Countercurrent system

 1.      COUNTERCURRET MECHANISM OF CONCENTRATING THE URINE

Normal osmolarity of urine: 300 mosm/lt

Osmolarity of diluted urine: Upto 50mosm/lt

Osmolarity of concentrated urine: Upto 1200mosm/lt

 

Requirements for concentrating the urine:

1.      ADH

2.      Hyperosmolar medullary interstitium

 

Factors contributing to hyperosmolarity of medullary interstitium:

1.      Countercurrent system

2.      Reabsorption at DCT & CD

3.      Urea recirculation

 

1.      Countercurrent system: A system in which the inflow runs in parallel, in opposite direction

                                         & in close proximity to the outflow

       Components of countercurrent system in kidney:

a)      Loop of Henle (contercurrent multiplier)

b)      Vasarecta (countercurrent exchanger)

     Role of Loop of Henle as countercurret multiplier in countercurrent mechanism:

      To generate osmotic gradient & hyperosmolarity of medullary interstitium

      Mechanism –

-          Diffusion of water out of thin descending limb of LOH (This makes the fluid in the tip of the loop to become more concentrated than the surrounding interstitum)

-          Passive reabsorption of solutes from the hypertonic fluid in the tin ascending limb (This helps in multiplication of interstitial osmolarity)

Active reabsorption of  sodium & chloride in the thick ascending limb of Loop of Henle (This helps in building up of a higher interstitial osmolarity

          To maintain the osmotic gradient & hyperosmolarity of medullary interstitium

           Mechanism –

                       Descending limb of vasa recta :

-          Solutes diffuse into the lumen

-          Water diffuses out

-          The osmolarity of blood increases from 300 milliosmoles to 1200 milliosmoles towards the tip of vasarecta

                           Ascending limb of vasa recta:

-          Solutes move out

-          Water diffuses in

-          The osmolarity of the blood decreases from 1200 milliosmoles to 300 milliosmoles from the tip upwards

               So the solutes are exchanged for water between the ascending and the descending limbs

               of vasarecta. This maintains the hypertonicity of medulla

1.       Reabsorption in the DCT & Collecting duct:

·         Sodium ions are actively absorbed from distal tubule and collecting duct under the influence of aldosterone.

·         Sodium ions are accompanied passively by chloride ions.

·         This also increases the medullary osmotic gradient

 

2.      Urea recirculation:

·         Large amounts of urea is reabsorbed in the medullary collecting duct.

·         The urea which moves into the interstitium is secreted in to the descending &

ascending limb of LOH.

Again it is reabsorbed in the medullary collecting