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Exchange surfaces and the lungs
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Large organisms need special surfaces to exchange substances quickly. The lungs are a perfect example.
Surface area to volume ratio
Small organisms have a large surface area to volume ratio, so diffusion through their surface is enough.
Larger organisms have a smaller ratio, so they need exchange surfaces and transport systems.
Larger organisms have a smaller ratio, so they need exchange surfaces and transport systems.
Alveoli
Alveoli are adapted for gas exchange:
Large surface area (millions of them).
Thin walls (one cell thick) for a short diffusion distance.
Rich blood supply to keep a steep concentration gradient.
Large surface area (millions of them).
Thin walls (one cell thick) for a short diffusion distance.
Rich blood supply to keep a steep concentration gradient.
Calculating the ratio
A cube of side 2 cm: surface area = 6 × 4 = 24 cm2, volume = 8 cm3.
Ratio = 24 : 8 = 3 : 1
Ratio = 24 : 8 = 3 : 1
Find the surface area to volume ratio of a 1 cm cube.
- Surface area = 6 × 1 = 6 cm2
- Volume = 1 cm3
- Ratio = 6 : 1
Answer: 6 : 1
Larger organisms have a smaller surface area to volume ratio, so they need exchange surfaces. Alveoli have a huge surface area, thin walls and a rich blood supply.
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