Edexcel iGCSE Biology-3.28B-3.29B ADH & Composition of Urine- Study Notes- New Syllabus
Edexcel iGCSE Biology-3.28B-3.29B ADH & Composition of Urine- Study Notes- New syllabus
Edexcel iGCSE Biology-3.28B-3.29B ADH & Composition of Urine- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
3.28B describe the role of ADH in regulating the water content of the blood
3.29B understand that urine contains water, urea and ions
Role of ADH in Regulating Water Content of the Blood
🌱 Introduction
The body must keep the water content of blood constant (homeostasis).
This is controlled by ADH (Antidiuretic Hormone).
ADH is produced in the hypothalamus and secreted by the pituitary gland.
How ADH Works
- Blood water content monitored by the hypothalamus (osmoreceptors).
- If water level changes, hypothalamus signals pituitary to adjust ADH release.
- ADH acts on collecting ducts:
- Makes walls more permeable to water.
- More water reabsorbed into blood by osmosis.
Situations
When body is dehydrated (low water, high salt concentration):
- Hypothalamus detects low water.
- Pituitary releases more ADH.
- Collecting duct → more permeable.
- More water reabsorbed into blood.
- Urine = small volume, concentrated.
When body is over-hydrated (too much water in blood):
- Hypothalamus detects high water.
- Pituitary releases less ADH.
- Collecting duct → less permeable.
- Less water reabsorbed into blood.
- Urine = large volume, dilute.
📊 Summary Table
| Condition | ADH level | Collecting duct permeability | Urine output |
|---|---|---|---|
| Dehydrated | High | More permeable | Small, concentrated |
| Over-hydrated | Low | Less permeable | Large, dilute |
⚡ Quick Recap
ADH = Antidiuretic Hormone.
Secreted by pituitary, acts on collecting duct.
High ADH = keep water → concentrated urine.
Low ADH = lose water → dilute urine.
👉 Mnemonic: “ADH = Always Drink Here” → tells kidney how much water to keep!
Composition of Urine
🌱 Introduction
Urine is the liquid waste produced by the kidneys.
It helps in excretion and osmoregulation.
Flow: Kidneys → Ureters → Bladder → Urethra → Out of body.
🔹 Main Components of Urine
- Water
• Largest component.
• Comes from excess water in diet & metabolism.
• Maintains fluid balance. - Urea
• Formed in liver from breakdown of excess amino acids (deamination).
• Major nitrogenous waste in humans. - Ions (salts/mineral ions)
• Examples: sodium (Na⁺), potassium (K⁺), chloride (Cl⁻).
• Amount varies depending on diet and blood balance.
• Excess removed to maintain homeostasis.
📊 Typical Composition of Urine
| Component | Source | Function/Reason for Excretion |
|---|---|---|
| Water | Excess from diet & respiration | Balance body fluids |
| Urea | Breakdown of amino acids in liver | Removes toxic nitrogen waste |
| Ions (Na⁺, K⁺, Cl⁻ etc.) | From food & metabolism | Maintain salt balance |
⚡ Quick Recap
Urine = Water + Urea + Ions.
Urea → from amino acids (liver).
Water & ions → controlled by kidney to keep blood balanced.
No proteins or glucose in normal urine (if present → disease sign).
👉 Mnemonic: WUI = Water, Urea, Ions = main urine parts.