Excretion and the kidney
How waste is removed and water balance is maintained.
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Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 13.1.1Describe the role of the kidneys in excretion and osmoregulation.
5 minute read
Excretion and the kidney
Excretion is the removal of the toxic waste products of metabolism and substances in excess of requirements.
The main excretory products
- Carbon dioxide from respiration, excreted by the lungs.
- Urea from the breakdown of excess amino acids, excreted by the kidneys.
- Excess water and salts, excreted by the kidneys (and in sweat).
Where urea comes from
Excess amino acids cannot be stored. In the liver they undergo deamination: the nitrogen-containing amino group is removed and converted into urea, and the rest can be respired. Urea is toxic, so it is carried in the blood to the kidneys and removed.
How the kidney works
Each kidney contains around a million tiny tubules. In each:
- Filtration — blood at high pressure is filtered. Small molecules (water, glucose, urea, salts) pass out; large ones (proteins) and blood cells stay in the blood.
- Selective reabsorption — useful substances are taken back. All the glucose is reabsorbed, along with the amount of water and salts the body needs.
- What remains — urea, excess water and excess salts — forms urine, which passes to the bladder.
The word selective carries the whole idea: filtration is indiscriminate, so reabsorption must be choosy.
Osmoregulation
The kidney also controls the water content of the blood. On a hot day, or after little drinking, more water is reabsorbed, producing a small volume of concentrated urine. After drinking a lot, less is reabsorbed, producing a large volume of dilute urine.
Think of it like this
The kidney tips out the whole handbag and then puts back only what is wanted. Filtering everything and selectively reclaiming is far more reliable than trying to pick out the rubbish item by item.
Worked examples
Method, step by step
Explain why a person who drinks very little water on a hot day produces a small volume of dark, concentrated urine.
- 1Sweating and low intake reduce the water content of the blood.
- 2The kidney responds by reabsorbing more water back into the blood from the tubules.
- 3Less water is left to dilute the urea and salts, so a small volume of concentrated urine is produced.
The blood has a low water content, so the kidneys reabsorb more water, leaving a small volume of concentrated urine and conserving water in the body.
Common misconceptions
- Thinking glucose is normally present in urine. All glucose is reabsorbed in a healthy person — glucose in urine is a sign of diabetes.
- Confusing excretion with egestion. Faeces are egested, not excreted, since they were never absorbed into the body's cells.
- Believing the kidneys make urea. The **liver** makes urea by deamination; the kidneys only remove it.
In the exam
- If asked why protein is not found in urine, the answer is that protein molecules are too large to pass through the filter.
- For urine concentration questions, state both the volume and the concentration — "a small volume of concentrated urine" is the expected phrasing.