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Gas Exchange in the Alveoli
In I/GCSE biology, the lungs absorb the oxygen into the blood, and remove the carbon dioxide from it, in the alveoli.
To make this efficient, the alveoli must have a structure that brings the blood and the air very close, over a large surface area.
It has been estimated that we have over 700 000 000 alveoli, in our two lungs, with 60m 2 of surface area.
The alveoli look like a bunch of grapes, covered with tiny capillaries like a net.
What Happens?
- Blood is pumped to the lungs from the heart, which passes through the capillaries, by the alveoli.
- The blood has carbon dioxide in it from the respiration that occurs in the cells.
- Around the lungs, the blood is separated from the air inside each alveolus by only two layers of cells. This is so the air diffusion can be quick and easy. This is less than a thousandth of a millimetre.
- The air in the alveolus has more oxygen in it than the blood in the capillaries do, so that the oxygen diffuses from the air and into the blood.
- As there is more carbon dioxide in the blood than in the air, the carbon dioxide also transfers, from the blood to the air.
- The blood has now gained oxygen, and lost carbon dioxide, so the heart pumps the blood around the body again.
What features make the lungs efficient according to the I/GCSE biology curriculum?
- Millions of alveoli over two lungs, meaning large surface area
- A thin cell wall between the alveolus and the capillaries, which makes diffusion easier
- There are lots of capillaries surrounding the alveoli, meaning there is a good blood supply. It maintains the blood gradient.
In IGCSE biology, you need to understand that the alveoli:
- Air containing oxygen enters the lungs and passes into each alveolus from the outside.
- Deoxygenated blood containing CO 2 comes from the rest of the body.
- Oxygen diffuses into the blood from the alveoli.
- Air containing more carbon dioxide leaves the lungs.
End of this topic!
