Cambridge O Level Biology 5090: Excretion (5090)

This study guide teaches Topic 13, Excretion, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in section 13.1 Excretion (13.1.1 to 13.1.3) and section 13.2 Urinary system (13.2.1 to 13.2.6). The 5090 syllabus is not tiered, so every candidate studies all of it. This topic lists no set practical, but you can be asked to interpret data and diagrams.

Use it with the revision notes for this topic and the practice questions. The course hub lists every topic, and the printable checklist lets you tick off each outcome. Assimilation and the hepatic portal vein come from Human nutrition, and carbon dioxide leaving the body is the gas exchange you met in Human gas exchange. To find your weak spots quickly, try one of the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
13.1.1	Describe excretion as the removal of toxic materials and the waste products of metabolism from organisms
13.1.2	State that carbon dioxide is a waste product of respiration, excreted through the lungs
13.1.3	State that urea is a toxic waste product made in the liver from the breakdown of excess amino acids
13.2.1	Identify the kidneys, ureters, bladder and urethra on diagrams and state the function of each
13.2.2	Explain the need for excretion, limited to the toxicity of urea
13.2.3	Outline the structure of a nephron and its blood vessels: Bowman’s capsule, glomerulus, tubules, loop of Henle, collecting duct
13.2.4	Outline what the nephron does: filtration at the glomerulus, reabsorption, and formation of urine (details of the processes not required)
13.2.5	Describe the role of the liver in the assimilation of amino acids by converting them to proteins
13.2.6	Describe deamination in the liver, forming urea

13.1.1 What excretion means

Excretion is the removal of toxic materials and the waste products of metabolism from organisms.

Metabolism is all the chemical reactions in a cell. Respiration and deamination are both metabolic reactions, and both make waste. The three excretory products in this topic are:

Excretory product	Made by	Leaves the body through
Carbon dioxide	Respiration, in every living cell	Lungs
Urea	Deamination, in the liver	Kidneys, in urine
Excess salts (ions) and excess water	Taken in with food and drink, beyond what the body needs	Kidneys, in urine

Excretion is not egestion

Egestion is the removal of undigested food as faeces, through the anus. Most of the material in faeces, such as fibre, was never absorbed into the blood and never took part in reactions inside cells. So removing it is not excretion. If a question asks which process removes urea, the answer is excretion; if it asks which process removes fibre, the answer is egestion.

13.1.2 Carbon dioxide and the lungs

Every living cell carries out respiration, which releases energy and makes carbon dioxide as a waste product. The route out is:

body cell → diffuses into blood plasma → carried to the lungs → diffuses into the alveoli → breathed out

The lungs are therefore an excretory organ as well as the gas exchange organ. The diffusion across the alveolar wall is taught in Human gas exchange. Carbon dioxide is one of the substances the plasma transports, along with urea.

13.2.5 The liver and amino acids: assimilation

Proteins in food are digested to amino acids, which are absorbed in the small intestine. The hepatic portal vein carries most absorbed molecules to the liver.

Assimilation is the uptake and use by cells of nutrients from the blood. Liver cells assimilate amino acids by converting them to proteins. They build the amino acids into new protein molecules, such as plasma proteins, which are released into the blood.

13.1.3 and 13.2.6 Deamination and urea

The body takes in more amino acids than it needs for making proteins, and it cannot store the excess. The liver breaks the excess down.

Deamination is the removal of the nitrogen-containing part of amino acids, in the liver, resulting in the formation of urea.

excess amino acid (in liver cell)
      │  deamination
      ├──► nitrogen-containing part ──► converted to UREA ──► into the blood
      └──► rest of the molecule ──► can be used, for example in respiration

Points to remember:

• The liver deaminates excess amino acids and makes urea. The kidneys do not make urea; they remove it.
• Urea is a toxic waste product of metabolism, so its removal counts as excretion.
• Urea dissolves in water, so it is carried in the blood plasma from the liver to the kidneys. Water acting as a solvent is part of Biological molecules.

Worked example 1: following one nitrogen atom. A nitrogen atom starts in a protein in a piece of fish. Describe its route until it leaves the body.

1. Protease enzymes digest the protein to amino acids in the stomach and small intestine.
2. The amino acids are absorbed into the blood in the small intestine.
3. The hepatic portal vein carries them to the liver.
4. If the amino acid is not needed, the liver deaminates it: the nitrogen-containing part is removed and turned into urea.
5. Urea enters the blood and leaves the liver in the hepatic vein, then travels via the vena cava, the heart, the lungs and the heart again to the aorta and the renal artery.
6. In the kidney, urea is filtered out of the blood at the glomerulus and stays in the tubule.
7. It leaves in urine, down the ureter to the bladder, and out through the urethra.

13.2.2 Why urea must be excreted

The syllabus limits this outcome to one reason: urea is toxic. If urea were not removed, it would be made faster than it was lost, so its concentration in the blood and tissue fluid would rise until it damaged body cells. Excretion keeps the concentration low.

Worked example 2: interpreting data. A person’s kidneys stop working. Their blood urea concentration is 0.25 g/dm³ on day 0 and 1.5 g/dm³ on day 5. Calculate the percentage increase and explain the change.

• Increase = 1.5 − 0.25 = 1.25 g/dm³.
• Percentage increase = 1.25 ÷ 0.25 × 100 = 500%.
• Explanation: the liver keeps deaminating excess amino acids and making urea, but the kidneys no longer remove it from the blood, so urea accumulates. Because urea is toxic, a high concentration is harmful to cells.

