Gas exchange explains how oxygen reaches the blood and carbon dioxide leaves it. This section has four lessons and a practice set, and belongs to the wider SPM Biology guide.
What does this section cover?
Start with relating respiratory surfaces to diffusion, which gives the reasons behind every exchange surface. Then study how ventilation works in humans.
Next is comparing gas exchange in selected organisms. The last lesson, interpreting oxygen and carbon-dioxide data, turns tables into explanations. The practice set mixes all four.
One idea, two structures
Here is an original comparison. Human alveoli and fish gills look very different, yet both answer the same four questions.
| Feature | Human alveolus | Fish gill |
|---|---|---|
| Large surface area | Many small air sacs | Many thin filaments carrying lamellae |
| Thin barrier | Wall one cell thick | Lamellae one cell thick |
| Moist surface | Thin layer of moisture lining the sac | Surrounded by water |
| Gradient kept steep | Breathing and blood flow | Water flow over the gills and blood flow |
The shapes differ because the organisms live in air or water. The explanation stays the same: a large, thin, moist surface with a steep gradient.
Who should start where?
If your answers say “it is thin so it is good” without a reason, begin with respiratory surfaces and diffusion. If you confuse the movements of the ribs and diaphragm, begin with ventilation.
If you mix up the structures of different organisms, begin with the comparison lesson. If tables of percentages leave you writing vague sentences, begin with the data lesson. A link to cellular respiration helps explain why the cells use the oxygen at all.
For a teacher to hear your explanations one question at a time, see online one-to-one Biology tuition.