Gas exchange in the alveoli
Gas exchange in the alveoli
- The alveoli are where oxygen enters the blood and carbon dioxide leaves it.
- Their structure makes them brilliant exchange surfaces.
- The swap happens entirely by diffusion.
Why alveoli are ideal
- a very large total surface area (millions of tiny sacs),
- very thin walls — alveolus and capillary each one cell thick → a tiny distance to cross,
- a rich blood supply from the capillary network,
- a moist lining, so gases dissolve before crossing.
Practice
Which feature makes alveoli good for gas exchange?
Large surface area, thin walls, a rich blood supply and a moist lining all speed up gas exchange.
Diffusion of the gases
- Oxygen is high in the alveolar air, low in the blood — so it diffuses air → blood.
- Carbon dioxide is high in the blood, low in the air — so it diffuses blood → air to be breathed out.
- Both move down their concentration gradients.
Practice
Oxygen diffuses:
Oxygen is higher in the alveolar air than in the blood, so it diffuses air → blood.
Practice
Carbon dioxide diffuses:
CO₂ is higher in the blood, so it diffuses blood → air and is then exhaled.
Keeping exchange fast
- Breathing brings fresh air, keeping alveolar oxygen high and CO₂ low.
- Flowing blood carries gases away as fast as they cross.
- Both keep the concentration gradients steep, so diffusion stays rapid.
Practice
Breathing and the flow of blood keep gas exchange fast because they:
Fresh air and moving blood keep concentrations different on each side, so the gradients stay steep.
You've got it
Key idea
- alveoli: large surface area, thin walls (one cell each), rich blood supply, moist lining
- oxygen diffuses air → blood; carbon dioxide diffuses blood → air (down the gradients)
- breathing + blood flow keep the gradients steep, so exchange stays fast