Class 11 Biology Excretory Products Elimination Concentration of Urine

Concentration of Urine

Removal of water makes the Urine/ filtrate more concentrated. Counter current mechanism plays the important role here.

Counter current mechanism

Filtrate flows in opposite direction through the Henle’s loop and therefore forms a counter current. The descending limb is permeable to water, and the ascending limb is impermeable to water, but permeable to electrolytes. The proximity between the Henle’s loop & vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium.

Cause of the gradient:

  1. NaCl is transported by the ascending limb of Henle’s loop which is exchanged

with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta.

  1. Urea enters the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.

This transport of substances facilitated by the arrangementof Henle’s loop & vasa recta is called the counter current mechanism

The overall process of urine formation:

  • Substances like glucose, amino acids, salts and good amount of water, are selectively re-absorbed as the urine flows along the tubular part of nephron
  • The amount of water absorbed depends upon the amount of wastes in the body & the amount of excess water in the body
  • Urine passes through the ureters to the urinary Bladder
  • Gets collected in Urinary bladder till the pressure increases
  • Finally passed out through the urethra


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