This document discusses urine osmolarity and the mechanisms involved in concentrating and diluting urine. It begins by introducing a clinical case of acute on chronic renal failure and the decision to check urine osmolarity. It then defines urine osmolarity and concentration and normal ranges. The mechanisms of excreting dilute or concentrated urine are described, involving antidiuretic hormone (ADH) and countercurrent multiplier and exchanger systems in the kidney. The role of urea and various transporters in concentrating the medullary interstitium is also covered. Key concepts like free water clearance and osmotic diuresis are defined. References are provided.

1. URINE
OSMOLARITY
PBL TUTOR(Group-IV)
Dr.Rupesh Shrestha
Presenter
Lalit karki

2. Case correlation
• Sudhir,13 years boy presents with puffy
face,haematuria,frothy urine and his urine
amount has markedly decresed.
• He was primarily confirmed as the case of acute
on chronic renal failure.
• Dr. Anish wanted to do urine for osmolarity and
fractional excretion of sodium.

3. Introduction
• Urine osmolarity is the number of dissolved particles per
unit of water in the urine.
• Urine concentration
• An index of the concentration of osmotically active
particles,particularly chloride, sodium, urea, glucose and
potassium in the urine.
• Minimum urine concentration:50 - 70 mOsm / L
• Maximum urine concentration :1200 - 1400 mOsm / L

4. Excreting a dilute urine
• Decrease water reabsorption
• Continue electrolyte reabsorption
Mechanism:
•decreased ADH
release and
reduced water
permeability
in distal and
collecting tubules

6. Excreting a concentrated urine
• Continue electrolyte reabsorption
• Increase water reabsorption
Mechanism:
• Increased ADH release which increases water
permeability in distal and collecting tubules
• High osmolarity of renal medulla
• Countercurrent flow of tubular fluid

7. Role of ADH

8. Counter current multiplier system
• High
permeability of
thin
descending
limb to water.
• The active
transport of Na
and Cl out of
thick ascending
limb
• Inflow of
tubular fluid
from the
proximal tubule
with outflow
into the distal
tubule.

9. Vasa recta-counter current exchangers
• Supply medullary
parts
• Prevents rapid
dissipation of
medullary
hyperosmolarity
• On descending part
Nacl and urea is
reabsorbed.and water
is secreted
• On ascending part
NaCl and urea are
secreted into the
interstitium, while
water is reabsorbed.

10. Urea concentrating medullary intestitium
Contribution is about
40-50%
4 urea transporters
(UT1-4)
UT-A1 and UT-A3 is
activated by
presence of ADH
Amount of urea in urine
and medullary interstitium
depends on filtration.

11. Concentrated urine

13. Concept of…
• Free Water Cleareance
To quantitate the gain or loss of water by
excretion of a concentrated or dilute urine.
C(H2O)=V-(Uosm x V)/Posm
-ve for hypertonic urine
+ve for hypotonic urine
• Osmotic diuresis
Presence of unreabsorbed solutes increase
the urine volume.
• Relation of urine concentration to GFR
Decrease in GFR increase the urine
concentration and vice-versa.

14. References
• Textbook of medical physiology, Guyton and Hall,12th
edition
• Ganong’s Review of medical physiology,23rd
edition

15. Group members:
-Aliza Hamal
-Anup Bhatta
-Bijay Shrestha
-Bikash Sapkota
-Lalit Karki
-Prabesh Raghubansi
-Prayash Panthi
-Suchana Sharma
-Sujata Pandey
-Sunder Chapagain