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Transcript of 3/10/2016concentration&dilution of urine1. Renal mechanisms of diluting and concentrating urine …
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Renal mechanisms of diluting and concentrating urine
The kidneys excrete excess water by forming dilute urine and conserves water by excreting
concentrated urine. Can excrete urine
with an osmolarity as low as 50mOsm/L Conversely, can excrete urine
with concentration of 1200 –1400mOsm/L.
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Renal mechanisms of diluting and concentrating urine
ADH alters renal excretion of water independent of rate of solute excretion.
ADH secretion when the solute in the body fluids become too concentrated.
It increases the permeability of late distal tubules and collecting ducts to
water.
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Excretion of dilute urine When there is a large excess of water in
the bodythe kidneys can excrete as much as 20L/day
of dilute urine.Continue to reabsorb solutes while failing to reabsorb large amount of
water in distal parts of the nephron. Glomerular filtrate has osmolarity that
of plasma.
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Excretion of dilute urine In proximal
tubules, solutes and water
are reabsorbed in equal
proportions, so that little change in
osmolarity occur.
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Excretion of dilute urine In the descending
loop of Henle water is
reabsorbed by osmosis
and the tubular fluid reaches equilibrium
with surrounding interstitial fluid.
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Excretion of dilute urine Ascending limb of
loop of Henle is impermeable to
water even in presence
of large amounts of ADH.
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Excretion of dilute urine Active reabsorption
of solutes, causing the tubular
fluid to become more dilute
as it flows into the early distal tubule,
osmolarity decreasing progressively to
100mOsm/L.
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Excretion of dilute urine In the distal tubules,
cortical collecting duct and medullary duct there is additional
reabsorption of NaCl. In the absence of
ADH these segments are
also impermeable to water.
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Excretion of dilute urine Tubular fluid
become even more dilute decreasing its
osmolarity to as low as 50 mOsm/L
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Excretion of concentrated urine Essential for survival of mammals
that live on land. Fluid intake is required
to match with fluid loss. The ability of the kidney
to form a small volume of concentrated urine minimizes the intake required to maintain
homeostasis.
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Excretion of concentration urine
When there is water deficit in the body the kidney forms small volume concentrated
urine by continuing to excrete solutes while increasing water reabsorption.
Obligatory urine volume 0.5l/day. Excretion of concentrated urine requires.
High ADH level.Hyperosmotic renal medullary interstitium.
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Excretion of concentrated urine
NaCl H2O Urea
Urea
H2O
H2O
NaCl
NaCl
NaCl
NaCl
H2O
300300
600 600 600
1200 1200 1200
100300
600
1200
cortex
medullar
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Stimuli for ADH secretion ECF osmolarity Blood volume/Arterial pressure Stimuli to CNS Drugs,
Alcohol inhibits ADH secretion
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Hyperosmotic renal medullary interstitium Provide osmotic gradient
necessary for water reabsorption to occur○ in the presence of high levels of ADH
Water moves through the tubular membrane by osmosis
○ into the renal interstitiumthen by vasa recta back into the blood.
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Causes of hyperosmotic renal medullary interstitium
Special anatomical arrangements of the loops of Henle
and vasa recta.
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Causes of hyperosmotic renal medullary interstitium Major factors that contribute to hyperosmolarity
of renal medullary interstitium are Active transport of Na+, and cotransport of K+, Cl-
and others out of the thick portion of loop of Henle. Capable of establishing concentration gradient
○ of 200 mOsm/L Active transport of ions
from the collecting duct into the medullary interstitium. Passive diffusion of urea
from inner medullary collecting ducts into the interstitiusm
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Causes of hyperosmotic renal medullary interstitium Diffusion of only small amount of water
from the medullary collecting ducts into the medullary interstitium. large amount being reabsorbed
○ in late distal tubules and cortical collecting ducts. Some passive reabsorption of NaCl
in the descending loop of Henle.
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Mechanisms Active pump
of thick ascending limb of loop of Henle
establishes 200mOsm/L concentration gradient
between the tubular fluid and interstitial fluid.
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Mechanisms Tubular fluid in the
descending limb of loop of Henle and
the interstitial fluid quickly reach
osmotic equilibrium. Hyperosmotic fluid
formed in the descending limb
flows into ascending limb.
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Mechanisms Once this fluid
is in the thick segment
additional ions are pumped into the
interstitium with water remaining
behind until osmotic
gradient of 200mOm/L○ is established.
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Mechanisms Tubular fluid in the
descending loop and hyperosmotic
medullary interstitial fluid
reaches equilibrium.
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Mechanisms
These are repeated over and over
with the net effect of adding more and
more solutes to the medulla in excess of water.
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Mechanisms With sufficient time
this process multiplies the concentration
gradient established by active
transport of ions in the thick segment of
loop of Henle raising the osmolarity to 1200 – 1400mOm/L
This process Counter current
multiplier.
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Mechanisms Passive reabsorption of urea
from inner medullary collecting ducts contributes 40% of the renal medullary
interstitium osmolarity when the kidney is forming maximally concentrated
urine.
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Mechanisms Very little urea
is reabsorbed in the loop of Henle.
Distal tubules and cortical collecting ducts are impermeable to
urea even in the
presence of high concentration of ADH.
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Mechanisms In the presence of
high [ADH], Urea] increases
rapidly in cortical collecting duct.
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Mechanisms As fluid flows
into inner medullary collecting ducts
further reabsorption
of water takes place,
[Urea] increase.
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Mechanisms Increased [Urea]
causes large amount of urea
to diffuse into the interstitium
because this segment
is highly permeable to urea
ADH increases this permeability
even more.
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Urea cycle Urea is recycled
several times before it is excreted.
Some diffusion of urea into the thin loop of
Henle from the medullary interstitium.
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Maintenance of hyperosmolarity of the renal medulla
Medullary blood flow is low The vasa recta serves as counter
current exchangers,Minimizing wash out of solutes from the
medullary interstitium. Blood enters and leaves the medulla by
the way of vasa recta at the boundary of cortex and medulla.
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Maintenance of hyperosmolarity of the renal medulla
As blood descends into the medulla toward the papillae it becomes
progressively more concentrated
party by solute ○ entry from the
interstitium
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Maintenance of hyperosmolarity of the renal medulla
and party by water diffusing ○ into the
interstitium By the time
blood reaches the tips of vasa recta
it has conc.of 1200mOm/L.
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Maintenance of hyperosmolarity of renal medulla
As blood ascends back toward the cortex it become
progressively less concentrated
water moving into vasa recta○ solutes diffusing
out.
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Maintenance of hyperosmolarity of renal medulla
Thus although there is large amount of fluid
and solute exchange ○ across the vasa
recta,there is little
○ net dilution of the concentration
○ of the interstitial fluid at each level of the medulla.
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Vasa recta mOsm/L
Renal medullaryinterstitium
Renal medullaryinterstitium
H2O
H2O
H2O
600
800
10001200
600
300
solutes
solutes
solutes
solutes
solutes
solutes
600600
800 900
1200
800
300300