The kidney and the urinary system

Structure and functions in living organisms · Excretion · note 2 of 5

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The kidney and the urinary systemSpec 2.72B, 2.73B

In short

The 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 kidneys do two jobs. Excretion: they remove urea from the blood. Osmoregulation: they control the water content and the ion content of the blood. Both jobs happen as the kidneys make urine.

Excretion: urea is filtered out of the blood and is not taken back, so it leaves the body in urine.
Osmoregulation: the kidney changes how much water (and how many ions) it reabsorbs back into the blood, so only the excess is lost in urine.

Parts of the urinary system
Part	Function
Kidneys	Filter the blood and make urine
Renal artery	Brings blood to the kidney
Renal vein	Takes blood away from the kidney
Ureters	Tubes that carry urine from the kidneys to the bladder
Bladder	Stores urine
Urethra	Tube that carries urine out of the body

The urinary system: two kidneys connected to the aorta by renal arteries and to the vena cava by renal veins, with a ureter from each kidney to the bladder and the urethra leading out of the bladder. — The **urinary system**. The renal artery brings blood to each kidney and the renal vein takes it away.

Common mistake:

The ureter carries urine from the kidney to the bladder. The urethra carries urine from the bladder out of the body. Do not swap them.

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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