Respiration

Gas exchange and respiration are easy to confuse. To keep them separate:

• Respiration is the chemical reaction inside cells that uses oxygen and produces CO₂ (covered above).
• Gas exchange is the physical movement of oxygen into the body and CO₂ out of the body, by diffusion across an exchange surface.

Why multicellular organisms need a gas exchange surface

A single-celled organism like Amoeba has a huge surface-area-to-volume ratio. Oxygen from the water can simply diffuse through its cell membrane straight into the cytoplasm fast enough to keep every part of the cell supplied. No special apparatus needed.

For a multicellular animal like a human, the inner cells of the body are far too deep for diffusion alone to supply them with oxygen at a useful rate. The body solves this by having:

• A specialised gas exchange surface (the lungs) with a huge surface area and very thin walls
• A transport system (the blood) to carry oxygen from the exchange surface to every cell

Features of a good gas exchange surface

Any good gas exchange surface, in any animal, has the same four adaptations:

1. Large surface area so a lot of oxygen can cross at once
2. Thin walls (one cell thick where possible) so the diffusion distance is short
3. Steep concentration gradient maintained by good ventilation on the outside and good blood supply on the inside
4. Moist surface, because the gases must dissolve before they can diffuse through the cells