Key facts & rules

• Excretion: removal of metabolic WASTE products (CO₂ from respiration, urea from protein breakdown).
• Egestion is NOT excretion — it is the removal of undigested food.
• Kidneys: filter blood (ultrafiltration), selectively reabsorb useful substances, produce urine.
• Urea: made in the liver from excess amino acids (deamination).
• Nephron stages: glomerular filtration → tubular reabsorption (glucose, amino acids, water) → secretion.
• ADH (antidiuretic hormone): increases water reabsorption in collecting duct → concentrated urine.
• Skin excretes water and salts in sweat (and small amounts of urea).
• Lungs excrete CO₂ and water vapour.