1. Lecture 4
RENAL HANDLING OF SODIUM, CHLORIDE AND WATER
“the core of renal physiology”

2. Some Generalizations:
► Maintaining salt (NaCl) and H20 balance is a key
function of the kidney.
► Na+, Cl- and H20 are all freely filtered.
AND….huge amounts of these are filtered.
AND….most of what’s filtered is reabsorbed
Note: Na, Cl & H20 are not normally secreted
Some Generalizations:
 Na+ Reabsorption is active, via the transcellular route and is powered by the
basolateral Na-K-ATPase.
 Cl- Reabsorption is passive (paracellular) and active (transcellular). Regardless
of route, it is always coupled somehow to Na+ reabsorption. Indeed,
parallel Cl- reabsorption is implied when describing Na+ reabsorption.
 H20 Reabsorption is by osmosis and secondary to reabsorption of solute,
particularly Na+ and those dependent on Na+ reabsorption.

3. Overview of Na+ Reabsorption along the Nephron
65% of filtered Na+ is reabsorbed
from the proximal tubule.
25% of filtered Na+ is reabsorbed
from the thick ascending limb.
5% of filtered Na+ is reabsorbed
from the distal tubule.
4-5% of filtered Na+ is reabsorbed
from the collecting duct.

4. Proximal Tubule: Na+ Reabsorption
Na+ Reabsorption Stepwise:
● 1. Na-K-ATPase keeps intracellular
Na level low. This means there is a
gradient across apical membrane.
● 2. Filtered Na+ is transported across
apical membrane several ways.
● 3. Na+ entering the cell is then moved
across basolateral membrane.
Important Points:
* Reabsorption of other solutes are
linked to Na+ reabsorption.
** Without the Na-K-ATPase, the Na+
gradient that powers reabsorption of
Na & other solutes would not exist.

5. Proximal Tubule: Cl- Reabsorption
Remember this diagram?
Cl- reabsorption parallels
reabsorption of Na+.
A Helpful Concept:
“Electronuetrality Rule”
any volume of solution
(no membranes separating stuff here)
will have equal numbers of
cations and anions.
►Thus, a solution with 140 mM Na+,
will have 140 mM of cation.
► In the plasma, the most abundant
anions are Cl- (~110 mM) & HCO3- (~24 mM).

6. Proximal Tubule: Cl- Reabsorption
Two Routes of Cl- Reabsorption:
Paracellular
2. Transcellular

7. Proximal Tubule: Cl- Reabsorption
Two Routes of Cl- Reabsorption:
Paracellular
Through “not so tight” tight junctions
Passive, down electrochemical gradient
Depends indirectly on Na+ transport
Most Cl- reabsorption via this route
2. Transcellular

8. Proximal Tubule: Cl- Reabsorption
Two Routes of Cl- Reabsorption:
Paracellular
Through “not so tight” tight junctions
Passive, down electrochemical gradient
Depends indirectly on Na+ transport
Most Cl- reabsorption via this route
2. Transcellular
Complicated apical process which
depends directly on Na+ transport
Apical transport is essentially
Cl-Na-symport
Basolateral transport via the
Cl-K-symporter

9. H20 Reabsorption Overview
H20 Reabsorption is by osmosis and secondary to reabsorption of solute.
→ If you drink excess H20 (no salt), then your kidneys must excrete the excess H20.
→ If you eat excess salt (no H20), then your kidneys must excrete the excess salt.
Evidence that the kidney’s do this is the body’s capacity to generate dilute
or concentrated urine.
*Obvious but important in order to produce dilute or concentrated urine the kidney’s must be able to “separate salt from water”.

10. Comparison of H20 and Na+ Handling
Four Significant Points:
 1. Equal amounts of H20 & Na+ are
reabsorbed from proximal tubule.
 2. H20 & Na+ are both reabsorbed
from loop of Henle, but from different
parts of the loop. Overall, the loop
reabsorbs more Na+ than H20.
 3. Na+ is reabsorbed from the distal
tubule. H20 is not.
 4. Both Na+ and H20 are reabsorbed
from collecting duct. The amounts
of each are variable & controlled.

