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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!!!