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.
• It's crucial for balancing body systems and preventing damage.
• Key organs involved: lungs, kidneys, liver.

Role of Organs in Excretion

Lungs

• Excrete carbon dioxide, a waste product of respiration.
• Blood transports CO2 to lungs for excretion.

Kidneys

• Excrete urea, excess water, and ions via urine.
  • Urinary System Components:
    • Kidneys: Filter blood.
    • Ureters: Connect kidneys to the bladder.
    • Bladder: Urine storage.
    • Urethra: Pathway for urine to exit the body.
  • Kidney Structure:
    • Cortex and Medulla.
    • Contains around a million nephrons.
  • Nephron Structure:
    • 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 in glomerulus forces out water, glucose, urea, and ions into Bowman's capsule.
    • Selective Reabsorption: Filtrate reabsorbs useful substances (all glucose, some ions, most water) back into blood; urea is not reabsorbed.

Liver

• Assimilation: Converts digested food molecules to usable forms.
• Processes excess amino acids through deamination:
  • Removal of nitrogen-containing part of amino acids to form urea.
  • Urea is transported to the kidney for excretion.
  • Important to remove urea due to its toxicity at high levels.

Conclusion

• Excretion is vital for removing waste and preventing toxic build-up.
• Understanding the role of lungs, kidneys, and liver is crucial.

• Note:
  • It's important to differentiate between ureter and urethra.
  • Details of processes like selective reabsorption are not required for the syllabus.