Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 1 of 11
SESSION 5: Tubular Transport
Na+, Cl- and H2O Handling in Loop of Henle and Distal Nephron
Countercurrent Multiplier Mechanism
Berne & Levy: pp. 707-710, 722-728
Note: there are several figures associated with this scribe. Although most of the explanations are
sufficient, if you’re a picture person, I would recommend pulling out the powerpoints for reference.
I. Session 5 overview
a. Mechanisms of salt & water handling in loop of Henle, distal convoluted
tubule, collecting duct
b. Countercurrent multiplier mechanism
i.
Helps conserve water during dehydration by depositing high
solute concentrations in the interstitial fluid
ii.
In the medulla
c. Luminal solute concentrations along nephron
i.
Know how and why it changes
d. Diuresis vs. antidiuresis
e. Obligatory water loss
f. Osmolar clearance, free water clearance
II. Renal control of salt and water balance
a. Crucial for regulation of:
i.
Blood pressure
1. Volume = water and the solutes in it
2. Volume is not controlled by water
3. It’s controlled by Na; more Na in the body = more
extracellular fluid
4. renal Na handling helps regulate systemic BP (volume)
ii.
Extracellular fluid solute concentration
1. solute concentration is regulated by the amount of water
that is excreted (or not excreted) in the urine
2. Na controls Volume; water controls the extracellular Na
concentration in the kidneys
iii. Concentrations of Na and K in body fluids
b. Normal function of these mechanisms allows:
i.
Water retention during dehydration
ii.
Excretion of dilute urine when well hydrated
iii. Sodium excretion when blood pressure rises or sodium retention
when blood pressure falls
c. Failure of renal control of salt and water balance can cause:
i.
Edema
1. more Na in the body causes more extracellular fluid and
edema
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 2 of 11
ii.
iii.
iv.
v.
Disorders of plasma K+ concentration: hyperkalemia,
hypokalemia
Undesirable changes in blood pressure
1. hypervolemic (high BP) or hypovolemic (low BP)
situations can occur
Acid/base disorders
Neurological problems: shrinking or swelling of the brain
1. intracellular volume changes are very dangerous in the
brain where there is little room for expansion
2. very important that extracellular solute concentration is
maintained within a narrow range
III. Sodium reabsorption
a. Proximal tubule:
i.
Na Cotransport with glucose, amino acids and phosphate
ii.
Na Countertransport with H+ (Na+/H+ exchange – Na reabsorbed
and H secreted)
b. Thick ascending limb of Henle: Na+,K+,2Cl- cotransport mechanism in the
luminal membrane
i.
Important for generating the high solute concentration in the
medullary interstitial fluid
c. Distal convoluted tubule: Na+,Cl- cotransport
d. Collecting duct: luminal membrane channels
i.
Na is reabsorbed by passing through luminal membrane channels
NOT cotransport as in other parts of the nephron
ii.
There is no Na secretion in the kidney, only reabsorption.
iii. The amount reabsorbed is under hormonal control, expecially in
the collecting duct (more on this later)
e. There is a high solute concentration (conc) in the interstitial fluid
IV. Water and Chloride
a. Both are dependent on Na reabsorption:
b. Water reabsorption:
i.
Always passive
ii.
can be transcellular or paracellular
1. paracellular is most important in the proximal tubule
iii. Follows osmotic gradients established by reabsorption of sodium
and other solutes
c. Chloride reabsorption:
i.
Always linked, either directly or indirectly, to Na+ reabsorption
1. a way to maintain electro-neutrality
2. can occur directly through cotransport as in loop of henle
and distal convoluted tubule
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 3 of 11
ii.
3. indirectly by a paracellular reabsorptive process as in the
proximal tubule
a. as water and Na left the lumen, the Cl became more
concentrated and passed down its gradient between
the cells.
Specific mechanisms differ in different segments
V. Loop of Henle
a. Descending limb:
i.
Freely
permeable to water ALWAYS, reagardless of
hydration state
ii.
Impermeable to Na+ and Cliii. Composed of thin squamous epithelium
b. Ascending limb:
i.
Always impermeable to water
ii.
