Edexcel iGCSE Biology-3.22B Kidney: Excretion & Osmoregulation- Study Notes- New Syllabus

Kidney – Excretion & Osmoregulation

🌱 Introduction

Kidney = Excretory + Osmoregulatory organ.
Main functions:
Excretion → removal of urea, excess salts, excess water.
Osmoregulation → controlling water balance in blood to keep conditions constant.

1️⃣ Excretion by the Kidney

• Step 1: Blood enters kidney via renal artery (rich in urea, salts, water).
• Step 2: Filtration in nephrons:
  Small molecules (urea, salts, glucose, water) are filtered out of blood in the glomerulus → Bowman’s capsule.
  Large molecules (proteins, blood cells) stay in blood.
• Step 3: Selective reabsorption in tubules:
  Useful substances reabsorbed:
  – All glucose (for respiration)
  – Some salts (as needed)
  – Most water (depends on body needs)
• Step 4: Formation of urine:
  Leftover = urea + excess salts + excess water → urine.
  Urine passes: collecting duct → ureter → bladder → urethra.

2️⃣ Osmoregulation by the Kidney

• Osmoregulation: controlling water content of blood.
• Carried out in collecting ducts of nephrons under control of ADH (antidiuretic hormone).
• When blood water is low (dehydrated):
  – Pituitary releases more ADH.
  – Collecting ducts become more permeable → more water reabsorbed into blood.
  – Urine = small volume, very concentrated.
• When blood water is high (too much water):
  – Pituitary releases less ADH.
  – Collecting ducts become less permeable → less water reabsorbed.
  – Urine = large volume, very dilute.

📊 Summary Table

⚡ Quick Recap 
Excretion → kidneys remove urea, salts, water → forms urine.
Osmoregulation → kidneys control water balance using ADH.
Low water → more ADH, concentrated urine.
High water → less ADH, dilute urine.
Mnemonic: “Kidneys Keep it Clean & Controlled”
Clean = Excretion (removing wastes).
Controlled = Osmoregulation (water balance).