3.2 Excretory System

Introduction

• Purpose: Removes waste products from the body to maintain homeostasis.
• Process: Excretion involves filtering waste from blood and expelling it as urine.

Excretory Organs (Beyond Kidneys)

• Lungs: Remove carbon dioxide (CO₂) during exhalation.
• Skin: Excretes water, salts, and some urea via sweat (thermoregulation).
• Liver: Detoxifies substances, converts ammonia to urea for kidney excretion.

The Urinary System

• Kidneys: Filter blood, produce urine.
• Ureters: Tubes carrying urine from kidneys to bladder.
• Urinary Bladder: Stores urine.
• Urethra: Tube for expelling urine from the body.

The Kidneys: Structure and Function

• External: Bean-shaped, reddish-brown organs; hilum (entry/exit point).
• Internal:
  • Renal Cortex: Outer layer (glomeruli, convoluted tubules).
  • Renal Medulla: Inner part (renal pyramids).
  • Renal Pelvis: Funnel-shaped, collects urine.
• Blood Vessels: Renal artery (supplies blood), Renal vein (carries filtered blood).

The Nephron: Functional Unit

• Microscopic structural and functional unit of the kidney (approx. 1 million per kidney).
• Two Main Parts:
  1. Renal Corpuscle (Malpighian Capsule):
     • Glomerulus: Capillary cluster for filtration.
     • Bowman's Capsule: Surrounds glomerulus, collects filtrate.
  2. Renal Tubule: Long, convoluted tube.
     • Proximal Convoluted Tubule (PCT)
     • Loop of Henle
     • Distal Convoluted Tubule (DCT)
     • Collecting Duct

Urine Formation: Three Steps

1. Ultrafiltration:
   • Occurs in Malpighian capsule.
   • High pressure forces water and small solutes from blood in glomerulus into Bowman's capsule.
   • Large molecules (proteins, blood cells) remain in blood.
2. Selective Reabsorption:
   • Essential substances (water, glucose, amino acids, salts) are reabsorbed from filtrate back into blood in renal tubule.
   • Highly selective process.
3. Tubular Secretion:
   • Waste products and excess ions (H⁺, K⁺, drugs) are actively transported from blood into renal tubule.
   • Further cleanses blood and regulates pH.

Urine Pathway

• Filtrate (now urine) from renal tubule → Collecting ducts → Renal pelvis → Ureters → Urinary bladder (storage) → Urethra (expulsion).