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The nephron and ultrafiltration

Structure and functions in living organisms · Excretion · note 3 of 5

Spec 2.74B, 2.75B
Biology only (what this means)Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels mean
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The nephron and ultrafiltrationSpec 2.74B, 2.75B

In short

A 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.

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

  • Bowman's capsule and glomerulus: where blood is filtered.
  • Proximal convoluted tubule: the first coiled part of the tubule, after the Bowman's capsule.
  • Loop of Henle: the long loop that dips down and comes back up.
  • Distal convoluted tubule: the second coiled part of the tubule.
  • Collecting duct: the tube that several nephrons drain into, which carries urine towards the ureter.
A nephron: blood from the renal artery enters the glomerulus inside the Bowman's capsule, where ultrafiltration happens; the filtrate passes along the proximal convoluted tubule (where glucose is reabsorbed into the capillaries), the loop of Henle and the distal convoluted tubule into the collecting duct (where water is reabsorbed), and urine flows on to the ureter; blood leaves to the renal vein. (opens full size in a new tab)
A nephron. Ultrafiltration happens at the Bowman's capsule, glucose is reabsorbed at the proximal convoluted tubule and water is reabsorbed at the collecting duct.

Ultrafiltration

Blood in the glomerulus is at high pressure. This forces small molecules out of the blood and into the Bowman's capsule. This is ultrafiltration. The liquid that enters the capsule is the glomerular filtrate. Large molecules and cells are too big to pass through, so they stay in the blood.

Composition of the glomerular filtrate
SubstanceIn the glomerular filtrate?
WaterYes
GlucoseYes
UreaYes
IonsYes
ProteinsNo (too large)
Blood cellsNo (too large)
Exam tip:

Say that filtration is based on size: small molecules pass into the capsule and proteins and cells do not. Use the word ultrafiltration.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

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.

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