The kidney and the nephron

Excretion in humans · Excretion in humans · note 2 of 3

Spec 13.1.5

Supplement

Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

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The kidney and the nephronSpec 13.1.5

In short

A 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.

Each kidney contains many tiny tubes called nephrons. A nephron filters the blood and forms urine. Blood enters a knot of capillaries called the glomerulus, which sits inside a cup-shaped Bowman's capsule. The glomerulus is supplied with blood by a branch of the renal artery.

Filtration (glomerulus): blood in the glomerulus is under high pressure. Water, glucose, urea and ions are forced out of the blood into the nephron. Large molecules such as proteins and blood cells stay in the blood.
Reabsorption (along the nephron): all of the glucose, some of the ions and most of the water are taken back into the blood.
Urine formation: what is left in the nephron is urine. It contains urea, excess water and excess ions. It flows to the ureter.

What happens to each substance
Substance	Filtered at the glomerulus?	Reabsorbed into the blood?	In urine?
Water	Yes	Most of it	Yes, the excess
Glucose	Yes	All of it	No
Ions	Yes	Some of them	Yes, the excess
Urea	Yes	Some, but much stays in the nephron	Yes
Proteins and blood cells	No	Not applicable	No

Simplified nephron: a branch of the renal artery supplies the glomerulus inside the Bowman's capsule, where water, glucose, urea and ions are filtered into the nephron; blood capillaries around the tubule take back all of the glucose, some of the ions and most of the water and drain towards the renal vein; urine containing urea, excess water and excess ions flows to the ureter. — Filtration at the glomerulus, reabsorption along the tubule, and what is left in the nephron is urine.

Exam tip:

The syllabus asks for an outline only; you do not need the names of the other parts of the nephron. Learn the three ideas: filtration of water, glucose, urea and ions; reabsorption of all glucose, some ions and most water; urine containing urea, excess water and excess ions.

Common mistake:

Glucose is filtered out of the blood, then all of it is taken back. Do not say glucose is not filtered. Proteins and blood cells are not filtered.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

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.

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