Gas exchange and breathing
How alveoli are adapted for exchange, and how breathing changes with exercise.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.1.1Describe the features of gas exchange surfaces and relate alveolar structure to function.
- 11.1.2Explain the effects of physical activity on rate and depth of breathing.
5 minute read
Gas exchange and breathing
Every efficient gas exchange surface in biology shares the same four features — learn them once and they apply to alveoli, gills, and leaves alike.
Features of a good exchange surface
- Large surface area — more space for diffusion at once.
- Thin (often one cell thick) — a short diffusion distance.
- Good blood supply — carries substances away, maintaining a steep concentration gradient.
- Moist — gases dissolve before diffusing across.
The alveoli
There are hundreds of millions of alveoli, giving an enormous total surface area. Each has a wall one cell thick, is surrounded by a dense capillary network, and has a moist lining. Oxygen diffuses from the alveolar air (high concentration) into the blood (low), and carbon dioxide diffuses the other way. The blood flow constantly removes oxygen and brings more carbon dioxide, so both gradients stay steep.
Breathing during exercise
Muscles respire faster, using more oxygen and producing more carbon dioxide. Rising CO₂ is detected, and breathing becomes faster and deeper. This increases the volume of air exchanged per minute, supplying more oxygen for aerobic respiration and removing CO₂ faster. Heart rate rises for the same reason — to transport the gases more quickly.
Oxygen debt
If exercise is vigorous enough that oxygen cannot be delivered fast enough, muscles respire anaerobically, producing lactic acid. This builds up an oxygen debt, which is why you continue breathing hard after stopping — the extra oxygen is needed to break down the lactic acid.
Think of it like this
The alveoli are a bunch of grapes rather than a balloon: the same volume, but far more skin. That is the whole point — surface area, not space, is what limits gas exchange.
Worked examples
Method, step by step
Explain why breathing becomes deeper and faster during exercise.
- 1Muscles contract more, so they respire aerobically at a higher rate.
- 2This uses more oxygen and produces more carbon dioxide, raising blood CO₂ concentration.
- 3The increased CO₂ is detected and breathing rate and depth increase.
- 4This supplies oxygen faster for respiration and removes the extra carbon dioxide.
Exercising muscles respire faster, raising carbon dioxide levels in the blood. This is detected, so breathing becomes faster and deeper to supply more oxygen and remove the excess carbon dioxide.
Common misconceptions
- Saying we breathe faster because "we need more air". The stimulus is rising carbon dioxide concentration, not a sensation of needing oxygen.
- Thinking breathing and respiration are the same thing. Breathing moves air; respiration releases energy inside cells.
- Believing the oxygen debt is repaid during exercise. It is repaid afterwards, which is why heavy breathing continues once you stop.
In the exam
- The four exchange-surface features earn marks in questions on lungs, gills, leaves and intestines. It is the highest-value list in the syllabus.
- For exercise questions, always name carbon dioxide as the detected stimulus and link the response to supplying oxygen for aerobic respiration.