Composed of 2 segments (both impermeable to water)
1. Thin segment: passive NaCl reabsorption
2. Thick segment: active Na+,K+,2Cl- cotransport
a. Near cortex
b. All the ions (Na, K and Cl) are reabsorbed at the
same time
c. Transport in Thick Ascending Limb of Henle’s loop (FIGURE)
i.
(See also B&L Fig. 41-8, p. 708)
ii.
peritubular interstitium is on the left
iii. tubular lumen on right
iv.
Na and Cl move in the cell from the lumen down the
concentration gradient (cg) that’s established by the Na/K
ATPase.
1. provide the driving energy for K and 2 Cl to move against
their cg.
2. note that this is an electrically neutral process
v.
Cl and K move through leak channels into the capillaries and to
the body
vi.
Some of the K is secreted back out of the cell to the tubular
lumen
vii.
This flow of ions creates of slightly more negative charge in the
interstitium because of the K leak
1. the transepithelial luminal potential is +6mV compared to a
negative interstitium
viii. The electrical gradient is the driving force for paracellular
movement, especially for Mg and Ca
VI. Transport in distal convoluted tubule
a. (See also B&L Fig. 41-9, p. 709)
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 4 of 11
b. Na and Cl move in together
VII. Collecting duct
a. Site of physiological (hormones) control of salt and water balance
b. This is the site of extracellular volume and BP “fine tuning”
c. Aldosterone stimulates Na+ reabsorption, K+ secretion, H+ secretion in the
CD
i.
Aldosterone is produced in the adrenal glands
d. Atrial natriuretic peptide (ANP) inhibits Na+ reabsorption
i.
From the anterior pituitary
ii.
Can be over-secreted if a tumor is present
e. Antidiuretic hormone (Vasopressin) stimulates water reabsorption
i.
During dehydration, ADH is Secreted from the posterior
pituitary, travels through blood stream to collecting ducts and
promotes water retention
f. There are 3 types of cells in the collecting tubule
i.
Principal cell – reabsorb Na and secrete K
ii.
α and β intercalated cells – control the pH (H+ movement)
g. Cation transport in collecting duct FIGURE
i.
(See also B&L Fig. 41-10, p. 710)
ii.
Note the large transepithelial potential. This is part of the
driving force for K and H secretion into the lumen of the
collecting duct
iii. The Na/K pump establishes a gradient for ion movement
1. allows Na to enter the cell along it’s electrochemical
gradient (ecg)
2. K exits along its ecg, maintaining electrical neutrality
iv.
K movement is indirectly linked to Na reabsorption
1. more Na absorbed, the more K secreted
VIII. Aldosterone (FIGURE)
a. A steroid hormone
b. mechanisms for increasing Na+ reabsorption in collecting duct:
i.
enters the cell and goes into the nucleus
ii.
activates mRNA that encode Na channel synthesis
iii. channels incorporated into the luminal membrane
iv.
increases capacity of principal cells to reabsorb Na
IX. Water permeability of collecting duct
a. physiologically controlled
b. Well-hydrated individuals:
i.
Collecting duct is impermeable to water
ii.
Water remains in tubular lumen; dilute urine is excreted
iii. There is little/no ADH present
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 5 of 11
1. ADH increases permeability of the collecting duct
c. Dehydrated individuals:
i.
Lots of ADH present
ii.
Collecting duct is highly water-permeable
iii. Water is reabsorbed; low volume of concentrated urine is
excreted
d. Moderately hydrated people would have a moderate amount of ADH
e. ADH mechanism: (FIGURE)
i.
Binds to membrane receptor that is linked to a G protein, Gs.
ii.
Activates adenylate cyclase which catalyzes the reaction:
ATPcAMP
iii. Phosphorylation by cAMP dependent kinase lead to
incorporation of water channels in the membrane
iv.
The intracellular membrane fuses with the luminal membrane,
allowing for the incorporation of the channels
v.
Water then easily enters the cell transcellularly from the tubular
fluid
vi.
It exits basolaterally and returns to the body
vii.
There is NO paracellular movement of water because the tight
junctions are very tight in the collecting duct.
X. Solute concentrations in peritubular interstitium (FIGURE)
a. The solute concentration in the interstitium of the cortex is about the same
as the plasma conc
b. The interstitial solute conc at the tip of the inner medulla is about 4x that
of the cortex
i.
This is unusually high compared to other parts of the body.
ii.
