detected by the brain, leading to an increased rate and greater depth of breathing
  11. Explain the role of goblet cells, mucus and ciliated cells in protecting the breathing system from pathogens and particles

Gas Exchange Surfaces

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What is Gas Exchange? 🔑

Gas exchange is the diffusion of oxygen and carbon dioxide into and out of an organism's body. In organisms, there are special areas called gas exchange surfaces where oxygen enters and carbon dioxide leaves.

Characteristics of Gas Exchange Surfaces

Gas exchange surfaces must be permeable to allow oxygen and carbon dioxide to move easily through them. They have several characteristics that make the process quick and efficient:

  • Thin walls - They are only one cell thick to allow gases to diffuse across them quickly
  • Close to an efficient transport system - To take gases to and from the exchange surface
  • Large surface area - So that a lot of gas can diffuse across at the same time (the total surface area of all alveoli in human lungs is over 100 m2100 \, \text{m}^2)
  • Good supply of oxygen - Maintained by ventilation (breathing movements)
  • Good blood supply - To maintain concentration gradients for efficient diffusion

💡 These features (large surface area, good blood supply, steep concentration gradient) are common to all exchange surfaces in biology, including villi in the small intestine and root hair cells in plants.

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