Excretion — Edexcel International GCSE Biology
Excretory products of the lungs, kidneys and skin, and how the kidney filters blood, reabsorbs useful substances and controls water with ADH.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Excretion is the removal from the body of the waste products of metabolism. In humans the lungs excrete carbon dioxide, a waste product of respiration. The kidneys excrete urea, a nitrogen-containing waste, with excess water and ions in urine. The skin loses water, salts and urea in sweat. Faeces are egested, not excreted.
The kidney and the urinary system
Biology onlyThe kidneys have two jobs: excretion, removing urea from the blood, and osmoregulation, controlling the water content and ion content of the blood. Both happen as the kidneys make urine. The renal artery brings blood to each kidney, ureters carry urine to the bladder, which stores it, and the urethra carries urine out of the body.
The nephron and ultrafiltration
Biology onlyA nephron is a tiny tube in the kidney that filters the blood and forms urine. In ultrafiltration, high blood pressure in the glomerulus forces small molecules, such as water, glucose, urea and ions, into the Bowman's capsule, forming the glomerular filtrate. Proteins and blood cells are too large to pass through, so they stay in the blood.
Selective reabsorption and urine
Biology onlySelective reabsorption is the return of useful substances from the glomerular filtrate to the blood. All the glucose is reabsorbed at the proximal convoluted tubule by active transport, because cells need it for respiration. Water is reabsorbed from the collecting duct by osmosis. What remains is urine, which contains water, urea and ions but no glucose.
ADH and the water content of the blood
Biology onlyADH (antidiuretic hormone) is released into the blood by the pituitary gland and controls the water content of the blood. When the blood water content is too low, more ADH is released, making the collecting duct more permeable to water. More water is reabsorbed into the blood, so a small volume of concentrated urine is made.
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Quick check questions
Name the excretory product of the lungs.
Show answer
Carbon dioxide.
Name three substances lost from the skin in sweat.
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Water, salts (ions) and urea.
Biology only Which part of the nephron does ultrafiltration take place in?
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The Bowman's capsule (from the glomerulus).
Biology only Where is glucose reabsorbed, and by what process?
Show answer
The proximal convoluted tubule, by active transport.
Biology only What does ADH do to the collecting duct?
Show answer
It makes the collecting duct more permeable to water, so more water is reabsorbed.
Frequently asked questions
What is the difference between excretion and egestion?
Excretion is the removal of the waste products of metabolism, such as carbon dioxide from the lungs and urea from the kidneys. Egestion is the removal of faeces, which are undigested food that was never part of the cells' chemistry. So faeces are not excretory products, but urine and the carbon dioxide we breathe out are.
How does the kidney filter blood?
Biology only The kidney filters blood by ultrafiltration in its nephrons. Blood in the glomerulus is at high pressure, which forces small molecules such as water, glucose, urea and ions into the Bowman's capsule, forming the glomerular filtrate. Proteins and blood cells are too large to pass through, so they stay in the blood.
Why is there no glucose in normal urine?
Biology only Normal urine contains no glucose because all the glucose in the glomerular filtrate is reabsorbed into the blood at the proximal convoluted tubule. This happens by active transport, which uses energy from respiration. Cells need glucose as a fuel for respiration, so the body must not lose any of it in urine.
What does ADH do in the kidneys?
Biology only ADH makes the collecting duct of the kidney more permeable to water, so more water is reabsorbed into the blood by osmosis. When the blood water content is too low, the pituitary gland releases more ADH, and a small volume of concentrated urine is made. When it is too high, less ADH is released and the urine is dilute.
What is osmoregulation in the kidneys?
Biology only Osmoregulation is the control of the water content and ion content of the blood by the kidneys. The kidney changes how much water and how many ions it reabsorbs back into the blood, so only the excess is lost in urine. It happens alongside excretion, in which urea is removed from the blood.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026