Very important to the kidney’s regulation of water
XI. Countercurrent multiplier mechanism
a. Concentrates solute in medullary interstitium
b. High solute concentration enables kidneys to excrete highly concentrated
urine and conserve water during periods of dehydration
i.
Highly conc urine = low volume
c. This mechanism requires integrated function of 3 components:
i.
Descending and ascending limbs of Henle’s loop
1. together act as the countercurrent multipliers
2. Descending Loop of henle is permeable to water, but not
Na and Cl
3. Ascending loop is impermeable to water, but permeable to
Na and Cl
ii.
Vasa recta capillaries
1. along the loop of henle
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 6 of 11
iii.
2. countercurrent exhangers that help maintain the cg (high
solute concentration)
Collecting ducts
1. the osmotic equilibrators of luminal fluid with interstitial
fluid
2. The collecting duct adjusts water reabsorption according
the level of ADH in able to equilibrate the luminal fluid
concentration with the interstitial fluid concentration.
XII. Solute concentrations during antidiuresis FIGURE
a. Don’t memorize the numerical values for the exam; know the concepts:
b. The kidney conserves water (low volume urine) in the presence of high
ADH levels
c. descending limb: the fluid is isotonic
d. As it moves toward the medulla, water is reabsorbed and solutes are left in
the lumen, becoming more conc
e. ascending limb: The opposite occurs; its impermeable to water, but Na
and Cl are reabsorbed (removed from the lumen) and the solute conc
becomes hypotonic
i.
the luminal conc is 1/3 lower than that of the plasma conc
ii.
Reabsorption occurs via a Na/Cl cotransporter (active process)
iii. Establishes an osmotic gradient across the epithelium so that the
conc in the lumen is ~200mOsm less than the interstitial fluid .
1. this gradient is called the Single effect is enlarged by the
countercurrent multiplier to create an axial gradient (from
the cortex to the tip of the medulla)
f. distal convoluted tubule: continues the dilution
g. collecting duct: the action of ADH causes water to be reabsorbed from
the lumen and carried back to the body by the vasa recta
i.
At any level of the medulla the conc of the fluid is the same as
the interstitial fluid; this is because as water leaves, the solute
concentration rises so that luminal fluid conc equals the solute
conc of the interstitial fluid.
ii.
VERY conc urine is produced(1200-1400 osmolality)
h. 3 factors are essential to producing concentrated urine (allowing
kidney to conserve water when needed)
i.
these are what produce the single effect mentioned above:
ii.
transport of solute
iii. water is reabsorbed from the descending limb, causing the fluid
lumen to become more concentrated
iv.
fluid is always moving through the nephron, thus the ascending
loop is always receiving a high conc solution. Transporters work
to move the solute out, resulting in a concentrated interstitium.
This allows for the kidneys to conserve water when necessary.
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 7 of 11
i. Controversy exits as to whether the distal convoluted tubule responds to
ADH or aldosterone. Books say different things, thus we won’t be tested
on that. The collecting duct definitely responds to these chemicals. The
questions on the test will be very clear.
XIII. Urea is concentrated in inner medulla FIGURE
a. Number in the rectangles indicate the % of urea filtered
b. 50 % is taken up in the proximal tubule
c. Enough urea is secreted so that the amount going up to the distal
convoluted tubule is about the same as that initially filtered
d. The thick ascending limb, distal convoluted and collecting duct are always
impermeable to urea.
e. Urea remains “trapped” in the lumen until it gets to the inner medulla
i.
In the inner medulla, ~80% is reabsorbed
ii.
Of that, 30% is taken up by the vasa recta and returned to the
body. 50% provides the source of urea that was secreted in the
thin ascending limb.
f. time is spent in the interstitial fluid space when passing between the tubule
lumen
i.
this contributes to a higher solute conc, allowing for further
reabsorption of water (water conservation)
ii.
of a 1200 mOsm solute at the tip of the medulla, about 600 of
that is urea (while the other 600 would be NaCl).
g. ~20% of urea MUST be excreted to maintain balance and prevent an
inappropriate increase in circulating urea.
i.
Kidney failure: urea buildup in blood stream
h. ADH promotes urea reabsorption from the inner medullary collecting
duct.
i.
When the body is trying to conserve water, urea will also be
conserved and deposited in the interstitium. This make the
interstitium more conc and via osmosis, water follows.
