ask mattrab Visit www.askmattrab.com for more academic resources.

Urine formation

Process of Urine formation

The basic filtration units are the nephrons. Each kidney has a million Nephrons packed together.

Urine formation occurs in three steps:

1.   Filtration

2.   Reabsorption

3.   Secretion


·        Filtration of blood takes place in the Glomerulus

·        Termed as Glomerular filtration

·        Blood reaches glomerulus by Afferent arteriole

·        Increased pressure of blood in glomerulus causes 3-layer filtration of blood

·        Blood is filtered through 3 layers:

    ·        Endothelium of glomerular blood vessels

    ·        Epithelium of Bowman’s capsule (Podocytes)

    ·        Basement membrane

·        Fine filtration of blood through the 3-layered membranes is termed as ‘Ultra filtration’

·        1100-1200 ml of blood is filtered by the kidneys per minute

·        Amount of the filtrate formed by the kidneys per minute is called Glomerular Filtration Rate (GFR)


·        Almost 99% of the filtrate is reabsorbed by renal tubules

·        Useful substances are reabsorbed by epithelial cells of tubules & sent back to blood

·        Wastes are absorbed from blood & sent to the filtrate

·        Water moves under osmotic pressure

·        Role of different tubules

2.   Proximal Convoluted Tubule (PCT):

·        Large amount of reabsorption of nutrients, water & electrolytes occur here

·        Presence of brush border epithelium increases surface area for absorption

3.   Henle’s loop

·        Minimum reabsorption occurs here

·        Important role in osmoregulation

4.   Distal Convoluted Tubule (DCT)

·        Conditional reabsorption occurs here

5.   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+.