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!!!