1. Role Of Distal Tubule
Fluid Leaves the loop of Henle and enters the distal convoluted tubule in the renal cortex
This fluid has an osmolarity of 100 mOsm/L.
NaCl is transported out of the tubule but, this part is relatively impermeable to water.

3. Role Of The Cortical Collecting Tubule
ADH acts at this level, when there is high conc. of ADH
C-C Tubule becomes highly permeable to water
Large amount of water is reabsorbed into the cortical interstitium

4. In the absence of ADH, segment is impermeable to water.
Decrease reabsorption of water but, reaborption of solutes further dilutes the urine.

6. Role Of The Medullary Collecting Ducts
There is further water reabsorption which also can vary in amount according to the conc. of ADH
The reabsorbed water is carried away by the Vasa recta into the venous blood.

8. Contribution Of Urea In Hyperosmolarity
Urea plays a very important role in the development of the medullary osmotic gradient.
Urea is completely filtered in the glomeruli

10. It is not reabsorbed from the renal tubule but, in the presence of ADH when water is reabsorbed in the collecting duct, urea conc. Increases.
There is diffusion of urea from the collecting duct into the medullary interstitium which leads to increased osmolarity in the medullary interstitium.

12. Increase conc. of urea in the medullary interstitium causes movement of urea in the ascending limb due to conc. gradient.
From here it goes back into the collecting duct through the distal convoluted tubule.

13. Thus, urea recirculates repeatedly and this helps to maintain hyperosmolarity in the medullary interstitium.

14. Factors Developing Medullary Gradient interstitium
Reabsorption of sodium from the ascending segment into the medullary interstitium.
Reabsorption of sodium from the collecting duct into medullary interstitium.
Diffusion of urea from the collecting duct into the medullary interstitium.

18. Role Of Vasa Recta In The Maintenance Of Medullary Gradient

19. Hairpin Arrangement Of The Vasa recta
Vasa recta is highly permeable to water and sodium like other capillaries.
When the blood enters the vasa recta it is isotonic to the plasma.

20. When the capillaries descend down into the deeper parts of medulla
When the capillaries descend down into the deeper parts of medulla. Water diffuses passively from the blood
Sodium and Urea passively, diffuse into the blood.
While passing through the ascending limb of the vasa recta

21. Sodium and urea diffuse out of the blood and enter the interstitial fluid of the medulla. Water diffuses out into the blood.
Since, sodium and urea are exchanged for water between the ascending and descending limb of the vasa recta. This system is called the Counter Current Exchanger

24. Slow Blood Flow
There is very slow of blood through the parenchyma of the medulla as it receives only 5 % of the total blood supplied to the kidney.
Because of sluggish blood flow the removal of sodium from the medullary interstitium is reduced.

25. End Of Todays Lecture!!!