Oxygen and carbon dioxide cross the alveolar wall by diffusion. The squamous epithelium makes the path short, breathing and blood flow keep the gradients steep, and cartilage, smooth muscle and elastic fibres keep air moving in and out.
In the alveoli, oxygen diffuses from the air (high partial pressure, about 13 kPa) into the blood in the capillaries (about 5.3 kPa), and carbon dioxide diffuses from the blood into the alveolar air, each down its own concentration gradient. Diffusion is fast because the surface area of the millions of alveoli is huge, the diffusion distance is less than 1 μm (the squamous epithelium of the alveolus plus the capillary endothelium), and the gradients are kept steep by ventilation, which replaces the alveolar air, and blood flow, which carries oxygenated blood away. Cartilage holds the trachea and bronchi open; smooth muscle contracts or relaxes to change the diameter of the airways, especially the bronchioles; elastic fibres stretch as the alveoli fill and recoil to help push air out.
Red dots are oxygen and grey dots are carbon dioxide. Read the graph of pO2: the red line is the alveolar air and the blue line is the blood. Untick Ventilation, then Blood flow, and watch the two lines.
Gases cross by diffusion, not active transport, and each gas moves down its own gradient. Say down a concentration (partial pressure) gradient, and name both layers of cells: the alveolar epithelium and the capillary endothelium.
Press Breathe and read the ALVEOLI read-out as the alveoli stretch and recoil. Then press Smooth muscle contracts (asthma) and watch AIRWAY WIDTH and the airflow, and press Relax.
| Tissue | Where | Function |
|---|---|---|
| cartilage | trachea (C-shaped rings), bronchi (plates) | strong but flexible support that keeps the airways open and stops them collapsing when air pressure inside falls during inhalation; the C shape lets the oesophagus expand when food is swallowed |
| smooth muscle | trachea, bronchi, bronchioles | contracts to narrow the airways and relaxes to widen them, controlling airflow; relaxes during exercise so more air reaches the alveoli |
| elastic fibres | all airways and around the alveoli | stretch when we breathe in, then recoil when we breathe out, helping to push air out of the alveoli; recoil also widens airways after smooth muscle contraction |
| squamous epithelium | alveolar walls | a single layer of very thin, flat cells giving a very short diffusion distance |
In an asthma attack the smooth muscle around the bronchioles contracts strongly, narrowing them; less air reaches the alveoli and breathing out becomes difficult. Inhaled drugs that relax the smooth muscle widen the airways again.
Bronchioles have no cartilage, so it is the smooth muscle and elastic fibres in their walls that control their width. Cartilage does not contract; muscle does. Elastic fibres do not contract either: they recoil passively after being stretched.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
On the alveoli graph, roughly where does the red line for alveolar air start?
What happens in the alveoli tab when Blood flow is unticked?
After pressing Smooth muscle contracts (asthma), what does AIRWAY WIDTH fall to?
While breathing, what does the ALVEOLI read-out show as air flows out?
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which feature most directly reduces the diffusion distance between alveolar air and blood?
Why does carbon dioxide diffuse from the blood into the alveoli?
What is the role of elastic fibres in the alveolar walls?
During an asthma attack, why does less oxygen reach the blood?
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