IP Biology Upper Sec 07: Excretion in Humans

These notes align with SEAB GCE O-Level Biology (6093) content used in IP programmes (exams from 2026).

Status: SEAB O-Level Biology 6093 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for this note.

What you must know

• Excretion = removal of metabolic waste ($\ce{CO2}$, urea, excess salts/water), not egestion. Main organs: lungs, kidneys, skin.
• Nephron: renal artery → afferent arteriole → glomerulus → Bowman’s capsule (ultrafiltration of small molecules) → proximal convoluted tubule (glucose, amino acids, most water/salts reabsorbed) → loop of Henle (water/salt), distal tubule (ion balance), collecting duct (water).
• Selective reabsorption uses diffusion, osmosis, and active transport (glucose reabsorbed fully in PCT).
• Kidney failure support: dialysis (diffusion across partially permeable membrane) or transplant.

Detailed notes

• Excretion vs egestion: metabolic wastes made in cells ($\ce{CO2}$