The document summarizes the countercurrent mechanism in the kidney. It describes: 1) How the loop of Henle acts as a countercurrent multiplier, trapping solutes in the medullary interstitium through active transport and increasing the osmolarity to 1200-1400 mosm/L. 2) How ADH increases collecting duct permeability, allowing water reabsorption and concentration of a urine as high as the medullary interstitium. 3) How the vasa recta acts as a countercurrent exchanger, maintaining the osmolarity gradient through solute exchange with the descending and ascending limbs.

1. Counter Current
Mechanism
BY: DEVANSH JAIN
ROLL NO: 33

2. Countercurrent system
A counter current system is a system of U shaped tubules in which the flow of
fluid is in opposite direction in two limbs of U shaped tubules.
When there is a water deficit in the body the kidney forms concentrated
urine by continuing to excrete Solutes while increasing water reabsorption
and decreasing the volume of urine formed. The human kidney can produce a
maximal urine concentration of 1200 to 1400 mosm/L.

3. Requirements for excreting a
concentrated urine
Basic requirement for forming a concentrated urine
1)High level of ADH which increases the permeability of DCT and collecting
ducts to water thereby allowing these tubular segment to reabsorb
water.
2)High osmolarity of medullary interstitium which provide the osmotic
gradient necessary for water reabsorption to occur in the presence of
high level of ADH.
Renal medullary interstitium surrounding the collecting duct in normally
hyperosmotic so when ADH level are high, water moves through the tubular
membrane by osmosis and enters into the renal interstitium.

4. Major factors that contribute to build up solute
concentration into renal medulla.
There are three major factors
1) Active transport of sodium ions and co-transport of potassium, chloride
and other ions out of the ascending Loop of Henle into medullary
interstitium.
2) Active transport of ions from collecting duct into medullary interstitium.
3) Facilitated diffusion of urea from collecting duct into
medullary interstitium.

5. Counter current system has two divisions
1) counter current multiplier formed by Loop of Henle.
2) counter current exchanger formed by vasa recta.

6. Counter current multiplier
Loop of henle is responsible for development of hyperosmolarity of
medullary interstitium.
Loop of henle of juxtamedullary nephron plays a major role as counter
current multiplier because loop of this nephron is long and extended up to
the deeper part of medulla.
Steps involved in causing hyperosmolarity of medullary interstitium.
1) first assume that the loop of Henle is filled with fluid with a concentration
of 300 mosm/liter.
2) the active ion pump of ascending limb on Loop of henle raises the
interstitial concentration upto 400 mosm/liter.

7. 3) Now the tubular fluid in the descending limb of the loop of Henle and the
interstitial fluid quickly reach equilibrium because of the osmosis of water out
of the descending limb and due to the concentration gradient the sodium and
chloride ions diffuses from medullary interstitium into the descending
limb.
4) Apart from this there is regular addition of more and more new sodium
and chloride ions add into descending limb by constant
filtration.
5) Now there is additional flow of fluid into the loop of Henle from PCT which
causes the hyperosmotic fluid in the descending limb to flow into the
ascending limb. Once this fluid is in the ascending limb, additional ions are
pumped into the interstitium which increases the interstitial osmolarity to
500 mosm/l.

8. 6) Then once again the fluid in the descending limb reaches equilibrium with
the hyperosmotic medullary interstitial fluid.
7) These steps are repeated over and over again with the net effect of adding
more and more solute to medullary interstitium this process gradually traps
solutes and multiply the concentration gradient established by the active
pumping of ions out of the ascending Loop of Henle. eventually Rising the
interstitial fluid osmolarity to 1200 to 1400 mosm/l.

9. Role of DCT and collecting duct in excreting
concentrated urine.
1) When the tubular fluid leaves the loop of Henle and flows into the DCT in
the renal cortex the fluid is diluted with an osmolarity of about 100mosm/l.
2) As the fluid flows into the cortical collecting tubule the amount of water
reabsorbed is critically dependent on concentration of ADH.
3) When there is high concentration of ADH collecting duct becomes highly
permeable to water so large amount of water are reabsorbed from collecting
ducts to Cortex interstitium.
4) This large amount of water are reabsorbed into the cortex interstitium
rather than into the renal medullary interstitium which helps to preserve the
high medullary interstitial fluid osmolarity.

10. 5) So the fluid at the end of collecting duct has essentially the same
osmolarity as the interstitial fluid of renal medulla that is about 1200 mosm/l.

11. Contribution of Urea to hyperosmotic renal
medullary Interstitium.
Urea contributes about 40 to 50% of osmolarity of medullary interstitium
Unlike sodium chloride ions urea is passively reabsorbed from the tubules by
facilitated diffusion
Mechanism:
1) As the water flows up to the ascending limb of Henle and into the DCT and
then into the collecting tubules. In the presence of high concentration of ADH
water is reabsorbed rapidly from collecting duct.
2) Therefore the urea concentration increases in the collecting tubules.
Continuous reabsorption of water from collecting tubule increases the urea
concentration in the tubules because of which due to concentration gradient
urea diffuses from the collecting tubules into the renal medullary interstitium.

12. 3) This diffusion is facilitated by specific urea transporters UT-A1 and UT-A3.

13. Counter current exchanger
Vasa recta function as countercurrent exchanger it is responsible for the
maintenance of medullary gradient developed by countercurrent multiplier
system
Mechanism
1) Blood enters and leaves the medulla by the way of vasa recta .the vasa
recta like other capillaries are highly permeable to solutes in the blood.
2) As the blood descends into the medulla through descending limb of vasa
recta it becomes more concentrated as simultaneously solutes enters into the
descending limb from interstitium and water goes out into the interstitium .
3) By the time the blood reaches the concentration of about 1200 mosm/l
same as that of medullary interstitium.

14. 4) As blood ascends towards the cortex through ascending loop of vasa recta
it's concentration decreases as solutes moves into medullary interstitium and
water comes into the limb.
5) The blood flows very slowly through vasa recta. So, a large quantity of
solutes can accumulate in the descending limb and by the time this solutes
flows into the ascending limb and then to medullary interstitium thus
increases the concentration .And the cycle repeats.
6) Thus vasa recta do not create the medullary hyperosmolarity , but they do
prevent it from being dessipated.