Mechanism of urine formation

Urine formation process includes:

  1. Glomerular filtration
  2. Selective reabsorption
  3. Active secretion

 

  1. Glomerular filtration/ultrafiltration :
  • Filtration of blood under pressure is called ultrafiltration
  • Takes place in glomerulus
  • Hydrostatic pressure of blood in glomerulus is 60 mm Hg which is opposed by osmotic pressure of plasma protein (30 mm Hg)
  • Similarly, HP of glomerular filtrate is 20 mm Hg. Thus, net filtration pressure is 10 mm hg which cause filtration of blood and primary urine is formed
  • Primary urine or filtrate contains some useful and some harmful substances.

 

  1. Selective reabsorption
  • Absorption of useful substance from primary urine into blood is called selective reabsorbtion
  • This takes place in PCT, DCT, Loop of henle, collecting tube

 

Proximal convoluted tubule(PCT)

  • Most of the useful substance like glucose ,amino acids , vitamins, etc are absorbed from filtrate
  • Some harmful substances like urea, uric acid also get absorbed.
  • Absorb water (90%)
  • Minerals also get absorbed.
  • Isotonic filtrates enter descending loop of henle.

 

Descending loop of henle (DLH)

  • Meant foe conservation of water
  • Impermeable to salt
  • Only water is reabsorbed.
  • Hypertonic filtrates enter ascending loop of henle.

 

Ascending loop of henle (ALH)

  • Permeable to salts only
  • Thus more salt are transported to medullar region and some may enter descending limb so there is cyclic movement of salts in loop of henle.
  • Hypotonic urine enters DCT.

 

DCT ( distal convoluted tubule)

  • Permeability is determined by ADH hormone
  • Wall is impermeable to salt and water.

 

Collecting tubule

  • Large amount of water is reabsorbed from filtrate into blood making the urine hypertonic.

 

  1. Active secretion
  • Secretion of harmful substance from blood into filtrate is called active secretion
  • Nutrients and water are reabsorbed into blood and waste ions, urea, and uric acids, get secreted into filtrate from blood.

 

#urine is yellowish in color. Color of urine is determined by urochrome formed in liver.

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