11. What defines when and where H20 moves along the nephron?
Answer: H20 moves only down osmotic gradients (no H20 pumps here)
H20 moves only if it can (a H20 permeable pathway must exist)
The Osmotic Gradients
Proximal Tubule: Na+ & Na-dependent solute reabsorption creates gradient
Loop & Collecting Duct: High salt (NaCl) and urea levels in medulla provide gradient
Possible H20 Permeation Pathways
H20 may move through lipid bilayers, aquaporins or tight junctions.
Basolateral membranes: always highly H20 permeable because they contain
a certain type of aquaporin.
Apical membrane & tight junction: H20 permeability vary along the nephron.
Distal Tubule Zero “
Collecting Duct Low, but regulated “
Ascending Limb Zero “
Descending Limb High “
Proximal Tubule High Apical H20 Permeability

12. Overview of “Urine Concentration” Control
Cortex
Medulla
High
Solute
Level
What happens when H20 permeability
of collecting duct is very low?
Dilute tubular fluid moves down
collecting duct and remains dilute.
H20
Perm
Low
Tubular
Fluid Dilute
Here

13. Overview of “Urine Concentration” Control
Cortex
Medulla
High
Solute
Level
Tubular
Fluid Dilute
Here
What happens when H20 permeability
of collecting duct is very low?
Dilute tubular fluid moves down
collecting duct and remains dilute.
H20
Perm
Low ▼
Result
dilute urine

14. Overview of “Urine Concentration” Control
Cortex
Medulla
High
Solute
Level
Tubular
Fluid Dilute
Here
What happens if H20 permeability
of collecting duct is high?
High solute level in medulla means
there is a large osmotic gradient
that favors H20 movement out.
H20 moves out of tubular fluid and
the fluid becomes more concentrated.
H20
Perm
High

15. Preview of “Urine Concentration” Control
Cortex
Medulla
High
Solute
Level
Tubular
Fluid Dilute
Here
What happens if H20 permeability
of collecting duct is high?
High solute level in medulla means
there is a large osmotic gradient
that favors H20 movement out.
H20 moves out of tubular fluid and
the fluid becomes more concentrated.
H20
Perm
High
Key Points to Remember (so far):
1) H20 is moving down osmotic gradient.
2) It only moves if there is a H20 permeable
pathway available.
3) How much H20 moves will depend on….
- Gradient size
- Degree of H20 permeability. ▼
Result
Concentrated Urine
(H20 was conserved)

16. Na+, Cl- and H20 Handling Varies in Different Tubular Segments
Proximal Tubule … There is “iso-osmotic” reabsorption
Loop of Henle … There is “separation of salt & H20”
Distal Tubule & … Reabsorption is “regulated” (by hormones)
Collecting Duct

17. “Isosmotic reabsorption” from PROXIMAL TUBULE
→ Proximal Tubule is in the cortex
→ The interstitium of the cortex is iso-osmotic to plasma
All Glucose
Reabsorbed
This figure was shown earlier.

18. “Isosmotic reabsorption” from PROXIMAL TUBULE
What’s happening with Cl- ? It’s concentration rises !
Cl- is also being reabsorbed….
So…Why does Cl- level rise?
Cl- level rises because…
Early on: HC03- is the primary
anion following the cations.
Later : HC03- levels drop & Cl-
starts following the cations.
►Remember, Cl- is reabsorbed
passively via the paracellular route.
VOLUME
Cl- level levels out later along the proximal tubule.
This means Cl- and H20 reabsorption are matching
each other later on in proximal tubule.

