RENAL SYSTEM
PHYSIOLOGY
Dr Shahab Shaikh PhD, MD
Lecture – 7: Concentration and Dilution of Urine
••••••••••••••••••••••••••••••••••
College of Medicine
Al Maarefa Colleges of Science & Technology
OBJECTIVES
• Describe the factors that determine the ability of
loop of Henle to creat osmotic medullary
gradient
• Identify
countercurrent multiplier and
countercurrent
exchange
systems
in
concentrating and diluting urine
• Explain changes in osmolarity of tubular fluid in
the various segments of the loop of Henle when
concentrated urine is being produced.
• Describe the role of ADH on the ability of the
kidney to produce either a dilute or a
concentrated urine.
Urine Formation
• Three Basic Mechanisms
(Renal Processes) of
Urine Formation include:
1.Glomerular filtration - GF
2.Tubular reabsorption - TR
3.Tubular secretion - TS
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Urine Formation
“The composition of the blood
(internal environment) is determined
not by what the mouth ingest but by
what the kidney keep.”
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Urine Formation
• Kidneys maintain water homeostasis by adjusting the
volume of urine produced.
• The ability of the kidney to concentrate or dilute
urine is an important function in regulating
Extracellular volume (ECV), Extracellular Fluid
osmolarity.
• The kidney can excrete dilute urine with an
osmolarity as low as 30 to 50mosm/liter or
concentrated urine with osmolarity as high as about
1200 to 1400mosm/liter.
• Kidneys can vary Urine Osmolarity between 30-1200
mosm/kg and urine volume between 0.5-20 L/day.
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Urine Formation
• This function of concentrating or diluting the
urine is determined in kidneys by the amount of
water reabsorption in the renal tubules.
• Water reabsorption is obligatory in the
proximal(65%) tubules and Loop of Henle(15%).
• Thus the final adjustment of the urine volume and
osmolarity depends on the extent of water
reabsorption in the Collecting Ducts.
• Water reabsorption from the collecting ducts
depends on:
• The blood level of Antidiuretic Hormone (ADH).
• The Medullary interstitium (MI) hypertonicity
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Urine Concentration
The basic requirements for forming
a concentrated (or diluted) urine is:
1. Secretion
of
antidiuretic
hormone (ADH):
– Regulates the permeability of the
distal tubules and collecting ducts
to water.
– High ADH causes high reabsorption
of water  Concentrated Urine.
2. High osmolarity of the renal
medullary interstitial fluid:
– Provides the osmotic gradient
necessary for water reabsorption
to occur in the presence of high
level of ADH.
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Urine Concentration
Counter Current Mechanism:
• Provides the osmotic gradient necessary for water
reabsorption by creating and maintaining high
osmolarity of the renal medullary interstitial fluid.
• Is a function of Juxta-medullary nephrons.
• Consists of …
1. Counter Current Multiplier system
– Creates the high osmolarity of the
renal medullary interstitial fluid
2. Counter Current Exchanger system
– Maintains the high osmolarity of
the renal medullary interstitial
fluid
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Counter Current Multiplication
• Creates the hyperosmotic Medullary
Interstitium by solute deposition in
medullary interstitium.
• NaCl reabsorbed from the thick
ascending limb of loop of Henle is
deposited in medullary interstitium.
• Active transport of NaCl along thick
ascending loop results in the
movement of water from the
descending limb increasing the
osmolarity of descending limb fluid.
• Urea reabsorbed from collecting
duct (ADH) to medullary interstitium
also contribute to medullary
hyperosmolarity.
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Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
COUNTERCURRENT MULTIPLICATION
BENEFITS:
1. It establishes a vertical osmotic gradient in the
medullary interstitial fluid. This gradient, in
turn, is used by the collecting ducts to
concentrate the tubular fluid so that a urine
more concentrated than normal body fluids
can be excreted.
2. Second, the fact that the fluid is hypotonic as
it enters the distal parts of the tubule enables
the kidneys to excrete a urine more dilute
than normal body fluids.
COUNTERCURRENT EXCHANGER
• It helps to maintain the Hyperosmolarity of medulla
while providing blood to renal medulla.
• Occurs by the Vasa Recta supplying the long loop of
Henle of Juxtaglomerular nephrons.
• As the blood passes through
the descending part of Vasa
Recta it looses water and gain
solutes due to increasing
osmolarity
in
medullary
interstitium.
• Reverse happens when the
blood returns back through
the ascending part of vasa
recta.
• Thus Hyperosmolarity of medulla remains undisturbed.
Role of Vasopressin(ADH)
• In the distal tubule and collecting duct water reabsorption is
variable based on the secretion of ADH.
• Osmotic gradient already exists between the tubules and
interstitium in the renal medulla created by the
countercurrent mechanism.
• When ADH is secreted in blood, it increases the
permeability of the tubule cells to water by increasing the
Aquaporin-2 water channel proteins for the transport of
water into interstitium by osmosis. Thus Concentrating
Urine.
• During a water deficit, the secretion of vasopressin
increases. This increases water reabsorption.
• During an excess of water, the secretion of vasopressin
decreases. Less water is reabsorbed. More is eliminated
making urine dilute.
Vasopressin: Mechanism of action
Vasopressin: Water Regulation
The Urea Recirculation
The Concentration of Urine
Regulation of H2O reabsorption in response to H2O deficit
The Dilution of Urine
Regulation of H2O reabsorption in response to H2O excess
Clinical Aspect
• Diuresis is condition in which urine production by the
kidneys is increased
• Osmotic diuresis is due to filtration of excessive
osmotically active substances in renal tubules. Eg.
Diabetes Mellitus, Mannitol
• Syndrome of Inappropriate Antidiuretic Hormone
secretion or SIADH is characterized by excessive
release of antidiuretic hormone from the posterior
pituitary gland or another source resulting in water
retention and dilutional hyponatremia.
• Diabetes Insipidus is a condition characterized by
excessive thirst and excretion of large amounts of
severely diluted urine due to either a problem with
the production of ADH (Central D.I) or decreased
kidney's response to ADH (Nephrogenic D.I).
References
• Human physiology by Lauralee Sherwood, 8th
edition
• Text Book Of Physiology by Guyton & Hall, 11th
edition
• Review of Medical Physiology by Ganong. 24th
edition
THANK YOU
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