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2.9.3Structure & Functions in Living Organisms

Kidney: Excretion and Osmoregulation — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

5 min read 4 parts Lesson 17 of 21 Unit quiz

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision
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Part 1 of 4Functions of the Kidney

2.9.3 Kidney: Excretion and Osmoregulation

The kidneys are a pair of organs at the back of the abdomen that filter the blood continuously.

Excretion
  • Kidneys remove urea, a toxic waste made in the liver from the breakdown of excess amino acids, from the blood.
Excess ions
  • Salts present in more than the body needs are also removed in the urine, so the ion balance of the blood is controlled.
Osmoregulation
  • This is the control of water and solute concentration in the blood, which is an example of homeostasis.
Urine
  • The final product is urine, a solution of urea, excess ions and excess water that is stored in the bladder.

Part 2 of 4Why Water Balance Matters

Body cells and blood plasma are mostly water, and water moves in and out of cells by osmosis.

Why Water Balance Matters
Too much water
  • Blood plasma becomes too dilute, so water enters cells by osmosis; they swell and may burst.
Too little water
  • Blood plasma becomes too concentrated, so water leaves cells by osmosis;
    • they shrink and may be unable to work or die.
Stable conditions
  • Keeping the blood concentration steady keeps the volume and function of cells steady.
Blood pressure
  • The water content of the blood also affects the volume of blood and therefore blood pressure.

Part 3 of 4Water Gain and Loss

The kidneys adjust the only water loss that the body can control.

1
Water enters the body in food and drink, and a small amount is made inside cells during aerobic respiration.
2
Water leaves in exhaled air from the lungs, and this loss cannot be controlled directly.
3
Water also leaves in sweat from the skin, together with some ions and a little urea; this depends on temperature and exercise.
4
Water leaves in urine, and the volume of urine is changed by the kidneys to keep total body water steady.
5
On a hot day or after exercise, less water is lost in urine so that the body does not become dehydrated.

Part 4 of 4Exam Checklist & Tips

2.9.3 – Key Points for the Exam

  • Excretion The removal of toxic waste products of metabolism and substances in excess from the body.
  • Osmoregulation The control of the water and solute concentration of the blood.
  • Homeostasis The maintenance of a constant internal environment.
  • Urea A nitrogen-containing waste made in the liver from excess amino acids.
  • Osmosis The movement of water across a partially permeable membrane from a dilute to a more concentrated solution.

Common Mistakes

  • Saying that the kidneys make urea; urea is made in the liver and removed by the kidneys.
  • Confusing excretion with egestion; egestion is the removal of undigested food as faeces.
  • Stating that cells burst only when they lose water;
    • losing water makes cells shrink, gaining too much makes them swell.
  • Forgetting that the lungs and skin also lose water, but only kidney loss is regulated.

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