The kidneys and urea

Homeostasis and response · Hormonal coordination in humans · note 5 of 11

Spec 4.5.3.3

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The kidneys and ureaSpec 4.5.3.3

In short

The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances. Water, glucose, ions and urea are filtered out of the blood, then all of the glucose, some ions and some water are reabsorbed. The urea, excess ions and excess water leave the body in the urine, so healthy urine contains no glucose.

The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water. The waste urea and the excess water and ions leave in the urine.

Blood is filtered in the kidney. Water, glucose, ions and urea pass out of the blood.
Useful substances are selectively reabsorbed back into the blood: all of the glucose, some ions and some water.
The rest, including urea, excess ions and excess water, passes out as urine.

You do not need to know the other parts of the urinary system, the structure of the kidney or the structure of a nephron.

Example concentrations (arbitrary units, for practice)
Substance	Blood before filtration	Fluid after filtration	Urine
Glucose	5	5	0
Ions	9	9	12
Urea	2	2	60

This kind of table can be shown as a bar chart. The glucose bar drops to zero in urine because all of it is reabsorbed. The urea bar is much higher in urine because water is reabsorbed while urea is not.

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Excess amino acids and urea

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The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. In the liver, these amino acids are deaminated to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion.

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Common mistake:

The kidney does not make urea. Urea is made in the liver, carried in the blood to the kidneys, and removed in the urine.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

What does insulin do to blood sugar?

Insulin lowers blood sugar (blood glucose concentration). The pancreas produces insulin when blood glucose is too high, for example after a meal. Insulin causes glucose to move from the blood into the cells, and in liver and muscle cells excess glucose is converted to glycogen for storage. Blood glucose then falls back to the normal level.

What is the difference between insulin and glucagon?

Higher tier Insulin lowers blood glucose and glucagon raises it. Both are made by the pancreas. Insulin is released when blood glucose is too high and makes glucose move from the blood into cells. Glucagon is released when it is too low and causes glycogen to be converted into glucose and released into the blood. They work in a negative feedback cycle.

Why are hormones called chemical messengers?

Hormones are called chemical messengers because they are chemicals secreted by glands directly into the bloodstream, which carries them to a target organ where they produce an effect. Compared with nerve impulses, hormones act more slowly but their effects last longer. They reach every part of the body but only affect their target organ.

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