CounterCurrent System -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

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

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

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

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

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

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

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

Role Of Vasa Recta In The Maintenance Of Medullary Gradient

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

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

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

Slow Blood Flow There is very slow flow 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

Increase Medullary Blood Flow Vasodilators Large increase in arterial pressure. These wash out the hyperosmotic inetrstitium Reduction in the urine concentrating ability

End Of Todays Lecture!!!