Physiological principles of concentration of Urine.

1. Concentration of
Urine
Dr. Aniket Shilwant
BAMS. MD (Ayurveda Physiology)

3. Introduction to Concentration of Urine
▪ GFR per day - 180 L
▪ Osmolarity of GFR is same as of plasma = 300 mOsm/L.
▪ Normal urine almost four times concentrated than Plasma.
▪ Osmolarity of Urine = 1,200 mOsm/L.
▪ Osmolarity of urine depends upon two factors:
1. Water content in the body
2. Antidiuretic hormone (ADH)

4. Osmolarity of Urine
FORMATION OF DILUTE URINE
Increased Water content – More Diuresis – Dilute Urine – ADH Inhibition
FORMATION OF CONCENTRATED URINE
Decreased Water content – Water retention – Concentrated Urine – ADH Stimulation
Formation of concentrated urine is complex procedure as that of dilute urine.
It involves two processes:
1. Development and maintenance of medullary gradient by countercurrent system
2. Secretion of ADH

5. Medullary Gradient
▪ Interstitial fluid at Renal Cortex = 300 mOsm/L
▪ Interstitial fluid at Renal Medulla = 1200 mOsm/L
▪ This gradual increase in the osmolarity of the medullary interstitial fluid is called
the medullary gradient.
▪ It plays an important role in the concentration of urine.
▪ Medullary Gradient – Developed and Maintained by – Countercurrent mechanism

7. Countercurrent Mechanism
▪ Countercurrent Flow
▪ System of U shaped tubules having fluid moving in opposite direction in limbs.
▪ Two divisions –
▪ Countercurrent Multiplier – Loop of Henle
▪ Countercurrent Exchanger – Vasa Recta

8. Role of Loop of Henle in development of
Medullary Gradient
▪ Hyperosmolarity in Inner Medulla – Active reabsorption of Na, Cl ions from Ascending limb into
Medullary interstitium.
▪ Increased concentration of Na, Cl in interstitium – Diffusion into Descending limb –
Again reaches Ascending limb
▪ Only small portion of ions (electrolytes) are excreted
▪ Regular addition of more and more new ions into descending limb due to filtration
▪ Cycle of = Reabsorption from Ascending limb – More addition in Descending limb
▪ Multiplies the quantity of electrolytes in filtrate. This is – Countercurrent Multiplier Mechanism

9. Role of Loop of Henle in development of Medullary Gradient
Factors for Hyperosmolarity of Medullary Interstitial Fluid
I. REABSORPTION OF SODIUM FROM COLLECTING DUCT
Reabsorption of Na from collecting duct into medullary interstitium - Addition to osmolarity of inner medulla.
II. RECIRCULATION OF UREA
Filtered Urea is reabsorbed in PCT but Secreted in Loop of Henle.
Filtered Urea is more present in DCT
Collecting Duct is impermeable to Urea.
Water reabsorption from DCT, Collecting Duct – Increased Urea conc. in Collecting Duct
Diffusion of Urea from Collecting duct into Medullary Interstitium (Urea Transporters – Activated by ADH)
Again Urea enters Ascending limb – DCT – Collecting Duct
Recirculation of Urea
Restricted and limited proportion of Urea is excreted in Urine

10. Role of Vasa Recta in development of Medullary
Gradient
▪ Vasa recta acts like countercurrent exchanger because of its position.
▪ Vasa recta runs parallel to loop of Henle.
▪ Descending limb of VR parallel with Ascending limb of Henle
▪ Ascending limb of VR parallel with Descending limb of Henle
▪ Reabsorption of Na, Cl from Ascending limb of Henle – Medullary Interstitium – Descending limb of
VR
▪ Diffusion of Water from Descending limb of VR – M. Interstitium – Ascending limb VR - Cycle
▪ Blood flows very slowly in VR
▪ More Na, Cl accumulation in Descending limb of VR moves slowly to Ascending limb
▪ Till then concentration of Na, Cl increases much more – Diffusion of Na, Cl into Medullary Interstitium
AL DL AL
LH VR VR

11. Role of Vasa Recta in development of Medullary
Gradient
▪ Benefit of Hairpin bend of VR ?
So, When blood passes through the ascending limb of vasa recta –
▪ Diffusion of Sodium chloride out of blood into medullary interstitium
▪ Diffusion of water into the blood
▪ Recycling of Urea is done same as Henle
▪ Sodium, Chloride ions + Urea are exchanged for Water in between
Ascending and
Descending limbs of VR.
This is – Countercurrent Exchanger Mechanism

13. Resources
▪ Textbook of Medical Physiology – Guyton & Hall
▪ Ganong’s review of Medical Physiology
▪ Essentials of Medical Physiology – Sembulingam
▪ Images – Internet

14. Dr. Aniket A. Shilwant
Associate Professor, Sharir Kriya Dept.
GJPIASR, CVM University, New Vidyanagar, Anand
ayuraniket18@gmail.com
Thank You All !!!