Edexcel iGCSE Biology-3.23B Structure of the Urinary System- Study Notes- New Syllabus

Urinary System – Structure

🌱 Introduction

The urinary system (also called the excretory system) removes nitrogenous waste and controls water and salt balance.
Main parts: Kidneys, Ureters, Bladder, Urethra.
Together, they make, transport, store, and release urine.

Kidneys

• Shape: Bean-shaped, reddish-brown organs, about the size of a fist.
• Location: On either side of the spine, just below the ribcage.
• Structure inside:
  Cortex (outer region) → contains filtering units (nephrons).
  Medulla (inner region) → pyramids where urine is collected.
  Pelvis → funnel-shaped chamber that drains urine into ureter.
• Function:
  Filter blood to remove urea, excess water, and salts.
  Maintain water balance (osmoregulation).
  Produce urine.

Ureters

• Structure: Narrow, muscular tubes (about 25–30 cm long).
• Function: Carry urine from each kidney to the bladder.
• How: Peristaltic movements (wave-like contractions) push urine down.

Urinary Bladder

• Structure: Hollow, muscular sac that can expand.
• Function:
  Temporary storage of urine (up to 400–600 mL in adults).
  When full → stretch receptors send signals → urge to urinate.
• Control: Has sphincter muscles that regulate urine release.

Urethra

• Structure: Tube leading from bladder to outside of body.
• Function: Carries urine out of the body.
• Difference:
  Males → longer urethra, also carries semen.
  Females → shorter urethra, only urine passes.

📊 Summary Table

⚡ Quick Recap
Kidneys → Filter blood + make urine.
Ureters → Carry urine down.
Bladder → Stores urine.
Urethra → Removes urine outside.
👉 Flow: Kidneys → Ureters → Bladder → Urethra (KUBU).