13.2.1 The urinary system

Identifying the organs

A front view of the human urinary system looks like this:

   [ kidney ]          [ kidney ]      ← at the back of the abdomen, one each side
        \                  /
      ureter            ureter         ← two narrow tubes running down
          \              /
           [  bladder  ]               ← muscular bag low in the abdomen
                 |
              urethra                  ← single tube to the outside

On a diagram: there are two kidneys and two ureters, but only one bladder and one urethra. The ureters join the bladder; the urethra leaves it at the bottom.

Functions

Organ	Function
Kidney	Removes urea and excess salts and water from the blood, as urine
Ureter	Carries urine from the kidney to the bladder
Bladder	Stores urine
Urethra	Carries urine from the bladder out of the body

Use the syllabus wording for the kidney: “removes urea and excess salts and water from the blood as urine”. Blood reaches each kidney in the renal artery and leaves in the renal vein. Blood in the renal vein contains less urea than blood in the renal artery.

13.2.3 The structure of a nephron

Each kidney contains a very large number of tiny tubes called nephrons. Each nephron makes urine. The parts, in the order fluid passes through them:

 arteriole in ──►(glomerulus)──► arteriole out ──► capillaries around tubules ──► renal vein
                  inside
        ┌─[ Bowman's capsule ]
        │
        └─► first coiled tubule ─► loop of Henle ─► second coiled tubule ─► collecting duct ─► ureter

Part	What it is
Glomerulus	A knot of blood capillaries, supplied by a branch of the renal artery
Bowman’s capsule	The cup-shaped start of the nephron, wrapped around the glomerulus
Tubules	The coiled tubes that carry fluid away from the capsule and on from the loop
Loop of Henle	A U-shaped section of tubule between the two coiled parts
Collecting duct	A tube that receives fluid from many nephrons and carries urine on towards the ureter

Associated blood vessels. Blood enters the glomerulus in a small arteriole (a branch of the renal artery) and leaves in another arteriole. That vessel then branches into capillaries that wrap around the tubules, which is where reabsorbed substances re-enter the blood. These capillaries drain into the renal vein.

13.2.4 How a nephron makes urine

The syllabus wants an outline only: the details of these processes are not required.

(a) Filtration at the glomerulus

Water, glucose, urea and ions are filtered out of the blood in the glomerulus into the Bowman’s capsule. The fluid in the capsule is the filtrate. Blood cells and large protein molecules stay in the blood.

(b) Reabsorption along the nephron

As the filtrate flows along the tubules, useful substances are taken back into the blood in the surrounding capillaries:

• all of the glucose
• some of the ions
• most of the water

(c) Urine

What is left flows down the collecting duct as urine, containing urea, excess water and excess ions. Healthy urine contains no glucose, because all of it was reabsorbed.

Worked example 3: comparing fluids. Concentrations (g/dm³) given for one person:

Substance	Blood plasma	Filtrate	Urine
Glucose	1.0	1.0	0
Urea	0.30	0.30	18
Protein	70	0	0

(a) Explain why glucose is present in the filtrate but not in the urine.

• Glucose molecules are small, so glucose is filtered out at the glomerulus. All of it is then reabsorbed into the blood, so none is left in urine.

(b) Explain why there is no protein in the filtrate.

• Protein molecules are too large to be filtered at the glomerulus, so they stay in the blood.

(c) Calculate how many times more concentrated urea is in urine than in plasma, and explain why.

• 18 ÷ 0.30 = 60 times.
• Urea is filtered but not reabsorbed, while most of the water is reabsorbed. The same mass of urea ends up in a much smaller volume of water.

Worked example 4: how much water is reabsorbed? Use these given figures: the kidneys produce filtrate at 120 cm³ per minute and urine at 1.2 cm³ per minute.

• Water reabsorbed each minute ≈ 120 − 1.2 = 118.8 cm³ (treating filtrate and urine volumes as water).
• Percentage reabsorbed = 118.8 ÷ 120 × 100 = 99.0%.
• Filtrate per day = 120 × 60 × 24 = 172 800 cm³ = 172.8 dm³; urine per day = 1.728 dm³.

This is why the syllabus says most of the water is reabsorbed.

Putting the topic together

Organ	Role in excretion
All body cells	Make carbon dioxide in respiration
Liver	Assimilates amino acids into proteins; deaminates excess amino acids; makes urea
Blood plasma	Carries carbon dioxide to the lungs and urea to the kidneys
Lungs	Excrete carbon dioxide
Kidneys	Filter blood and excrete urea, excess salts and excess water as urine

Common errors

• Saying the kidneys make urea. The liver makes it; the kidneys remove it.
• Calling faeces an excretory product of digestion. Removing undigested food is egestion.
• Mixing up ureter (kidney to bladder, two of them) and urethra (bladder to outside, one).
• Writing that glucose is “not filtered”. It is filtered; it is then all reabsorbed.
• Writing that “all the water” or “all the ions” are reabsorbed. The syllabus says most of the water and some of the ions.
• Describing deamination as “removing amino acids”. It removes the nitrogen-containing part of each amino acid.
• Confusing urea (the chemical) with urine (the solution of urea, ions and water).
• Leaving out the reason for excreting urea: the 13.2.2 answer is that urea is toxic.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 13: Excretion (13.1 Excretion; 13.2 Urinary system), published by Cambridge Assessment International Education.