Excretion: lungs, kidneys and skinSpec 2.71
In short
Excretion is the removal from the body of the waste products of metabolism. In humans the lungs excrete carbon dioxide, a waste product of respiration. The kidneys excrete urea, a nitrogen-containing waste, with excess water and ions in urine. The skin loses water, salts and urea in sweat. Faeces are egested, not excreted.
- Excretion
- The removal from the body of the waste products of metabolism.
Cells make waste products as they carry out chemical reactions. Some of these are poisonous or harmful if they build up, so they must be removed. Three organs carry out excretion in humans: the lungs, the kidneys and the skin.
| Organ | Excretory products |
|---|---|
| Lungs | Carbon dioxide (and water vapour), lost when we breathe out |
| Kidneys | Urea, with excess water and ions, lost in urine |
| Skin | Water, salts (ions) and urea, lost in sweat |
Carbon dioxide is a waste product of respiration. Urea is a nitrogen-containing waste product. The blood carries both to the organs that remove them.
Faeces are not excretory products. They are undigested food that has never been part of the cells' chemistry, so removing them is egestion, not excretion.
Always match the product to the organ: carbon dioxide from the lungs, urea from the kidneys. Sweat also contains urea, but its main job is cooling.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026