19. “Isosmotic reabsorption” from PROXIMAL TUBULE
What’s happening with Na+ ? It’s concentration stays constant !
“Isosmotic reabsorption” from PROXIMAL TUBULE

20. “Isosmotic reabsorption” from PROXIMAL TUBULE
What’s happening with Na+ ? It’s concentration stays constant !
“Isosmotic reabsorption” from PROXIMAL TUBULE
Clearly, Na+ is being reabsorbed.
Na+ reabsorption is what
drives reabsorption of HC03-
and the nutrients.
Na+ concentration inside the tubule
stays constant because H20 is
also reabsorbed along the tubule.
(i.e. fluid volume decreases)
VOLUME
This is called….
“Isosmotic Volume Reabsorption”
H20 always follows solute….the main extracellular solute is Na+.
When ever a little Na+ moves, a little H20 follows it.
In other words, Na+ & H20 reabsorption keep pace….osmolarity & [Na+] stay constant
as tubular fluid volume decreases.

21. Loop of Henle Separates Salt & H20
Important Fact:
The Loop ( the loop overall ) always reabsorbs more Na+ than H20.
This means that the fluid leaving the loop is always more dilute
than the fluid that entered it.
Same diagram as before:
2nd
 1. Na+ & H20 reabsorption physically separated (in different loop segments)
 2. H20 reabsorbed from descending limb.
(always occurs via osmosis)
 3. Na+ reabsorbed from ascending limb (either passive – in the thin region or active
in the thick region)
1st
3rd

22. Loop of Henle : Na+ Reabsorption (Thick Ascending Limb)
1. Na+ reabsorption powered by gradient
generated by basolateral Na-K-ATPase.
There is a unique apical Na+ transport.
Na-K-2Cl symport
Na-K-2Cl symporter requires all 3 ions to operate. Apical K channel assures there
will be lumenal K available to keep it going.
Note: Na-K-2Cl symporter is target of a very
common loop-diuretic (furosemide = lasix)

23. Distal Tubule : Na+ Reabsorption
*Like ascending limb of loop, distal tubule is not permeable to H20
but Na+ reabsorption occurs
Na+ reabsorption powered by gradient
generated by basolateral Na-K-ATPase.
Another unique apical Na+ transport.
Na-Cl symport
Na-Cl symporter is the target of another
type of diuretic (thiazide)
Note: presence in DT of apical Ca channels. Parathyroid hormone regulates these. (more later about this)

24. Collecting Duct : Na+ Reabsorption
Two Types of Cells Present
→ Principal cells (70% of total cells present) are specialized to handle Na+ & H20.
→ Intercalated cells (2 subtypes called / or A/B) are specialized to handle Cl- & pH.
Again…Na+ reabsorption powered
by the basolateral Na-K-ATPase.
Another unique apical Na+ transport a Na+ channel
(These are not the same Na channels type involved in AP in nerves and muscles).
This Na+ channel is regulated by the
hormone aldosterone.
Several diuretics also target this Na+
channel or its aldosterone regulation.
(more about this later)
Now a closer look

25. Collecting Duct : Na+ & H20 Reabsorption
Here is another view of the Principal Cell.
1. The H20 permeability of collecting
duct is inherently very low but can be
very high if Antidiuretic Hormone
(ADH) is present. (ADH = vasopressin)
2. ADH is a peptide hormone released
from posterior pituitary. It acts on the
apical membranes of principal cells.
(again, more about this later)

26. Collecting Duct : Na+ & H20 Reabsorption
Important Points to Remember:
First… Collecting duct is only site in the nephron
where H20 permeability is hormonally regulated.
ADH increases H20 permability. (normally its very low)
Second…Collecting duct of inner medulla is a little bit
different. Even in absence of ADH, it has some
small amount of H20 permeability. So, some H20
will always be reabsorbed there.
Third… ADH action is not all-or-none. A little ADH increases
H20 permeability a little. More ADH increases it more.
Fourth… ADH increases apical H20 permeability by stimulating fusion of aquaporin-containing
membrane vesicles. In absence of ADH, these are withdrawn by endocytosis.
Question #1: What happens to the tubular fluid if no ADH is present?
Collecting duct H20 permeability is low.
A large volume dilute urine will be excreted.