XIV. Water diuresis FIGURE
a. ex: very hydrated people or when ADH receptors are blocked (beer)
b. A high volume of dilute urine is excreted
c. Numbers in rectangles on figure are the % of filtered water
d. Values in circles are the solute conc
e. The collecting duct epithelium is impermeable to water
f. low ADH
g. 285 is the typical plasma osmolality value
h. water is still absorbed in the proximal tubule and the descending tubule
i. the solute conc increases, but not up to 1200
j. in a well hydrated person, the solute conc in the inner medulla decreases
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 8 of 11
i.
solute is washed out of the kidney interstitium, decreasing the
kidneys ability to retain water. (--this is ok in a well hydrated
person)
k. Collecting duct epithelium becomes impermeable to water because ADH
is low
i.
Na and Cl are still reabsorbed
ii.
Water remains
iii. the luminal fluid to become very dilute
iv.
15% of the original water absorbed in excreted in the urine
v.
this is very high volume and dilute urine
XV. Antidiuresis
a. high ADH
b. only about 1% of the filtered water is excreted
c. interstitial solute conc is high
d. ADH makes the collecting duct epithelium highly permeably to water.
Water is reabsorbed in this segment and a low volume, highly conc urine
is excreted.
e. AHD also allows urea to be reabsorbed in the collecting duct: The
concentrated urine can also be attributed to increased reabsorption of urea
from the collecting duct, wherein the water follows. (as explained above)
XVI. Luminal osmolality along nephron length GRAPH
a. proximal tube always reabsorbs 2/3 of the water regardless of the state of
hydration because it is so permeable to water
b. descending limb:
i.
During diuresis (low ADH), the tubular fluid osmolality is low
ii.
Antidiuresis (high ADH), the tubular fluid osmolity is high
(conc urine)
c. Ascending Limb: a dilute fluid is produced because solute is reabsorbed
and water remains behind.
d. Distal convoluted tubule
i.
Further reabsorbs Na and Cl, making luminal fluid even more
dilute
e. Collecting duct
i.
The largest difference in hydration states is seen here
ii.
Diuresis: water remains in the collecting duct leading to a low
urine concentration because less urea is reabsorbed to the
interstitium.
iii. Antidiuresis: water and urea are removed from the collecting
duct to match the interstitial solute conc and the urine becomes
very concentrated.
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 9 of 11
XVII. Vasa recta maintain solute gradient
a. Act as countercurrent exchangers and work to maintain solute conc in the
interstitium established by the loop of henle
b. They also function to remove the materials reabsorbed into the interstitium
and returns it to the body
c. Arranged in a hairpin loop along the loop of henle
d. Water and NaCl are exchanged between descending and ascending limbs
e. Solute gradient is maintained while small amounts of NaCl and water are
returned to systemic circulation
f. The vasa recta carries water back to the body
g. The numbers on the figure is the osmolality of plasma
h. Because the vessels are permeable to both water and solute, they can
equilibrate with the intersitium as seen in the descending portion of the
vasa recta
i. At the tip of the medulla, the interstitial conc is higher than the blood,
drawing water out of the vasa recta
j. Then the solute conc in the blood is lower than that of the interstitium, it
will then draw solute into the blood stream
k. Water and NaCl are exchanged between the descending and ascending
limbs
l. Solute gradient is maintained while small amounts of NaCl and water are
returned to systemic circulation.
m. This occurs throughout the vasa recta so that at the tip of the medulla, the
solute conc is high
n. Both the volume and solute conc leaving the vasa recta is higher than
when it went in
o. Effect on RBCs: they’re going to lose water and become shriveled and
spiked. They recover as they move back up to the cortex.
XVIII. Flows, solute concentrations in antidiuresis FIGURE
a. Shows blood flow and osmolality of going into the vasa recta and leaving
b. Leaving: higher flow (volume) and conc
c. Vasa recta remove the volume of water and solute that was reabsorbed
from the loop of henle and collecting duct.
XIX. Obligatory water loss
a. How much urine we must excrete to rid the body of metabolic waste
product
b. Body generates 600 milliosmol wastes/day.
c. Maximum urine concentration = 1200 mOsm/l
d. What is minimum urine excretion to eliminate wastes?
i.
