Alveoli: Where Oxygen and Carbon Dioxide Are Exchanged
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
In the alveoli, oxygen diffuses from the air into the blood across the thin alveolar wall, dissolving in the moist lining first. Carbon dioxide diffuses in the opposite direction, from blood into the alveolar air. Both gases move down their concentration gradients.
Alveolar gas exchange is one of the most predictable exam questions. To score full marks, state the direction of each gas, name diffusion as the process, and explain the concentration gradient for each gas separately.
Oxygen exchange
Inhaled air in the alveoli has a higher concentration of oxygen than the blood arriving in the surrounding capillaries (which has released its oxygen to body tissues). Oxygen dissolves in the moist film lining the alveolus and diffuses across the thin wall (one cell thick for the alveolus, one cell thick for the capillary) into the blood plasma and then into red blood cells.
Inside red blood cells, oxygen binds to haemoglobin to form oxyhaemoglobin. This binding reaction removes free oxygen from the plasma, keeping the oxygen concentration in the blood low and maintaining the steep concentration gradient for continued diffusion.
Carbon dioxide exchange
Blood arriving at the alveoli from respiring tissues has a higher concentration of carbon dioxide than the air in the alveoli. Carbon dioxide diffuses out of the blood, across the alveolar wall, and into the alveolar air. It is then exhaled.
The concentration gradient for carbon dioxide is maintained by ventilation: each breath removes carbon dioxide-rich air from the alveoli and replaces it with fresh air containing little carbon dioxide. Without breathing, carbon dioxide would accumulate in the alveoli and the gradient would flatten, slowing exchange.
Ventilation removes CO₂-rich air and replaces it with fresh air
Both gases cross the same thin, moist surface by diffusion. No energy is required: the process is entirely passive and depends on concentration gradients.
Try a focused question (3 marks)
Explain how oxygen passes from the air in the alveoli into the blood.
Show the answer and marking guidance
Oxygen concentration is higher in the alveolar air than in the blood in the capillaries.
Oxygen dissolves in the moist lining of the alveolus.
Oxygen diffuses across the thin wall (short diffusion distance) down its concentration gradient into the blood.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying oxygen is actively transported into the blood: gas exchange at alveoli is by diffusion only.
Describing oxygen moving into the blood without mentioning the concentration gradient.
Forgetting that gases dissolve in the moist lining before diffusing across the wall.
Not explaining how the gradient is maintained (blood flow for O₂, ventilation for CO₂).