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Counter Current Multiplication.
And Micturition Reflex
Dr.Mohammed Sharique Ahmed Quadri
Assistant professor physiology
Al Amaarefa College
• 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.
• Discuss the micturition reflex.
Urine Excretion
• Depending on the body’s state of hydration, the
kidneys secrete urine of varying concentrations.
• Too much water in the ECF establishes a
hypotonic ECF.
• A water deficit establishes a hypertonic ECF.
• This function is determined by the amount of water
reabsorption in the renal tubules.
Urine Concentration & Dilution
• Water reabsorption is obligatory in the proximal(65%)
tublues and Loop of Henle(15%).
• The final adjustment of the urine volume and osmolarity
depends on the extent of facultative water reabsorption in
the Collecting Ducts, which is depends on:– The blood level of antidiuretic hormone (ADH).
– The Medullary interstitium (MI) hypertonicity
(Medullary concentration gradient).
Urine Concentration
Requirements for forming a concentrated urine
 High level of ADH (increase permeability)
 High osmolarity of the renal medullary
interstitial fluid (osmotic gradients)
Driving force for H2O reabsorption
• Driving force for water reabsorption
throughout the entire length of tubules is an
osmotic gradient between tubular lumen and
surrounding interstitial fluid.
Urine Excretion
• A large, vertical osmotic
gradient is established in the
interstitial fluid of the medulla
(from 100 to 1200 mosm/liter)
• This osmotic gradient exists
the tubular lumen and the
interstitial fluid.
Urine Concentration & Dilution
What is the process by which renal medullary
interstitial fluid becomes hyperosmotic?
This process involves the operation of the
medullary countercurrent system
Medullary Countercurrent system
• Juxta medullary nephrons
– long loop of henle estabilishes a vertical osmotic
gradient (Countercurrent multiplier) ,
– their vassa recta preserve this gradient while
providing blood to renal medulla,( Countercurrent
– collecting ducts of all nephrones use the gradient in
conjunction with the hormone vassopressin, to
produce urine of varying concentration (osmotic
equilibrating device) .
• Collectively this entire functional organization is
known as medullary countercurrent system
Countercurrent Multiplication
• Comparing the descending and ascending limbs
of the loop of Henle
• The descending ling is highly permeable to
water but does not extrude sodium for
• The ascending limb actively transports NaCl
out of the tubular lumen into the surrounding
interstitial fluid. It is impermeable to water.
Therefore, water does not follow the salt by
• There is a countercurrent flow produced by the
close proximity of the two limbs.
Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
Countercurrent Multiplication
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
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.
Role of Vasopressin
• Vasopressin-controlled, variable water reabsorption occurs in
the final tubular segments.
• 65 percent of water reabsorption is obligatory in the proximal
tubule. In the distal tubule and collecting duct it is variable,
based on the secretion of ADH.
• The secretion of vasopressin increases the permeability of the
tubule cells to water. An osmotic gradient exists outside the
tubules for the transport of water by osmosis.
• Vasopressin works on tubule cells through a cyclic AMP
• 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.
Mechanism of action of Vasopressin
Regulation of H2O reabsorption in
response to H2O deficit
Regulation of H2O reabsorption in
response to H2O Excess
Countercurrent exchange within the vasa
recta conserves the medullary vertical
osmotic gradient.
Renal Failure
• Causes of renal failure
– Infectious organisms
– Toxic agents
– Inappropriate immune responses
– Obstruction of urine flow
– An insufficient renal blood supply
• Urine stored in bladder is eliminated by micturition
• Urine in bladder stimulates stretch receptors
• Stimulated stretch receptors signal smooth muscle in
bladder wall by parasympathetic neurons
• Contraction of bladder pushes urine out of the body
• Micturition reflex
– Relaxation of external urethral sphincter muscle
allowing urine to pass through urethra and out of the
• Under voluntary control but cannot be delayed
• Urinary incontinence
– Inability to prevent discharge of urine
Reflex and Voluntary Control of Mictrurition
Abnormalities of micturition
Atonic Bladder Caused by Destruction of Sensory Nerve
• Micturition reflex cannot occur if the sensory nerve
fibers from the bladder to the spinal cord are
• Person loses bladder control
• Instead of emptying periodically, the bladder fills to
capacity and overflows a few drops at a time through
the urethra. This is called overflow incontinence.
• A common cause of atonic bladder is crush injury to
the sacral region of the spinal cord.
• Certain diseases can also
• Automatic Bladder Caused by Spinal Cord Damage
Above the Sacral Region.
• Typical micturition reflexes can still occur.
• They are no longer controlled by the brain.
• During the first few days to weeks the micturition
reflexes are suppressed because “spinal shock” (sudden
loss of facilitative impulses from the brain stem and
• Gradually typical micturition reflexes return; then,
periodic (but unannounced) bladder emptying occurs.
• Some patients can still control urination in this condition
by stimulating the skin (scratching or tickling) in the
genital region, which sometimes elicits a micturition
• Uninhibited Neurogenic Bladder Caused by Lack
of Inhibitory Signals from the Brain.
• Which results in frequent and relatively
uncontrolled micturition.
• From partial damage in the spinal cord or the
brain stem that interrupts most of the inhibitory
• Facilitative impulses passing continually down
the cord -even a small quantity of urine elicits an
uncontrollable micturition reflex, thereby
promoting frequent urination.
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
• Text book of physiology by Linda .s
contanzo,third edition