IGCSE Biology: Excretion in Humans

Introduction

• Excretion is the process of removing toxic materials, waste products of metabolism, and substances in excess from the body.
• Essential for balancing body systems and preventing damage.

Main Organs Involved in Excretion

• Lungs:
  • Excrete carbon dioxide, a waste product from respiration.
  • Blood carries CO2 to the lungs for exhalation.
• Kidneys:
  • Excrete urea, excess water, and ions via urine.
  • Structure includes:
    • Kidneys: Filter blood.
    • Ureters: Tubes connecting kidneys to the bladder.
    • Bladder: Stores urine.
    • Urethra: Tube for urine excretion.
  • Important to distinguish between ureter and urethra.
• Liver:
  • Involved in amino acid breakdown and urea formation.

Kidney Structure and Function

• Nephron: Functional unit located between the cortex and medulla.
  • Components include:
    • Glomerulus: Knot of capillaries.
    • Bowman's Capsule: Cup-shaped structure.
    • Loop of Henle
    • Proximal Convoluted Tubule
    • Distal Convoluted Tubule
    • Collecting Duct
  • Processes:
    • Ultrafiltration: High blood pressure forces small molecules (water, glucose, urea, ions) from the glomerulus to Bowman's capsule.
    • Selective Reabsorption: Useful substances (all glucose, some ions, most water) reabsorbed into blood, none of the urea is reabsorbed.
    • Remaining filtrate forms urine.

Liver's Role in Excretion

• Assimilation: Conversion of digested food molecules into useful molecules (e.g., amino acids to proteins).
• Deamination: Breakdown of excess amino acids in the liver:
  • Removal of nitrogen-containing parts to form ammonia then urea.
  • Urea, formed from excess amino acids, is transported to kidneys for excretion.

Conclusion

• Urea excretion is crucial due to its toxicity.
• Summary of topic 13: Excretion in Humans from the IGCSE syllabus.
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