Concept preview

What you will be able to do

1. 01Explain how oxygen and carbon dioxide diffuse at the alveoli and at the tissues in terms of partial-pressure gradients.
2. 02Describe how oxygen is carried in the blood, mostly bound to haemoglobin as oxyhaemoglobin.
3. 03Describe how carbon dioxide is carried, mostly as bicarbonate ions.
4. 04Explain how the medulla of the brain controls breathing rate in response to carbon dioxide and blood pH.

Once air has been pumped into the alveoli, the actual exchange of gases needs no pump at all — it happens entirely by diffusion, the natural drift of molecules from where they are crowded to where they are sparse. To describe how crowded a particular gas is, we use its partial pressure: the share of the total pressure that one gas in a mixture contributes on its own. Air is a mixture, so the pressure it exerts is split among its gases, and each gas diffuses according to its own partial pressure, quite independently of the others. Oxygen always moves from a region of high oxygen partial pressure to low, and carbon dioxide always moves from high carbon dioxide partial pressure to low, each following its own gradient. This is the master key to the whole topic: to work out which way a gas will move at any surface in the body, you simply compare its partial pressure on the two sides and let it diffuse from high to low.