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2.9.7Structure & Functions in Living Organisms

ADH and Composition of Urine — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

5 min read 4 parts Lesson 21 of 21 Unit quiz

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision
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Part 1 of 4How ADH Works

2.9.7 ADH and Composition of Urine

Water level in the blood is controlled by negative feedback.

1
Osmoreceptors in the hypothalamus detect changes in the water potential of the blood and send signals to the pituitary gland.
2
When the blood water potential is too low, the pituitary gland releases more ADH into the blood.
3
ADH increases the permeability of the collecting ducts to water, so more water is reabsorbed into the blood by osmosis.
4
A small volume of concentrated urine is produced, and the blood water potential rises back to normal.
5
When the blood water potential is too high, less ADH is released, the ducts become less permeable and a large volume of dilute urine is produced.

Part 2 of 4Composition of Urine

Urine is a solution that varies with what the body needs to remove.

Water
  • Excess water is lost in urine, with the volume adjusted by the action of ADH on the collecting ducts.
Urea
  • This toxic waste from the breakdown of amino acids is always present in urine.
Ions
  • Excess salts such as sodium chloride are removed in urine to keep the ion balance in the blood correct.
Absent
  • In a healthy person urine contains no glucose and no protein, because glucose is reabsorbed and protein is not filtered.

Part 3 of 4Why Urine Varies

Pale urine is dilute and dark urine is concentrated, as the same amount of urea is dissolved in different volumes of water.

Why Urine Varies
Drinking a lot
  • More water enters the blood, less ADH is released and a large volume of pale yellow urine is produced.
Hot weather
  • More water is lost in sweat, so more ADH is released and less, darker, more concentrated urine is made.
Exercise
  • Sweating and faster breathing lose water, so the kidneys conserve water by making less urine.
Dehydration
  • Very dark urine in small volume is a sign the body is short of water.

Part 4 of 4Exam Checklist & Tips

2.9.7 – Key Points for the Exam

  • ADH A hormone that makes the collecting ducts more permeable to water.
  • Hypothalamus A part of the brain that detects the water content of the blood.
  • Water potential
    • A measure of the tendency of water to move;
      • dilute solutions have a higher water potential.
  • Dilute urine Urine containing a large amount of water compared with solutes.
  • Negative feedback A response that reverses a change and returns a condition to its normal level.

Common Mistakes

  • Saying that ADH is released when the blood is too dilute; it is released when the blood is too concentrated.
  • Stating that ADH changes the size of the nephron; it changes the permeability of the collecting duct.
  • Forgetting to say the water moves by osmosis when explaining reabsorption.
  • Writing that dark urine contains more urea being made; the same urea is dissolved in less water.

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