PRELIMS
Human Excretory System
Excretion is the removal of nitrogenous waste substances (urea, ammonia, uric acid) formed during metabolism.
I. The Kidneys: Primary Excretory Organs
- Structure: Pair of bean-shaped organs (~140g each).
- Cortex: Outer part.
- Medulla: Inner part.
- Nephron: The structural and functional unit (~1.3 million per kidney).
- Structure sequence: Bowman's Capsule (surrounds Glomerulus) → Proximal Convoluted Tubule (PCT) → Loop of Henle (descending/ascending limbs, in medulla) → Distal Convoluted Tubule (DCT) → Collecting Duct.
- Glomerulus: A tuft of capillaries; site of filtration under blood pressure.
- Process (Urine Formation — three stages):
- Ultra-filtration (Glomerular Filtration): Occurs in Bowman's Capsule via the Glomerulus; blood pressure forces water, salts, glucose, urea into the capsule as filtrate.
- Reabsorption (Tubular Reabsorption): Useful substances (glucose, amino acids, most water, salts) are reabsorbed from the filtrate back into blood by the nephron tubule cells.
- Secretion (Tubular Secretion): Additional waste ions (H+, K+, ammonia, some drugs) are actively secreted from blood into the tubule, and the filtrate becomes concentrated urine.
- Filtration Rate (GFR): ~125 ml/minute (180 liters/day). Only ~1–1.5 liters is excreted as urine — over 99% of filtrate is reabsorbed.
II. Urine Composition and Properties
- Composition: Water (95%), Salt (2%), Urea (2.7%), Uric acid (0.3%).
- Color: Light yellow due to Urochrome (formed by hemoglobin dissociation).
- pH: Slightly acidic (pH 6).
- Kidney Stones: Formed primarily of Calcium Oxalate.
III. Accessory Excretory Organs
- Skin: Secretes Sebum (oil glands) and Sweat (water, salts).
- Liver: Converts toxic Ammonia and amino acids into Urea.
- Lungs: Excretes and water vapor.
IV. Excretory Parts in Other Animals
- Unicellular: General body surface (diffusion).
- Flatworms: Flame cells.
- Annelids: Nephridia.
- Arthropods: Malpighian tubules.
- Crustaceans: Antennal gland.
UPSC Relevance
The excretory system is a key area for GS-III Biology and Health.
- Functional Units: Identifying the Nephron and the role of Bowman's Capsule in ultra-filtration.
- Metabolic Byproducts: Understanding that Urea is produced in the Liver but filtered in the Kidney.
- Pathology: Linking Calcium Oxalate to kidney stones and the mechanism of Hemodialysis (artificial kidney).