Excretory System

Kidneys

• Paired, bean-shaped; located in lower back (dorsal side).
• Function:
  • Filter ~180 litres blood/day → produce 1–2 litres urine.
  • Regulate water, electrolytes, pH balance.
• Structure:
  • Outer surface → Convex
  • Inner surface → Concave
  • Cortex = exterior, Medulla = interior.

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Nephrons (Functional Unit)

• ~1 million per kidney.
• Glomerulus → filters blood.
• Bowman’s capsule → collects filtrate.
• Tubules (proximal, loop of Henle, distal) → reabsorb water & nutrients.
• Main role → real controller of blood composition.

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Urine

• Colour: Urochrome.
• Composition: ~95% water, 2% urea + fats, glucose, uric acid.
• Odour: Beetle-like due to organic substances.
• Nature: Acidic, relative density higher than drinking water.

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Hormones

• ADH → water retention.
• Aldosterone → sodium balance.

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Dialysis

• Artificial process → selective filtration of blood.

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Significance

• Removes toxins.
• Maintains homeostasis.
• Regulates blood pressure.

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Other Excretory Organs

• Lungs → CO₂ excretion.
• Skin → sweat & oil (sebum).
• Liver → bile pigments, metabolic wastes.
• Spleen → worn-out RBC breakdown.
• Intestine → salts, undigested waste.
• Tears (lacrymal glands) → alkaline.