How do the alveoli swap oxygen for carbon dioxide between the air and the blood?
Explain how gas exchange happens in the alveoli and describe how they are adapted for fast diffusion
A scaffolded answer to the N(A)-Level Biology outcome on gas exchange. How oxygen and carbon dioxide diffuse between the alveoli and the blood, and the four adaptations that make the alveoli efficient at this.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
This outcome wants you to explain how gas exchange happens in the alveoli: oxygen passing into the blood and carbon dioxide passing out, both by diffusion. You should be able to name the four adaptations of the alveoli and link each one to faster diffusion. The marks reward stating the gradient for each gas, the correct direction, and connecting each adaptation to how it speeds up gas exchange. This builds directly on what you know about diffusion.
The answer
What gas exchange is
Gas exchange is the swapping of two gases between the air in the lungs and the blood. The body needs to take in oxygen (for respiration) and get rid of carbon dioxide (a waste from respiration). Both gases move by diffusion, down their concentration gradients, with no energy needed.
Oxygen into the blood
The air breathed into an alveolus has a high concentration of oxygen. The blood arriving in the capillary has a low concentration of oxygen (it has been used up by the body cells). So oxygen diffuses down the gradient, from the alveolus across the thin walls into the blood. The blood then carries the oxygen to the body cells.
Carbon dioxide out of the blood
The blood arriving at the alveolus has a high concentration of carbon dioxide, made by the body cells during respiration. The air in the alveolus has less. So carbon dioxide diffuses the other way, out of the blood into the alveolus, and is then breathed out.
How the alveoli are adapted
The alveoli are very good at gas exchange because of four features:
- Large surface area. There are millions of alveoli, so a lot of gas can be exchanged at once.
- Thin walls. Each wall is one cell thick, so the gases have only a short distance to diffuse.
- Rich blood supply. Many capillaries carry gases away, keeping the concentration gradient steep.
- Moist surface. The walls are moist, so the gases dissolve and diffuse more easily.
Examples in context
Example 1. Why fresh air helps. Breathing keeps replacing the air in the alveoli with fresh air that is high in oxygen and low in carbon dioxide. This keeps the gradients steep, so oxygen keeps diffusing in and carbon dioxide keeps diffusing out. It shows why ventilation (breathing) and gas exchange work together.
Example 2. Why pneumonia makes you breathless. In pneumonia, the alveoli fill with fluid. This increases the distance gases must diffuse and reduces the surface area available, so less oxygen reaches the blood and the person becomes breathless. It shows how damaging the alveoli wrecks the very features that make gas exchange fast.
Try this
Q1. Name the process by which oxygen moves from the alveolus into the blood. [1 mark]
- Cue. Diffusion.
Q2. State the direction carbon dioxide moves during gas exchange. [1 mark]
- Cue. Out of the blood into the alveolus (to be breathed out).
Q3. Explain how the large surface area of the alveoli helps gas exchange. [2 marks]
- Cue. A large surface area lets more oxygen and carbon dioxide diffuse at the same time, so gas exchange is faster.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksExplain how oxygen moves from the air in an alveolus into the blood, and how carbon dioxide moves the other way.Show worked answer →
The air breathed into the alveolus has a high concentration of oxygen, while the blood arriving in the capillary has a low concentration of oxygen. So oxygen diffuses down its concentration gradient, across the thin alveolus wall and capillary wall, into the blood.
At the same time, the blood arriving has a high concentration of carbon dioxide (a waste from respiration), while the air in the alveolus has less. So carbon dioxide diffuses the other way, out of the blood into the alveolus, ready to be breathed out.
What markers reward: both gases moving by diffusion down their own concentration gradients, with the correct directions (oxygen in, carbon dioxide out) and naming the gradient. Saying the gases are pumped or use energy is wrong; it is diffusion.
Original4 marksDescribe four ways the alveoli are adapted for efficient gas exchange and explain how each adaptation helps.Show worked answer →
Adaptation 1: there are millions of alveoli, giving a very large surface area, so a lot of gas can be exchanged at once.
Adaptation 2: each alveolus has a very thin wall (one cell thick), giving a short distance for the gases to diffuse across.
Adaptation 3: the alveoli have a rich blood supply from many capillaries, which carries gases away and keeps the concentration gradient steep.
Adaptation 4: the alveolus walls are moist, so the gases can dissolve and diffuse more easily.
What markers reward: four correct adaptations, each linked to faster diffusion (large surface area, short distance, steep gradient, moist surface). A feature named without explanation scores only half.
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