The Composition of Urine — Excretion | IGCSE Biology

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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:

Finding any of these in urine is a sign of disease. Glucose suggests diabetes; protein suggests kidney damage or very high blood pressure.

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