COUNTER CURRENT MECHANISM (Concentration Mechanism of Urine)
Urine Concentration and Diluting Mechanisms
Transcript of 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