How It Works: Why the Concentration Gradient Is Everything

How It Works: Why the Concentration Gradient Is Everything

Gas exchange at the alveolus is driven entirely by diffusion, which means it can only happen when there is a concentration gradient — a difference in concentration of a gas on either side of the alveolar membrane. The moment that gradient disappears, gas exchange stops.

In the alveolus, oxygen concentration is kept high because you constantly breathe in fresh air. In the blood arriving at the alveolus (from the pulmonary artery), oxygen concentration is low because it has already been used up by respiring body cells. This difference is the gradient that drives oxygen to diffuse from the alveolus into the blood.

For carbon dioxide it is the reverse: blood arriving at the alveolus carries a high concentration of CO₂ (produced by cellular respiration), while the alveolar air has a low CO₂ concentration (you exhale CO₂ with every breath). So CO₂ diffuses from blood into the alveolar air space, and is breathed out.

The dense capillary network is critical for maintaining these gradients. Blood flows continuously past the alveolus — it is constantly being refreshed, taking away the oxygen it has collected and delivering new CO₂. Without this continuous blood flow, both gradients would collapse and gas exchange would cease. This is why poor circulation is just as dangerous as poor breathing.