MECHANISM OF CONCENTRATION OF THE FILTERATE

MECHANISM  OF CONCENTRATION OF THE  FILTERATE

        

                    Mammals have the ability to produce a concentrated urine. The human body does not

       maintain a constant water volume, the kidneys use a transportsystem called the counter-

      current mechanism..The Henle’s loop and vasa recta play a significant role in this.

      The particles and fluid removed from the blood, the filtrate, moves from the Bowman's

    capsule to the proximal tubule, loops of Henle, distal tubule, and collecting tubule.

    The flow of filtrate in the two limbs of  henle’s loop is in opposite directions and thus forms a

    counter current The flow of blood through the limbs of vasa recta is also in a counter current

    pattern. The name is based on the fact that concentration first increases in the direction of

   f low, then decreases as flow continues through the ascending parallel loop. The mechanism

   relies on the adjacent, parallel loops of Henle and vasa recta.

In the ascending loop, Na⁺ (or any solute) is actively pumped out of the tubule. As flow  

   continues up the loop, the tubular concentration decreases as does the interstitial (the fluid

   surrounding the loop) concentration. Because water is impermeable in the ascending loop, the

   volume at the bottom of the loop is the same as that entering the distal tubule. At the bottom

   of the loop, the tubular and interstitial concentrations are equal.

In the descending loop, the concentrations inside and outside the tubule are increasing

   with the current, with the maximum concentration being reached at the bottom of the loop.

  The increased concentration is the result of the passive diffusion of Na⁺ into the tubule and

  water out of the tubule. When the filtrate reaches the distal tubule, a net loss of Na⁺ and water

  has occurred through the loops of  Henle.

Inside the distal and collecting tubules, the filtrate is either diluted or concentrated to

form urine. The Henle's loop and vasa recta are close together and counter current mechanism in them help 

in maintaining an osmolarity towards the tissue surrounded by Henles loop in the renal medulla.,.i.e.,from 

300 mOsmoIL in the cortex to about  1200 mOsmoIL in the inner medulla.This  gradient  is mainly caused 

by NaCl and urea.NaCl is transported by the ascending limb of Henle’loop which is exchanged with the 

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

recta.Similarly,small amount of urea enter the thin segment of the ascending limb of Henle’s loop which is 

transported back to the interstitium by the collecting tubule.

This counter current mechanism helps to maintain a concentration gradient in the medullary

interstitium.Presence of such interstitial gradient helps in an easy passage of water from the

collecting tubule thereby concentrating the filtrate(urine).human kidneys can produce urine nearly

four times  concentrated than the initial filtrate formed.