Edexcel iGCSE Biology-3.21 Excretion in Humans- Study Notes- New Syllabus

Excretion in Humans – Lungs, Kidneys & Skin

🌱 Introduction

Excretion = removal of metabolic waste products from the body.
If not removed → wastes like CO₂, urea, salts can become toxic.
Major organs of excretion: lungs, kidneys, skin.

🫁 Lungs

• Excretory products: Carbon dioxide (CO₂) + water vapour.
• Origin: Produced during aerobic respiration in body cells.
• Equation: Glucose + O₂ → Energy + CO₂ + H₂O.
• Removal pathway:
  CO₂ diffuses into blood → carried as bicarbonates or dissolved CO₂.
  Blood reaches lungs → CO₂ diffuses into alveoli.
  Removed from body during exhalation.
• Importance: Prevents CO₂ buildup → avoids making blood too acidic.

🩸 Kidneys

• Excretory products:
  Urea → from breakdown of excess amino acids in liver.
  Excess salts → e.g., sodium, potassium.
  Excess water.
• Removal pathway:
  Blood enters kidney through renal artery.
  Filtration occurs in nephrons → urea, salts, water removed.
  Forms urine → passes through ureters → stored in bladder → excreted via urethra.
• Importance: Maintains homeostasis (constant internal environment):
  Controls water balance (osmoregulation).
  Removes toxic nitrogenous waste.

🧴 Skin

• Excretory product: Sweat (water + salts + small amounts of urea).
• Origin: Produced by sweat glands in dermis.
• Removal pathway: Sweat released through sweat ducts → pores on skin.
• Functions:
  Helps in excretion (small role compared to kidneys).
  Plays a major role in temperature regulation → cooling by evaporation.

📊 Summary Table

⚡ Quick Recap 
Lungs → excrete CO₂ + H₂O vapour (from respiration).
Kidneys → excrete urea, salts, water (via urine).
Skin → excretes sweat (water + salts + urea), mainly for cooling.
✨ Trick to remember:
“L-K-S = CO₂-Urine-Sweat”
Lungs → CO₂,
Kidneys → Urine,
Skin → Sweat.