9.1 Human Gas Exchange

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the features of gas exchange surfaces in humans (large surface area, thin surface, good blood and air supply)
  2. State the percentages of gases in atmospheric air
  3. Investigate and explain the differences between inspired (breathed in) and expired (breathed out) air
  4. Identify, on diagrams and images, the larynx, trachea, lungs, bronchi, bronchioles, alveoli and associated capillaries
  5. State the characteristics of, and describe the role of, the alveoli exchange surface in gas exchange
  6. Identify, on diagrams and images, the ribs, internal and external intercostal muscles and the diaphragm
  7. Explain how the ribs, intercostal muscles and diaphragm produce volume and pressure changes in the thorax, causing air to move in and out of the lungs
  8. Investigate and explain the effect of physical activity on rate and depth of breathing
  9. Explain the role of goblet cells, ciliated cells and mucus in protecting the gas exchange system

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1. Features of Gas Exchange Surfaces

Gas exchange is the process by which oxygen moves from the air into the blood, and carbon dioxide moves from the blood into the air. For this to happen efficiently, the exchange surface must have three key features:

Large Surface Area

  • A large surface area means that a lot of gas can be exchanged at once.
  • Think of it like this: if you have a big table, you can spread out many things at the same time. A bigger exchange surface allows more oxygen and carbon dioxide molecules to cross at the same time.
  • In the human lungs, the millions of tiny air sacs (called alveoli) together create a surface area of around 70 square metres — roughly the size of a tennis court — packed inside your chest.

Thin Surface

  • The surface must be very thin so that gases do not have far to travel.
  • Gases move by a process called diffusion — they naturally move from where there are more of them to where there are fewer of them.
  • A thinner surface means the distance gases have to diffuse is very short, making the process fast and efficient.
  • The walls of the alveoli are only one cell thick.

Good Blood and Air Supply

  • A good blood supply means that blood carrying low oxygen and high carbon dioxide is constantly arriving at the exchange surface, and blood with high oxygen is constantly being taken away. This keeps the concentration difference steep, which speeds up diffusion.
  • A good air supply means fresh air (rich in oxygen) is constantly being brought to the surface by breathing, replacing air that has had oxygen removed from it. This also keeps the concentration difference steep.
  • Together, these supplies maintain what is called a concentration gradient — the difference in the amount of a gas on each side of the surface that drives diffusion.

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