IGCSE Biology: Excretion in Humans

Introduction

• Excretion is the process of removing toxic materials, metabolic waste products, and substances in excess from the body.
• Important for maintaining balance in body systems and preventing damage.

Organs Involved in Excretion

• Lungs: Expel carbon dioxide, a waste product of respiration.
• Kidneys: Remove urea, excess water, and ions through urine.
• Liver: Processes waste and converts excess amino acids to urea.

Excretion via Lungs

• Carbon dioxide is transported by the blood to the lungs and excreted during respiration.

Excretion via Kidneys

• Urea: Produced from protein metabolism, must be excreted as it is toxic in high concentrations.
• Urinary System Components:
  • Kidneys: Filter blood.
  • Ureters: Tubes connecting kidneys to the bladder.
  • Bladder: Stores urine.
  • Urethra: Passage for urine to exit the body.
• Structure of the Kidney:
  • Cortex and Medulla: Contain nephrons, the functional units producing urine.
  • Nephrons include: Glomerulus, Bowman's capsule, Loop of Henle, proximal and distal convoluted tubules, and collecting duct.

Nephrons and Urine Formation

• Glomerulus & Bowman's Capsule:
  • High blood pressure in glomerulus forces water, glucose, urea, and ions into Bowman's capsule (ultrafiltration).
• Selective Reabsorption:
  • Nephrons reabsorb glucose, some ions, and most water back into the blood.
  • Urea remains in the filtrate and becomes part of urine.

Liver's Role in Excretion

• Assimilation: Conversion of digested food molecules into body molecules.
• Deamination:
  • Excess amino acids broken down, removing nitrogen to form ammonia, then urea in the liver.
  • Urea is transported in blood to kidneys for excretion.

Conclusion

• Urea excretion is crucial to prevent toxicity.
• Understanding excretion pathways helps in comprehending the balance and detoxification processes in the human body.

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