counter current mechanism,

1. Urine Concentration and
Urine Dilution
Dr Kiran Kumar C
MBBS.,MD.,FAGE.,C DIAB
Assistant Professor

2. • In the steady state, water intake and output must be equal

3. • The kidney adjusts its water output to compensate for water
intake/ water loss
• The kidney excretes a variable amount of solute, depending
especially on salt intake.
• For a normal diet, the excreted solute is ~600 mOsmol/day.
• For average conditions of water and solute intake and output, this
600 mOsmol is dissolved in a daily urine output of 1500 mL .

4. • The excretion of water is regulated separately from the excretion of
solutes
• Kidneys must be able to excrete urine that is either hypo osmotic or
hyperosmotic with respect to the body fluids.
• The reabsorption of solute in the proximal tubule results in the
reabsorption of a proportional amount of water.

5. • The thick ascending limb, is the major site where solute and water are
separated.
• Thus the excretion of both dilute and concentrated urine requires
normal function of the loop of Henle .
• Single Effect

7. Water diuresis
Excretion of dilute urine

9. Water antidiuresis
Excretion of concentrated urine

10. • Urine is concentrated by the AVP-dependent reabsorption of water
from the collecting duct.
• Reabsorption of NaCl from the ascending limb of Henle’s loop
generates a high [NaCl] in the medullary which then drives water
reabsorption from the collecting duct.

11. • Urea accumulates in the medullary interstitium which allows
the kidneys to excrete urine with the same high urea
concentration.

12. Counter current
multiplication
Excretion of concentrated urine

13. Requirement for excreting concentrated urine
• High levels of ADH
• A high renal medullary interstitium

19. The vasa recta’s countercurrent exchange mechanism and
relatively low blood flow minimize the washout of the
medullary hypertonicity

21. Osmotic gradient of medullary interstitium from corticomedullary
junction to papilla:
a. Length of loops of Henle: Species with long loops (e.g., desert
rodents)
concentrate more than those with short loops (e.g., beaver).
b. Rate of active NaCl reabsorption in the TAL: Increased luminal Na+
delivery to TAL enhances NaCl reabsorption, whereas low Na+ delivery
Reduces concentrating ability.
c. High Na-K pump turnover enhances NaCl reabsorption, whereas
inhibiting transport (e.g., loop diuretics) reduce concentrating ability.

22. • 2. Protein content of diet: High-protein diet, up to a point, promotes
urea accumulation in the inner medullary interstitium and increased
concentrating ability.
• 3. Medullary blood flow: Low blood flow promotes high interstitial
osmolality. High blood flow washes out medullary solutes.
• 4. Osmotic permeability of the collecting tubules and ducts to
water: AVP enhances water permeability and thus water
reabsorption.

23. • 5. Luminal flow in the loop of Henle and the collecting duct: High
flow (osmotic diuresis) diminishes the efficiency of the
countercurrent multiplier and thus reduces the osmolality of the
medullary interstitium.
• In the MCD, high flow reduces the time available for equilibration of
water and urea.
• 6. Pathophysiology: Central DI reduces plasma AVP levels, whereas
nephrogenic DI reduces renal responsiveness to AVP