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Learn0610Papers 1, 3 · CoreExcretion in humansKidneys and excretion
Excretion in humans · Subtopic 1 of 1

Kidneys and Excretion

Excretion is removing the waste products of metabolism. The lungs excrete carbon dioxide; the kidneys excrete urea, excess water and excess ions as urine. Urea is made in the liver by deamination of excess amino acids. In each kidney nephron, blood is filtered under high pressure in the glomerulus (ultrafiltration): small molecules pass into the tubule. Then all glucose, most water and some ions are selectively reabsorbed. What's left flows to the bladder. Kidney failure is treated by dialysis or a transplant.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define excretion and name excretory products and organs.
  • Describe deamination in the liver.
  • Describe ultrafiltration and selective reabsorption in a nephron.
  • Describe dialysis and kidney transplants.
Pause and think

Your kidneys filter about 180 litres of fluid a day, but you produce only about 1.5 litres of urine. Where does the rest go?

Section 2 of 14

Core idea

Start hereA filter that never stops

Your kidneys filter all your blood about 60 times a day, keeping what you need and removing what you don't.

The big idea

The liver turns excess amino acids into urea. The kidneys filter blood under pressure, then reabsorb the useful substances, leaving urea, excess water and ions as urine.

1. The nephron

How a nephron works

Filter everything small, take back the useful.

Worked examplePlasma, filtrate and urine

Glucose: plasma 0.1%, filtrate 0.1%, urine 0%. Urea: 0.03%, 0.03%, 2%.

Goal
Explain the figures.
Strategy
Filtration then reabsorption.
  1. same in plasma and filtrate\text{same in plasma and filtrate}
    Both are small molecules and are filtered freely.
  2. glucose→0\text{glucose}\rightarrow0
    All reabsorbed.
  3. urea×≈67\text{urea}\times\approx67
    Not reabsorbed; concentrated as water is reabsorbed.
AnswerGlucose fully reabsorbed; urea concentrated.
✓ CheckProteins would be 0 in both filtrate and urine ✓.
Key takeawayDialysis Blood flows past a partially permeable membrane. Dialysis fluid matches normal blood glucose and ion levels but contains no urea, so only urea (and excess ions) diffuse out.
Pause and think

Why does eating a lot of protein increase urea production?

Summary

  • Liver: urea.
  • Glomerulus: ultrafiltration.
  • Tubule: selective reabsorption.
Key words
Nephron
Filtering unit of the kidney.
Ultrafiltration
Filtration of blood under high pressure.
Deamination
Removal of nitrogen from amino acids.
HistoryWillem Kolff. In 1943, in occupied Netherlands, Willem Kolff built the first dialysis machine from sausage casings, orange-juice cans and a washing machine.
Fun factOne kidney is enough. You can live a normal life with one healthy kidney, which is why living people can donate a kidney to a relative.
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