The document explains the counter current mechanism in the nephron, detailing osmolarity, diffusion, and the structure and function of different nephron segments. It describes how the descending and ascending limbs of the loop of Henle contribute to creating an osmotic gradient, which is maintained by the vasa recta. Additionally, it highlights the roles of the collecting duct and the importance of urea transport in regulating water movement in the kidney.

1. COUNTER CURRENT
MECHANISM
Dhayanithi.C

2. CONTENTS
Osmolarity and diffusion
Osmosis & osmotic pressure
Structure of nephron
Counter current mechanism
Counter current multiplier and exchanger
Functions of ADH

3. OSMOSIS & OSMOTIC PRESSURE
the movement of water(solvent) from a region of high
water concentration through a semipermeable
membrane to a region of low concentration of water
It is a passive transport.
OSMOTIC PRESSURE
Osmotic pressure is the minimum amount of pressure
needed to prevent a pure solvent from flowing into a
solution across a semipermeable membrane.

5. OSMOLARITY & DIFFUSION
The number of osmoles per litre is osmolarity.
The osmolarity increases from 300 mOsmolL-1 in
the cortex to 1200 mOsmolL-1 in the inner
medulla by counter current mechanism.
DIFFUSION
the movement of a molecule down a concentration
gradient, from an area of its high concentration to an
area of its low concentration.

7. STRUCTURE OF NEPHRON

8. PCT
( PCT) is a segment of the renal tubule responsible
for the reabsorption and secretion of various solutes
and water
It reabsorbs Water and solutes

9. LOOP OF HENELE
long U-shaped portion of the tubule that conducts urine within
each nephron of the kidney
It reabsorbs water in thin descending
limb .
It reabsorbs solutes in thin ascending
limb.

10. DCT & CD
DCT
a short nephron segment, interposed between
the macula densa and collecting duct
CD
 reabsorb sodium and chloride across the apical membrane

12. COUNTER CURRENT MECHANISM?
The countercurrent mechanism is a mechanism in which
the exchange of two fluids can take place from one
direction to another with their concentrations.
Osmolality gradient is created by LOH -- CC multiplier
This is maintained by vasa recta - -> counter current
exchanger.
CD acts as osmotic equilibrating device.

13. COUNTER CURRENT MULTIPLIER
• descending limb of LOH –
permeable to
water.(Becomes Hypertonic)
• Thin ascending limb of LOH
– impermeable to water
• Thick ascending limb of
LOH- Na+ Cl- actively
transported by Na+KATPase

14. ROLE OF CD
 It is permeable to water and urea.(it becomes Hypertonic).
 Urea transporters helps in movement of Urea in CD.
 UT-A1 (influenced by ADH).
 UT-A2 (acts on PCT)
 Inner medullary part of CD is permeable to urea

15. COUNTER CURRENT EXCHANGER(VASA
RECTA)
• Osmolal gradient is mainted byVASA
RECTA.
• Descending limb ofVasa recta solute
diffusion takes place.
• Na+Cl- is permeable in descending limb
of vasa recta.
• Impermeable to ascending limb.
•
• Helps in maintain ECF volume

16. NET- EFFECT
 In LOH water comes out of descending limb into interstitium & solutes come out
of ascending limb. (vice- versa happens in vasa recta)
 The longer LOH accumulates more solutes across its length from outer layers into
Deeper layers of medulla,Thus osmolality increases along the length of LOH
 Higher osmolality in the medullary interstitium further enhanced by diffusion of
urea from the CD.
 Water moves out passively from CD but it is removed by vasa recta from
interstitium.
 Deeper portion of medulla- osmolality 1200-1400 mOsm/kg of H2O is maintained

17. THANKYOU…