Excretion in humans — Cambridge IGCSE Biology
Excretion of carbon dioxide, urea, water and ions; the urinary system, the nephron, and the liver's role in forming urea.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Excretion is the removal of the waste products of metabolism and substances in excess of requirements. The lungs excrete carbon dioxide made in respiration, and the kidneys excrete urea, excess water and excess ions in urine. Urine passes from the kidneys down the ureters to the bladder, where it is stored, then leaves through the urethra.
The kidney and the nephron
SupplementA nephron is a tiny tube in the kidney that filters the blood and forms urine. At the glomerulus, high pressure forces water, glucose, urea and ions out of the blood, while proteins and blood cells stay behind. All the glucose, some ions and most of the water are reabsorbed, leaving urea, excess water and excess ions as urine.
The liver, deamination and urea
SupplementDeamination is the removal of the nitrogen-containing part of amino acids to form urea. It happens in the liver, which breaks down amino acids in excess of requirements. The urea passes into the blood and is carried to the kidneys, which excrete it in urine. Urea must be excreted because it is toxic and would build up.
Exam questions
7 exam-style questions (21 marks), each with its mark scheme.
Answer the questionsFlashcards
13 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of excretion in humans on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Which organ excretes carbon dioxide?
Show answer
The lungs.
Name the tube that carries urine from the bladder out of the body.
Show answer
The urethra.
Supplement Name the two regions seen in a section through a kidney.
Show answer
The cortex (outer) and the medulla (inner).
Supplement Which substance is all reabsorbed into the blood from the nephron?
Show answer
Glucose.
Supplement Where is urea formed, and from what?
Show answer
In the liver, from excess amino acids.
Frequently asked questions
How does the kidney filter blood?
Supplement The kidney filters blood in tiny tubes called nephrons. Blood in the glomerulus is under high pressure, so water, glucose, urea and ions are forced out into the nephron, while proteins and blood cells stay in the blood. All the glucose, some ions and most of the water are reabsorbed, and what is left becomes urine.
What is the difference between the ureter and the urethra?
The ureter carries urine from the kidney to the bladder, while the urethra carries urine from the bladder out of the body. Each kidney has its own ureter, and the bladder stores the urine until it leaves. Students often mix the two up, so link the ureter with the kidney and the urethra with the way out.
Why is there no glucose in urine?
Supplement There is normally no glucose in urine because all of it is reabsorbed into the blood from the nephron. Glucose is small enough to be filtered out of the blood at the glomerulus, so it does enter the nephron. It is then all taken back into the blood as the filtered liquid passes along the nephron.
Where is urea made in the body?
Supplement Urea is made in the liver by deamination. The liver removes the nitrogen-containing part of amino acids that are in excess of requirements, and this part forms urea. The urea passes into the blood and is carried to the kidneys, which filter it out of the blood so that it leaves the body in urine.
Why is excretion important?
Excretion is important because it removes the waste products of metabolism and substances in excess of requirements, which could harm the body if they built up. The lungs remove carbon dioxide from respiration. The kidneys remove urea, excess water and excess ions in urine. Urea is toxic, so it must not build up in the blood.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026