The kidneys, ADH and kidney failure

The kidneys remove urea and control the water content of the blood. Each kidney contains about a million tiny tubes called nephrons.

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The urinary system and the nephron

Blood enters each kidney in the renal artery and leaves in the renal vein. Urine passes down the ureters to the bladder and out through the urethra.
In each nephron:
1. Filtration: high pressure in the glomerulus forces small molecules (water, glucose, urea, ions) into the Bowman's capsule. Proteins and blood cells are too big and stay in the blood.
2. Selective reabsorption: all the glucose is taken back into the blood in the first part of the tubule (the proximal convoluted tubule), partly by active transport. Useful ions are reabsorbed too.
3. The loop of Henle and collecting duct reabsorb water as needed.
4. What remains (urea, excess water and ions) is urine.
1 2 3 4 filtration in theglomerulus: water,glucose, urea, ions all glucosereabsorbed waterreabsorbed urine loop of Henle collecting duct
How a nephron makes urine: filtration, reabsorption of glucose, reabsorption of water.

ADH controls water

ADH (anti-diuretic hormone) is released by the pituitary gland.
If the blood is too concentrated (for example after sweating), more ADH is released. It makes the collecting ducts more permeable, so more water is reabsorbed. Urine is small in volume and concentrated.
If the blood is too dilute, less ADH is released, so less water is reabsorbed and lots of dilute urine is made. This is negative feedback.
Urea is made in the liver when excess amino acids are broken down (deamination).

Treating kidney failure

DialysisTransplant
How it worksBlood flows past a partially permeable membrane; dialysis fluid has normal glucose and ion levels but no urea, so urea diffuses outA healthy kidney from a donor is put into the patient
ForKeeps the patient alive while waiting for a donorA long-term cure; no need for regular dialysis
AgainstSeveral sessions a week, several hours each; expensive; diet restrictionsShortage of donors; risk of rejection, so drugs to suppress the immune system are needed for life
Worked example

On a hot day a runner sweats a lot. Explain how the kidneys help to keep the water content of her blood constant.

  1. Sweating makes the blood more concentrated.
  2. The pituitary releases more ADH.
  3. The collecting ducts become more permeable and reabsorb more water.
  4. She makes a small volume of concentrated urine.

Answer: More ADH is released, more water is reabsorbed in the collecting ducts, so less, more concentrated urine is made.

Key idea

The glomerulus filters small molecules into the Bowman's capsule; all glucose is reabsorbed; water is reabsorbed under the control of ADH from the pituitary. Urea comes from deamination in the liver. Kidney failure is treated by dialysis or a transplant.

Key words
ADH
Anti-diuretic hormone, released by the pituitary gland. It makes the collecting ducts more permeable so more water is reabsorbed into the blood.
Glomerulus
A knot of capillaries in the kidney where ultrafiltration happens.
Bowman's capsule
The cup-shaped start of a nephron that surrounds the glomerulus and collects the filtrate.
Artery
A blood vessel that carries blood away from the heart at high pressure. It has thick muscular and elastic walls.
Vein
A blood vessel that carries blood back to the heart. It has thin walls, a large lumen and valves.
Nephron
A kidney tubule where blood is filtered and useful substances are reabsorbed.

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2 more questions

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  1. In a nephron, where is blood plasma forced out under high pressure?

    1. From the loop of Henle into the blood
    2. From the bladder into the urethra
    3. From the glomerulus into the Bowman's capsule
    4. From the collecting duct into the ureter
    Show the answer

    Answer: From the glomerulus into the Bowman's capsule

    Small molecules are filtered out of the glomerulus into the capsule.

  2. Glucose is found in a patient's urine. What might be the cause?

    1. Too much exercise only
    2. Drinking water
    3. A cold
    4. Uncontrolled diabetes
    Show the answer

    Answer: Uncontrolled diabetes

    Too much glucose to reabsorb.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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