Excretion in Humans
The body’s chemical reactions produce waste that would become toxic if it built up. Excretion removes it.
This is a short topic built on two processes in the kidney — ultrafiltration and selective reabsorption — and on a set of words that are easy to confuse with each other.
1. What excretion is
Excretion is the removal of the waste products of metabolism, toxic materials, and substances in excess of requirements.
The main excretory products:
| Product | Where it comes from | How it leaves |
|---|---|---|
| Carbon dioxide | Respiration | Lungs |
| Urea | Breakdown of excess amino acids in the liver | Kidneys, in urine |
| Excess water and ions | Diet | Kidneys, in urine (and skin, in sweat) |
Excretion is not egestion. Excretion removes substances the body made during its own chemical reactions. Egestion removes undigested food, which was never absorbed into the body and never took part in metabolism. Faeces is egested, not excreted.
2. The liver and deamination
Excess amino acids cannot be stored by the body, so they must be broken down.
Deamination is the removal of the nitrogen-containing part of amino acids, which is then converted to urea.
The sequence:
- Excess amino acids reach the liver
- The nitrogen-containing amino group is removed — this is deamination
- This produces ammonia, which is highly toxic
- The liver converts ammonia into urea, which is much less toxic
- Urea is carried in the blood to the kidneys, where it is removed in urine
Depth of answer should match the marks. For one or two marks, “the breakdown of excess amino acids to form urea” is enough. For three, you need the full chain: nitrogen-containing group removed → ammonia → converted to urea. Naming ammonia as the intermediate is what usually separates a 2 from a 3.
The liver also breaks down alcohol and other toxins, and breaks down old red blood cells.
3. The urinary system
| Structure | Function |
|---|---|
| Kidneys | Filter the blood, producing urine |
| Ureters | Carry urine from the kidneys to the bladder |
| Bladder | Stores urine |
| Urethra | Carries urine from the bladder out of the body |
Ureter and urethra are different tubes and only one letter apart. Ureter: kidney → bladder (there are two). Urethra: bladder → outside (there is one). Also note the sperm duct carries sperm, and the urethra in males carries urine or semen — never both at once.
Blood is brought to the kidney by the renal artery and leaves by the renal vein. Blood in the renal vein contains less urea and, usually, less water and fewer ions than blood in the renal artery.
4. How the kidney works
Each kidney contains around a million tiny tubules called nephrons. Two processes happen in each:
Ultrafiltration
Takes place in the glomerulus — a knot of capillaries — and the Bowman’s capsule surrounding it.
- Blood arrives at high pressure
- Small molecules are forced out of the blood into the capsule: water, glucose, urea, ions
- Large molecules stay in the blood: proteins and blood cells — they are too big to pass through
The liquid formed is the filtrate.
Ultrafiltration is not selective. It filters purely by size — anything small enough goes through, useful or not. That is why glucose, which the body needs, ends up in the filtrate and must be recovered afterwards. Being clear about this makes selective reabsorption make sense.
Selective reabsorption
As the filtrate passes along the nephron, useful substances are reabsorbed back into the blood:
| Substance | What happens |
|---|---|
| Glucose | All of it is reabsorbed |
| Some water | Reabsorbed, according to the body’s needs |
| Some ions | Reabsorbed, according to the body’s needs |
| Urea | Not reabsorbed — it is a waste product |
What remains — water, urea and excess ions — is urine.
All the glucose is reabsorbed, but not all the water. A statement like “the nephron reabsorbs all the water and some of the ions” is wrong, because urine would then be impossible. The reliable version: all the glucose, some of the water, some of the ions.
Glucose in urine is a sign of diabetes. If blood glucose is very high, not all of it can be reabsorbed, so some appears in the urine — which is why urine tests are used to detect it.
5. Comparing the fluids
Questions frequently give a table of three fluids and ask you to explain the differences:
| Substance | Blood plasma | Filtrate (in Bowman’s capsule) | Urine |
|---|---|---|---|
| Protein | Present | Absent | Absent |
| Glucose | Present | Present | Absent |
| Urea | Present (low) | Present | Present (concentrated) |
| Water | Present | Present | Present (less) |
The two explanations you need:
- No protein in the filtrate — protein molecules are too large to pass through during ultrafiltration
- No glucose in urine — all glucose is selectively reabsorbed into the blood
- Urea is more concentrated in urine — because water has been reabsorbed while urea has not
6. Mistakes that cost marks
Confusing excretion with egestion.
Mixing up ureter and urethra.
Saying ultrafiltration is selective. It filters by size; reabsorption is the selective step.
Saying all the water is reabsorbed. All the glucose is; only some water.
Saying protein is reabsorbed. Protein never entered the filtrate — it is too large to be filtered.
Saying the kidney “cleans the blood” without naming ultrafiltration and selective reabsorption.
Giving deamination without ammonia in a three-mark answer.
Saying urea is made in the kidney. It is made in the liver; the kidney removes it.
Frequently asked questions
What is excretion? The removal of the waste products of metabolism, toxic materials, and substances in excess of requirements.
What is the difference between excretion and egestion? Excretion removes substances produced by the body’s own reactions. Egestion removes undigested food that was never absorbed.
Where is urea made? In the liver, by deamination of excess amino acids. The kidney removes it.
What is deamination? Removal of the nitrogen-containing part of excess amino acids, forming ammonia, which the liver converts to urea.
What is the difference between the ureter and the urethra? The ureter carries urine from kidney to bladder. The urethra carries it from bladder to outside.
What is ultrafiltration? Filtration of the blood at the glomerulus under high pressure, where small molecules pass into the Bowman’s capsule and large ones — proteins and blood cells — stay behind.
Why is there no glucose in urine normally? All of it is selectively reabsorbed into the blood.
Why does urine contain no protein? Protein molecules are too large to be filtered out of the blood in the first place.
Quick revision checklist
- I can define excretion and distinguish it from egestion
- I can name the excretory products and the organ that removes each
- I can explain deamination in full, including ammonia
- I know urea is made in the liver, not the kidney
- I can name the parts of the urinary system and their functions
- I can distinguish ureter from urethra
- I can explain ultrafiltration, including why proteins stay behind
- I know ultrafiltration filters by size, not usefulness
- I can explain selective reabsorption: all glucose, some water, some ions
- I can explain a plasma/filtrate/urine table and the reason for each difference
- I know why glucose in urine indicates diabetes
These notes cover topic 13 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. The corpus of lessons behind this project covered this topic more lightly than most, so this page follows the syllabus structure closely; the flagged errors and wording notes are drawn from the lessons that did cover it.
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