27. Collecting Duct : Na+ & H20 Reabsorption
Important Points to Remember:
First… Collecting duct is only site in the nephron
where H20 permeability is hormonally regulated.
ADH increases H20 permability. (normally its very low)
Second…Collecting duct of inner medulla is a little bit
different. Even in absence of ADH, it has some
small amount of H20 permeability. So, some H20
will always be reabsorbed there.
Third… ADH action is not all-or-none. A little ADH increases
H20 permeability a little. More ADH increases it more.
Fourth… ADH increases apical H20 permeability by stimulating fusion of aquaporin-containing
membrane vesicles. In absence of ADH, these are withdrawn by endocytosis.
Question #2: What happens to the tubular fluid when ADH is present?
Collecting duct H20 permeability is high.
A small volume concentrated urine will be excreted.

28. Origin of the Hyperosmotic Interstitium in Renal Medulla
A key question in renal physiology is:
”How does the kidney generate its hyperosmotic medullary in interstitium?” 28

29. Countercurrent mechanism in Physiology
Descending loop
Ascending loop
Countercurrent system
A countercurrent mechanism therefore merely maintains the status quo, it does not
create differences in temperature between the tube water and the bath water.

30. Origin of the Hyperosmotic Renal Medulla
Cortex
Medulla
mOsM
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Medulla is hyperosmotic
but there is also an
osmolarity gradient.

31. Origin of the Hyperosmotic Renal Medulla
Cortex
Medulla
mOsM
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
TDL MI
TAL
Countercurrent
Multiplication
H20
reabsorbed Na
H20
Na+
reabsorbed
The 3 Essential Elements: (for generating the medullary osmotic gradient)
►1) Loops of Henle form countercurrent multiplier system which depends:
on active transport of Na+ out of ascending limb;
the high permeability of its thin descending limb to water;
inflow of tubular fluid from proximal tubule with outflow into the distal tubule;
( Overall, the loop always reabsorbs more Na+ than H20 )

32. Origin of the Hyperosmotic Renal Medulla
Vasa Recta
Blood Osmolarity
IN = OUT
mOsM
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Hair-pin loop
arrangement
preserves
medullary
osmolarity
Cortex
Medulla
The 3 Essential Elements:
1) Loops of Henle form countercurrent multiplier system
( Overall, the loop always reabsorbs more Na than H20 )
► 2) Contribution of vasa recta capillaries _ countercurrent exchange system
( The vasa recta loop down into the renal medulla) 32

33. Origin of the Hyperosmotic Renal Medulla
The 3 Essential Elements:
1) Loops of Henle form countercurrent multiplier system
( Overall, the loop always reabsorbs more Na than H20 )
2) Contribution of vasa recta capillaries _ countercurrent exchange system
( The vasa recta loop down into the renal medulla)
► 3) Recycling of urea between loop and collecting duct.
( Urea provides about half of the high medullary osmolarity )

34. Origin of the Hyperosmotic Renal Medulla
Note:
→ Urea very useful in concentrating urine.
→High protein diet = more urea = more concentrated urine.
→Kidneys filter, reabsorb and secrete urea.
→Urea excretion rises with increasing urinary flow.
Cortex
Medulla
mOsM
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Urea Recycling:
Keeps “dumping” urea
(solute) back into medulla.
Rate of “dumping” depends
on urea level in the tubular
fluid (faster when higher).
Urea
Recycling
The 3 Essential Elements:
1) Loops of Henle form countercurrent multiplier system
( Overall, the loop always reabsorbs more Na than H20 )
2) Contribution of vasa recta capillaries _ countercurrent exchange system
( The vasa recta loop down into the renal medulla)
► 3) Recycling of urea between loop and collecting duct.
( Urea provides about half of the high medullary osmolarity )

35. Tubular Fluid Osmolarity Changes Along Nephron
Percent filtered
H20 Remaining
Iso-osmotic
to plasma
Dilute
urine
Conc.
Approaches osmolatity of interstitium in cortex
Approaches osmolatity of interstitium in medulla
Water restriction
High
water
intake
Hypo-osmotic
to plasma