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.
In a healthy person, urine is roughly:
| Substance | What it is | Where it came from |
|---|---|---|
| Water (about 95%) | Solvent | Filtrate that was not reabsorbed |
| Urea | The body's main nitrogen-containing waste | Made in the liver from excess amino acids |
| Excess ions | Mostly sodium, potassium and chloride | Filtered from blood; only the excess is excreted |
| Other small wastes | Creatinine, broken-down drug molecules, hormone breakdown products | Various metabolic reactions |
Urine should not normally contain:
- Glucose (all reabsorbed in the PCT)
- Amino acids (all reabsorbed)
- Proteins (too large to be filtered)
- Blood cells (too large to be filtered)
Finding any of these in urine is a sign of disease. Glucose suggests diabetes; protein suggests kidney damage or very high blood pressure.
What changes the colour and volume of urine
The volume and concentration of urine can change rapidly:
- More fluid drunk = larger volume of paler urine
- Hot weather or vigorous exercise = more water lost as sweat, so a smaller volume of darker urine
- Cold weather = less sweating, so more urine
- Salty diet = body needs to lose excess salt, often as concentrated urine
- Caffeine and alcohol = both block or reduce ADH release, producing larger volumes of paler urine and contributing to dehydration