ICSE Class 10 Biology: Excretion — Kidney & Osmoregulation 2026

Excretion — Definition and Importance

• Excretion: removal of metabolic waste products (urea, CO₂, excess water, salts)
• Accumulation of waste is toxic: urea is formed from amino acid catabolism in liver (ornithine cycle)
• Organs of excretion: kidneys (urea, water, salts), lungs (CO₂, water vapour), skin (urea, salts)

Kidney — External Structure

• Bean-shaped, ~10 cm; located retroperitoneally below diaphragm
• Renal artery brings blood in; renal vein takes blood out; ureter carries urine to bladder
• Adrenal (suprarenal) gland sits on top of each kidney

Nephron — Structure

• Bowman's capsule: cup-shaped, surrounds glomerulus (capillary knot)
• Proximal convoluted tubule (PCT) → Loop of Henle → Distal convoluted tubule (DCT) → collecting duct
• ~1 million nephrons per kidney; functional unit of the kidney

Urine Formation — Three Processes

• Ultrafiltration: high pressure in glomerulus forces water, glucose, urea, salts into Bowman's capsule (glomerular filtrate)
• Selective reabsorption: all glucose, 99% water, Na⁺ reabsorbed in PCT and DCT; urea mostly not reabsorbed
• Tubular secretion: H⁺, K⁺, some drugs secreted into filtrate in DCT

Osmoregulation

• ADH (antidiuretic hormone) from pituitary: increases water reabsorption in collecting duct
• High blood osmolality → more ADH → concentrated urine; low osmolality → less ADH → dilute urine
• Aldosterone from adrenal cortex: promotes Na⁺ reabsorption in DCT → increases water retention

Kidney Failure and Dialysis

• Kidney failure: inability to filter blood; uraemia (urea buildup) is fatal
• Haemodialysis: blood passed through artificial kidney machine; dialysis fluid has same concentration as normal plasma
• Kidney transplant: permanent solution; donor kidney connected surgically

ICSE Exam — Excretion

• Draw a labelled nephron and state the process occurring at each part
• Explain how ADH regulates urine concentration
• State the waste products excreted by the lungs and skin