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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Define excretion correctly (metabolic waste, not undigested food). Know ultrafiltration and selective reabsorption in the nephron. Understand ADH's role in osmoregulation (Extended).
Topic 13 covers the definition of excretion, kidney structure and nephron function, the role of ADH in osmoregulation (Extended), and dialysis vs transplant. The nephron filtration and reabsorption process is commonly tested.
Priority 1: Excretion definition
Excretion is the removal of metabolic waste products from the body. Key examples: CO₂ (from respiration, removed via lungs) and urea (from deamination of excess amino acids in the liver, removed via kidneys). Do not confuse with egestion (removal of undigested food).
Priority 2: Nephron function
Ultrafiltration: blood is filtered at the glomerulus: small molecules (water, glucose, urea, ions) pass into the Bowman's capsule; large molecules (proteins, blood cells) stay in the blood. Selective reabsorption: useful substances (all glucose, most water, some ions) are reabsorbed from the tubules back into the blood. Urea and excess water continue to the collecting duct as urine.
Priority 3: ADH and osmoregulation (Extended)
ADH (antidiuretic hormone) controls water reabsorption in the collecting duct. When blood is too concentrated (dehydrated): more ADH released → collecting duct becomes more permeable → more water reabsorbed → small volume of concentrated urine. When blood is too dilute (overhydrated): less ADH → less water reabsorbed → large volume of dilute urine.
Frequently asked questions
Is the kidney an excretory organ?
Yes: the kidney removes urea (a metabolic waste product of amino acid deamination in the liver) dissolved in water as urine.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.