4BI1
Excretion
Structures and Functions in Living Organisms · 6 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 12% of your exam marks.
stable
High
Stable12%
The kidney and urea production appear regularly; dialysis is a common application question.
Different organs deal with different kinds of waste:
| Organ | What it excretes | How |
|---|---|---|
| Kidneys | Urea, excess water, and excess mineral ions | In urine |
| Lungs | Carbon dioxide and water vapour | In exhaled breath |
| Skin | Excess water, excess mineral ions (especially sodium), and a little urea | In sweat |
Note: the liver, although not strictly an excretory organ itself, is where is made. When excess amino acids are broken down (a process called ), the nitrogen-containing portion is converted into urea, which the blood then carries to the kidneys to be removed in urine.