Urine Concentration and Diluting Mechanisms

7/31/2019 Urine Concentration and Diluting Mechanisms

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Urine concentration and dilution

Dr. Niranjan Murthy HL

Associate Professor

Dept of Physiology

SSMC, Tumkur


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Daily urine output- 1.5 to 2 ltrs

Dilute urine- 15% of filtered volume

Concentrated urine- 0.5% of filtered volume


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Water Reabsorption

By osmosis:

1. Obligatory reabsorption- PCT- 65%

2. Countercurrent mechanism- 20%3. Facultative reabsorption- ADH influence- 15%


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Obligatory reabsorption:

Seen in PCT

65% of filtered load Follows solute reabsorption

AQP 1


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Countercurrent mechanism

Countercurrent multiplier- loop of henle

Countercurrent exchanger- vasa recta


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Requirement for countercurrent mechanism:

1. Countercurrent flow

2. Active transport3. Differential water permeability


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Countercurrent multiplier


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Descending limb:

Freely permeable to water

Thin ascending limb: Permeable to sodium, chloride and urea

Thick ascending limb:

Active transport of sodium, chloride andpotassium


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Active reabsorption of sodium and chloride

from thick ascending limb facilitates water

reabsorption-

1. from cortical collecting duct in presence of

ADH by delivering hypotonic fluid to cortical

collecting duct

2. from medullary collecting duct by

establishing osmotic gradient


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Countercurrent mechanism increases

medullary interstitial concentration and

gradient which is necessary for water

reabsorption in collecting tubule


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Role of urea:


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Role of urea:

Contributes to 50% of osmolality at medullary

interstitium

Permeable in presence of ADH

Recirculation

Urinary concentration without expenditure ofenergy


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Facultative reabsorption of water:

Facilitated by ADH and Aldosterone

DCT and collecting duct


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ADH: antidiuretic hormone

Peptide hormone

Posterior pituitary V2 receptors in distal tubule

AQP2

Contributes to 15% of water reabsorption


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Gradient for water reabsorption in distal

tubule is established by-

1. Sodium Chloride co-transport in thick

ascending limb

2. Sodium potassium counter-transport in

collecting duct by the influence of

aldosterone


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Countercurrent exchanger

Maintains

medullary osmotic

gradient established

by countercurrentmultiplier


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Small lag in equilibration between the blood

and the adjacent peritubular fluid

Volume of blood leaving vasa recta is slightly

greater than that entering because of-

1. Lag in equilibration

2. Oncotic forces


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Blood flow to medulla is 10% of total RBF

Blood flow is sluggish

Protection for medullary cells against high

osmotic gradient:

Formation of highly osmotic inositol, sorbitol,etc by medullary epithelium


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Other areas of countercurrent mechanism in

body:

Testis

Intestinal villi

Skin


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Free water clearance

Cosm = (Uosm x V) / Posm

CH2O

= V

Cosm

= V

(Uosm

x V) / Posm

If negative- concentrated urine

If positive- dilute urine


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Factors affecting concentrating and

diluting mechanisms of kidneys

1. ADH levels

2. Active reabsorption of NaCl by thick

ascending limb of loop of Henle (loop

diuretics)length of loop and percentage of

nephrons with long loops- Psammomys (35%)

3. Availability of urea- protein diet

4. Rate of flow through loop of henle and

collecting duct


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5. Rate of flow through vasa recta

6. Presence of prostaglandins

PGE2: increases blood flow vasa recta andreduces active reabsorption of NaCl

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Urine Concentration and Diluting Mechanisms