IGCSE Biology: Excretion in Humans Practice Questions
Excretion is the removal of the toxic waste products of metabolism, together with substances in excess of requirements. The kidneys remove urea, excess water and excess salts, filtering the blood and then reabsorbing what the body needs.
Topic 13 of Cambridge IGCSE Biology 0610 is one of the most misused vocabulary topics on the paper. Excretion, egestion and secretion are three different things, and the difference between filtration and selective reabsorption decides several marks. The questions below work through both.
What you need to know for Excretion in Humans
- ExcretionThe removal from organisms of the toxic materials, the waste products of metabolism and substances in excess of requirements. Egestion is different: it is the removal of undigested food, which has never entered the cells.
- DeaminationExcess amino acids cannot be stored, so in the liver the nitrogen containing part is removed and converted into urea. The remainder can be respired or converted to fat or carbohydrate.
- Filtration in the kidneyBlood at high pressure in the glomerulus is filtered. Water, glucose, urea and salts pass into the renal capsule. Blood cells and large protein molecules are too big to pass through and stay in the blood.
- Selective reabsorptionAs the filtrate passes along the nephron, all the glucose, most of the water and some ions are reabsorbed back into the blood. Urea is not reabsorbed.
- UrineThe liquid remaining after reabsorption. It contains urea, excess water and excess salts, and it should contain no glucose or protein.
- Dialysis and transplantDialysis uses a partially permeable membrane and dialysis fluid to remove urea and excess salts from the blood. A transplant replaces the failed kidney with a donor kidney.
IGCSE Biology Excretion in Humans questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Describe how urea is formed in the body and explain why the body cannot simply store excess amino acids.
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- Proteins in the diet are digested to amino acids, which are absorbed into the blood and used to build new proteins.
- Amino acids cannot be stored in the body, so any that are surplus to requirements must be broken down.
- In the liver the nitrogen containing amino group is removed. This process is deamination.
- The amino group is converted into urea, which is toxic in high concentrations and is carried in the blood to the kidneys for excretion. The remainder of the molecule is respired or converted to fat or carbohydrate.
Explain the difference between filtration and selective reabsorption in a kidney nephron, and name one substance involved in each.
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- Blood enters the glomerulus at high pressure, forcing small molecules through into the renal capsule. Water, glucose, urea and salts all pass through.
- Blood cells and large protein molecules are too large to pass through, so they remain in the blood. Filtration is not selective, it simply separates by size.
- As the filtrate flows along the nephron, useful substances are reabsorbed back into the blood. All the glucose is reabsorbed, along with most of the water and some ions.
- Reabsorption is selective because only certain substances are taken back. Urea is left behind and passes out in the urine.
A urine sample from a patient contains glucose. Explain why this is a sign that something is wrong.
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- Glucose is small enough to be filtered out of the blood at the glomerulus, so it is normally present in the filtrate.
- In a healthy person all of the glucose is then reabsorbed into the blood during selective reabsorption, so none appears in the urine.
- Glucose in the urine therefore means either the blood glucose concentration was so high that reabsorption could not keep up, which suggests diabetes, or the reabsorption mechanism is damaged.
- Blood glucose concentration should be tested to distinguish between the two.
State two advantages of a kidney transplant compared with regular dialysis.
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- Dialysis requires several sessions each week, each lasting hours, which restricts work, school and travel. A transplanted kidney works continuously.
- Dialysis patients must follow a strict diet controlling protein, salt and fluid intake. After a successful transplant the diet is much less restricted.
- A transplant is also cheaper in the long term than years of dialysis.
- The main disadvantage is that the transplanted kidney may be rejected, so immunosuppressant drugs are needed.
Common mistakes in this topic
- Confusing excretion with egestion. Undigested food leaving the anus has never been inside a cell, so it is egested, not excreted.
- Saying urea is produced in the kidney.
- Writing that protein is filtered into the nephron. Protein molecules are too large.
- Forgetting that carbon dioxide is also an excretory product, removed by the lungs.
- Describing reabsorption as happening in the glomerulus. Filtration happens there, reabsorption happens further along the nephron.
Exam tips
- Write a one line definition of excretion, egestion and secretion and learn all three together. Cambridge tests the distinction directly.
- For nephron questions, follow the liquid: blood, glomerulus, capsule, tubule, collecting duct, ureter, bladder.
- Say all the glucose, most of the water and some of the ions. The quantifiers carry marks.
- In dialysis questions, explain that the dialysis fluid contains the same concentration of glucose and salts as healthy blood so those are not lost by diffusion.
- Link the concentration of urine to water intake and the weather. Hot days give small volumes of concentrated urine.
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Excretion in Humans FAQs
What is the difference between excretion and egestion?
Excretion is the removal of the toxic waste products of metabolism and substances in excess of requirements, such as urea, carbon dioxide and excess salts. Egestion is the removal of undigested food as faeces. Undigested food has never been absorbed into cells, so it is not a product of metabolism.
Where is urea made in the body?
Urea is made in the liver. Excess amino acids cannot be stored, so the liver removes the nitrogen containing part in a process called deamination and converts it to urea. The urea is then carried in the blood to the kidneys, which remove it from the blood and excrete it in urine.
What is selective reabsorption?
Selective reabsorption is the return of useful substances from the filtrate in the nephron back into the blood. All the glucose, most of the water and some mineral ions are reabsorbed. Urea is not reabsorbed, so it stays in the tubule and leaves the body in the urine.
Why does dialysis fluid contain glucose and salts?
Dialysis fluid contains glucose and mineral ions at the same concentration as healthy blood. Because there is no concentration gradient for those substances, they are not lost from the blood by diffusion. Urea is absent from the fluid, so there is a steep gradient and urea diffuses out of the blood.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.