Topic 13.1 · Core
What Is Excretion?
Excretion sounds like a simple word until an exam asks you to apply it precisely. For this syllabus point you need to know exactly which substances count, which organ removes each one, and why forming a substance is a completely different job from excreting it.
The definition Cambridge wants
Excretion is the removal from the body of waste products of metabolism and substances in excess of requirements. That definition has two halves, and an exam answer needs to show you understand both. A waste product of metabolism is something produced by chemical reactions happening inside cells — carbon dioxide, made during respiration, is the clearest example. A substance in excess of requirements is different: it isn’t waste at all, it’s something the body genuinely needs, just not in the amount currently present. Excess water and excess ions fall into this second half, which is exactly why they count as excretory products even though the body couldn’t function without water or ions altogether.
Excretion is not egestion
Egestion is the removal of undigested food from the body as faeces. Because that material was never produced by metabolism in the first place, egestion doesn’t meet the definition above, and faeces should never be used as your example of an excretory product in an answer about excretion. Keep the test simple: did this substance come from metabolism, or from food that was never absorbed at all? Only the first counts here.
Two organs, two very different outputs
| Excretory organ | What it excretes |
|---|---|
| Lungs | carbon dioxide |
| Kidneys | urea, excess water, excess ions |
Carbon dioxide is produced continuously during respiration, carried to the lungs, and leaves the body in the air you breathe out — the detailed route through the gas-exchange system belongs to a different chapter, but for this one the relationship you need is simply respiration produces carbon dioxide, and the lungs excrete it. The kidneys’ output is a little more varied: urea, excess water and excess ions all become part of urine, but they aren’t the same kind of substance. Urea is pure waste that has to go. Water and ions are useful to the body and stay in circulation right up until there’s more of them present than the body currently needs — “excreted” doesn’t mean “useless”, it means “no longer required at this amount”.
Where urea actually comes from
This is the distinction that catches the most students out. The liver forms urea, from excess amino acids, in a process called deamination — the kidneys never make urea, they only remove it from the blood once the liver has already produced it. A student who writes “urea is made in the kidney” has confused production with excretion, and that single mix-up is enough to lose a mark that otherwise looked secure. The full formation story — assimilation, deamination, and why urea has to be removed at all — is covered on the deamination and urea page; what matters here is simply keeping the two organs’ jobs apart: liver forms it, kidneys excrete it.
Where the boundary sits
Some biology resources also mention sweat as a route for losing water and ions. That’s reasonable biology, but it isn’t an additional required Chapter 13 output objective in the current 2026–2028 syllabus, so don’t treat it as a third excretory organ alongside the lungs and kidneys. Detailed liver waste routes and bile pigments sit outside the required scope here too — the two relationships that matter for this syllabus point are lungs–carbon dioxide and kidneys–urea/water/ions, and everything else in this chapter builds on getting those two exactly right.
Once you’re confident on what’s excreted and by which organ, the next step is knowing where those organs actually sit and how urine moves through them — covered on the human urinary system page.