Class 11 Biology Excretory Products Elimination Process of Urine formation

Process of Urine formation


Urine is the liquid by-product secreted by kidneys. Its main purpose is to remove wastes like urea, uric acid & excess water from blood. The process of filtration occurs in Kidneys. The basic filtration units are the nephrons. Each kidney has a million of Nephrons packed together.

Urine formation occurs in three steps:

  1. Filtration
  2. Reabsorption
  1. Secretion



    1. Filtration of blood takes place in the Glomerulus
    2. Termed as ‘Glomerular filtration’
    3. Blood reaches glomerulus by Afferent arteriole
    4. Increased pressure of blood in glomerulus causes 3-layer filtration of blood
  1. Blood is filtered through 3 layers:
    1. Endothelium of glomerular blood vessels
    2. Epithelium of Bowman’s capsule (Podocytes)
    3. Basement membrane
  2. Fine filtration of blood through the 3-layered membranes is termed as ‘Ultra filtration’
    1. 1100-1200 ml of blood is filtered by the kidneys per minute
    2. Amount of the filtrate formed by the kidneys per minute is called Glomerular Filtration Rate (GFR)



    1. Almost 99% of the filtrate is reabsorbed by renal tubules
    2. Useful substances are reabsorbed by epithelial cells of tubules & sent back to blood
    3. Wastes are absorbed from blood & sent to the filtrate
    4. Water moves under osmotic pressure
    5. Role of different tubules
  1. Proximal Convoluted Tubule (PCT):
    • Large amount of reabsorption of nutrients, water & electrolytes occur here
    • Presence of brush border epithelium increases surface area for absorption
  2. Henle’s loop
    • Minimum reabsorption occurs here
    • Important role in osmoregulation
  • Distal Convoluted Tubule (DCT)
    • Conditional reabsorption occurs here
  1. Collecting duct
    • Large amount of reabsorption of water occurs here


  1. Cells of the tubules secrete substances into the filtrate
  2. Maintains ionic & acid base balance of body fluids
  3. Role of different tubules in Secretion:
  • Proximal Convoluted Tubule (PCT)
    • Selective secretion of H+, NH3, K+
  • Henle’s loop
    • Descending limb: Transport of water
    • Ascending limb: Active/ passive transport of electrolytes
  • Distal Convoluted Tubule (DCT)
    • Selective secretion of H+, NH3, K+
  • Collecting duct
    • Selective secretion of H+, K+

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