(600 mOsm/day)/ (1200 mOsm/l) = 0.5 l/day
e. What if the kidneys couldn’t concentrate urine?
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 10 of 11
i.
ii.
(600 mOsm/day) / (300 mOsm/l) =
2.0 l/day
(300 mOsm/l would make the urine about the same conc as
plasma)
f. The concentrating ability of the kidneys conserves 1.5L of water/day.
XX. Osmolar clearance
a. Reflects the ability of the kidneys to clear the plasma of all solutes
(electrolytes, urea, etc…)
b. Osmolar clearance (Cosm) = Uosm * V / Posm
c. Example: if Uosm = 600 mOsm/l, V = 2 ml/min, Posm = 300 mOsm/l,
Cosm = ?
i.
Ans: 4 ml/min
d. When kidneys excrete excess solute, Cosm increases; when solute is
retained, Cosm falls
XXI. Free water clearance (CH2O)
a. Definition: excretion of water in excess of amount needed to excrete
isosmotic urine, i.e. clearance of pure water by the kidneys
b. CH2O = V - Cosm
i.
(where V is urine volume)
c. If Uosm < Posm, CH2O is positive; pure water is cleared from the body
i.
Urine is less conc than plasma
d. If Uosm > Posm, CH2O is negative; pure water is retained
i.
Urine is more conc than plasma
e. EX: from above, use values and plug into equation:
i.
CH2O = 2ml/min – 4ml/min
ii.
= -2ml/min
1. kidneys are generating 2ml of pure water and depositing it
into the plasma
2. urine was more conc than plasma and this is a negative
water clearance
f. know how to perform this calculation for the exam!!**
i.
if the urine is more concentrated than plasma, you’re going to get
a negative number!
XXII. Fractional excretion (Fex)
a. The fraction of the filtered amount of a substance that is excreted in urine
b. Fex = (amt excreted)/ (amt filtered) = (Ux * V)/ (Px * GFR)
c. If creatinine clearance = GFR
i.
Fex = (Ux * V )/ ( Px * (UcrV / Pcr )
ii.
Cancel V and rearrange to get:
iii. (Ux * Pcr)/ (Ucr * Px)
1. don’t need the urine conc with this equation
XXIII. Case presentation
Renal 14
Wed 3/19/03 10am
Dr. Mallet
R Edwards
Proscribe Samera Kasim
Page 11 of 11
a. A 65 y.o. man with a history of hypertension and a myocardial infarction 6
months ago presents to your cardiology clinic with complaints of shortness
of breath and swollen ankles. Physical exam reveals 4+ pitting edema in
the lower extremities, crackles and rales in both lungs, and jugular venous
distension. You decide to treat his edema with a loop diuretic.
The following lab values are obtained before treatment:
PNa
130 mEq/l
UNa
260 mEq/l
PCr
1 mg/dl
UCr
50 mg/dl
2 hr after diuretic administration:
PNa
140 mEq/l
PCr
1 mg/dl
UNa
UCr
210 mEq/l
15 mg/dl
b. Pt has CHF
c. How does this type of diuretic increase urine output?
i.
Loop diuretics inhibit Na/K/Cl cotransport mechanism in the
thick ascending limb
d. Aside from a timed urine collection, how can you quickly determine if the
diuretic is working properly?
i.
Determine the fraction excretion of Na before and after the
diuretic is administered
e. Did the diuretic affect FENa? How much?
i.
Before treatment: FENa = (UNa * Pcr)/ (Ucr * PNa)
(260 * 1 )/ (50 * 130)
= 0.04 = 4%
 (96% of Na is being reabsorbed)
ii.
after treatment: (210 * 1 )/ (15 * 140)
= 0.1 = 10%
f. Why didn’t UNa increase after giving the diuretic?
i.
Some of the Na that was not reabsorbed is reabsorbed later in the
nephron in exchange with K.
ii.
Thus, urine will have higher K conc
XXIV. SUMMARY: Renal salt and water handling
a. Mechanisms of Na+, Cl- & H2O reabsorption beyong proximal tubule
b. Different transport mechanisms in descending vs. ascending limbs of
Henle’s loop
c. Hormonal control of collecting duct function
d. Countercurrent multiplier & exchanger mechanisms
e. Measures of solute and water handling: Cosm, CH2O, fractional excretion