Where gas exchange happens
Breathing brings air to the lungs, but the actual gaseous exchange takes place in millions of tiny air sacs called alveoli. Each alveolus is wrapped in a dense network of blood capillaries, so oxygen and carbon dioxide need only diffuse a very short distance.
Exchange at the alveolus
- Inhaled air has a higher concentration of oxygen than the blood, so oxygen diffuses from the alveolus into the blood.
- Blood returning to the lungs has a higher concentration of carbon dioxide, so carbon dioxide diffuses from the blood into the alveolus and is breathed out.
Key idea
Diffusion always follows the concentration gradient — each gas moves from where it is more concentrated to where it is less concentrated.
Adaptations of the alveolus
The alveoli are superbly suited for exchange:
- Huge surface area — hundreds of millions of alveoli together give a large area.
- Very thin walls — the alveolar wall and capillary wall are each just one cell thick, giving a short diffusion path.
- Moist lining — a film of moisture lets gases dissolve.
- Rich blood supply — capillaries constantly carry gases away, keeping the gradient steep.
Transport of the gases
Oxygen is carried by the red pigment haemoglobin in red blood cells, forming oxyhaemoglobin. Most carbon dioxide is carried in the plasma as hydrogencarbonate ions, with smaller amounts bound to haemoglobin or dissolved. At the body tissues the opposite exchange occurs: oxygen diffuses from blood into cells, and carbon dioxide diffuses from cells into blood.
The enormous combined surface area of the alveoli — many times larger than the area of the skin — together with the constant flow of blood through the capillaries allows the lungs to supply enough oxygen even during hard exercise.
Example
During exercise, muscle cells respire faster and release more carbon dioxide, steepening the gradient so that gas exchange speeds up.