Excretion in Humans
SEAB syllabus topic 7: what excretion is, the urinary system, how the nephron makes urine by ultrafiltration and selective reabsorption, and how dialysis treats kidney failure, with the why, the traps and the exam checks.
Excretion removes the wastes that the cells make, such as urea and carbon dioxide, so that they do not harm the body.
The nephron makes urine by filtering the blood under high pressure and then taking back the useful substances.
Dialysis does the job of failed kidneys by removing urea from the blood by diffusion through a partially permeable membrane.
Excretion and nitrogenous waste
define excretion and explain the importance of removing nitrogenous and other compounds from the body
Excretion is the removal from the body of the waste products of metabolism, toxic materials and substances in excess of requirements.
- Metabolism
- Metabolism: all the chemical reactions that take place in the cells of a living organism supports K325-7(a)
- Metabolic waste
- Metabolic waste: a substance made by the chemical reactions in cells that the body does not need, such as urea and carbon dioxide
- Toxic substance
- Toxic substance: a substance that can harm or poison the body if it builds up
- Nitrogenous waste
- Nitrogenous waste: a waste substance that contains nitrogen, such as urea supports K325-7(a)
- Deamination
- Deamination: the removal of the amino group from an amino acid in the liver, which is then turned into urea
- Urea
- Urea: the nitrogenous waste made in the liver from excess amino acids, which is carried in the blood plasma to the kidneys and removed in the urine
- Egestion
- Egestion: the removal of undigested food from the alimentary canal as faeces; it is not excretion, because undigested food has never been through the metabolism of the body's cells supports K325-7(a)
Why wastes must be removed
Urea
Urea is toxic, so if it built up in the blood and the tissues it would damage the cells, and this is why the kidneys must remove it.
Carbon dioxide
Carbon dioxide dissolves in the blood to form an acid, so a build-up would lower the pH of the blood and harm the cells.
Excess water and mineral salts
If excess water and excess mineral salts were not removed, the concentration of the blood and the tissue fluid would change, and cells could gain or lose water by osmosis.
Bile pigments
Bile pigments are made in the liver when old red blood cells are broken down, and they would turn the skin yellow if they built up.
Small organisms and large organisms
Unicellular organisms
A unicellular organism has a large surface area to volume ratio, so its wastes can leave by diffusion across the cell surface membrane.
Multicellular organisms
A multicellular organism has a smaller surface area to volume ratio, so diffusion alone is too slow and it needs excretory organs to remove its wastes.
Wastes and how they leave the body
| Waste substance | Where it comes from | How it leaves the body |
|---|---|---|
| Carbon dioxide | Carbon dioxide is made in the cells during respiration. | The lungs remove carbon dioxide from the body when we breathe out. |
| Urea | Urea is made in the liver by the deamination of excess amino acids. | The kidneys remove urea from the blood and it leaves the body in the urine, and the skin removes a small amount of urea in sweat. |
| Excess water | Excess water comes from food and drink, and some is also made in respiration. | Excess water leaves the body in the urine, in sweat from the skin and as water vapour in the breath. |
| Excess mineral salts | Excess mineral salts come from the food and drink that we take in. | Excess mineral salts leave the body in the urine and in sweat. |
| Bile pigments | Bile pigments are made in the liver from the breakdown of haemoglobin in old red blood cells, and they pass into the bile. | Bile pigments pass along the alimentary canal and leave the body in the faeces. |
Depth: why and how
- Why is urea made from amino acids? The body cannot store excess amino acids, so the liver removes the amino group by deamination, which forms urea, and the rest of each amino acid is used in respiration or stored as glycogen or fat.
- Why is urea carried in the blood plasma? Urea is soluble in water, so it dissolves in the plasma and is carried from the liver to the kidneys.
- Why are the kidneys, lungs, skin and liver all needed for excretion? Each waste substance is made in a different way and leaves the body by a different route, so no single organ can remove them all.
- Cross-link to Nutrition in humans: the liver carries out the deamination of excess amino acids and forms urea, as in K325-4(f).
- Cross-link to Respiration in humans: carbon dioxide is a waste product of respiration, and the lungs remove it in gaseous exchange.
- Cross-link to Homeostasis, co-ordination and response in humans: removing excess water and mineral salts helps to keep the internal environment of the body stable.
Common mistakes
Excretion is the removal of faeces from the body.
Faeces are mainly undigested food (with water and bacteria), which is egested, not excreted, because it never entered the cells; the small amount of bile pigments they carry is a waste product of metabolism.
Urea is made in the kidneys.
Urea is made in the liver by deamination, and the kidneys remove it from the blood.
The kidneys are the only excretory organ.
The lungs, the skin and the liver also take part in excretion, in addition to the kidneys.
Sweating is just for cooling the body.
Sweat also removes excess water, mineral salts and a small amount of urea, so sweating has an excretory role as well as a cooling role.
Excretion CUBS: Carbon dioxide is removed by the lungs, Urea, the nitrogenous waste, is removed by the kidneys, Bile pigments leave in the faeces, Salts and excess water leave in urine and sweat.
The four letters C, U, B and S are four separate wastes and the organ or route for each; they are not an order of events, and the salts and water can also leave in other ways, as the table shows.Excretion removes the waste products of metabolism, such as urea and carbon dioxide, together with toxic materials and excess substances, because they would harm the cells if they built up.
Exam check
Q1 Which substance is a nitrogenous waste product?
- A Carbon dioxide
- B Urea
- C Glucose
- D Starch
Show answer
MCQ answer key B (Urea): urea contains nitrogen, and it is made in the liver when the amino group is removed from excess amino acids.
Q2 Which of these is an example of excretion?
- A Passing out undigested food as faeces
- B Absorbing glucose in the small intestine
- C Breathing in oxygen
- D Removing carbon dioxide from the body through the lungs
Show answer
MCQ answer key D (Removing carbon dioxide from the body through the lungs): carbon dioxide is a waste product of respiration, so removing it through the lungs is excretion, whereas passing out undigested food is egestion.
Q3 Why must urea be removed from the body?
- A It is toxic and would damage cells if it built up
- B It is needed for making proteins
- C It is a source of energy for the cells
- D It is undigested food
Show answer
MCQ answer key A (It is toxic and would damage cells if it built up): urea is toxic, so it would damage the cells if it built up in the body.
Structured Amino acids that the body does not need are broken down in the liver.
- Name the process in the liver that removes the amino group from an amino acid. [1]
- Name the nitrogenous waste product that is formed. [1]
- Name the organ that removes this waste substance from the blood, and name one other excretory organ with the substance that it removes. [2]
- Explain why the removal of urea is important. [2]
Total: 6 marks
Show mark scheme
(a) 1 mark
- Mark scheme: deamination
(b) 1 mark
- Mark scheme: urea
(c) 2 marks
- Mark scheme: the kidneys
- Mark scheme: any one of: the lungs and carbon dioxide, OR the skin and water, mineral salts or a little urea in sweat, OR the liver and bile pigments
(d) 2 marks
- Mark scheme: urea is toxic
- Mark scheme: so it would damage the cells if it built up in the body
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- The urinary system
- The nephron: ultrafiltration and selective reabsorption
- Dialysis for kidney failure
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Notes version 1.0 · Last updated 30 Sept 2026 · Topic 7 of 14