concentration and dilution of urine

1. Concentration and
dilution of urine
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
R.D. Gardi Medical College, Ujjain, Madhya Pradesh.
Email: dr.goothy@gmail.com

2. The student should be able to
Describe functions of ADH
Describe the concentration of tubular fluid
Counter current multiplier mechanism
Counter current exchange system
Disorders of renal concentrating or diluting ability

3. Question
Glucose reabsorption occurs in the
1. PCT
2. LH
3. DCT
4. CD

4. Question
In absence of ADH greatest fraction of filtered water is absorbed in the
1. DCT
2. LH
3. PCT
4. CD

5. Question
The glucose transporter on the terminal side of PCT is
1. SGLT2
2. GLUT2
3. GLUT1
4. None of the above

6. Descending and Ascending Limbs of a Long Henle’s Loop
The descending limb
(1) is highly permeable to H2O (via abundant, always-open AQP-1 water
channels)
(2) does not actively extrude Na+ that is, it does not reabsorb Na+. (It is
the only segment of the entire tubule that does not do so.)
The ascending limb
(1) actively transports NaCl out of the tubular lumen into the surrounding
interstitial fluid and
(2) is always impermeable to H2O, so salt leaves the tubular fluid without
H2O osmotically following along.

7. Counter current multiplier mechanism
 Even though the flow of fluids is continuous through the loop of
Henle, we can visualize what happens step by step, much like an
animated film run so slowly that each frame can be viewed.

8. Counter current multiplier mechanism
 Even though the flow of fluids is continuous through the loop of
Henle, we can visualize what happens step by step, much like an
animated film run so slowly that each frame can be viewed.
Initially, before the vertical osmotic gradient is established, the medullary
interstitial fluid concentration is uniformly 300 mOsm/L, as are the rest of the
body fluids.

10. Step-1
 The active salt pump in the ascending limb can transport NaCl
out of the lumen until the surrounding interstitial fluid is 200
mOsm/L more concentrated than the tubular fluid in this limb.
 When the ascending limb pump starts actively extruding NaCl,
the medullary interstitial fluid becomes hypertonic.
Water cannot follow osmotically from the ascending limb because this limb is
impermeable to H2O.
However, net diffusion of H2O does occur from the descending limb into the
interstitial fluid.

11. Step-1
 The tubular fluid entering the descending limb from the proximal tubule is isotonic.
Because the descending limb is highly permeable to H2O, net diffusion of H2O
occurs by osmosis out of the descending limb into the more concentrated
interstitial fluid.
The passive movement of H2O out of the descending limb continues until the
osmolarities of the fluid in the descending limb and the interstitial fluid become
equilibrated.
Thus, the tubular fluid entering the loop of Henle immediately starts to become
more concentrated as it loses H2O.
At equilibrium, the osmolarity of the ascending limb fluid is 200 mOsm/L and the
osmolarities of the interstitial fluid and descending limb fluid are equal at 400
mOsm/L

13. Step-2
 If we now advance the entire column of fluid in the loop several
frames (step 2 ), a mass of 200 mOsm/L fluid exits from the top of the
ascending limb into the distal tubule, and a new mass of isotonic fluid
at 300 mOsm/L enter the top of the descending limb from the proximal
tubule.
At the bottom of the loop, a comparable mass of 400 mOsm/L fluid
from the descending limb moves forward around the tip into the
ascending limb, placing it opposite a 400 mOsm/L region in the
descending limb, but the 200 mOsm/L concentration difference has
been lost at both the top and the bottom of the loop.

15. Step-3
 The ascending limb pump again transports NaCl out while H2O
passively leaves the descending limb until a 200 mOsm/L
difference is reestablished between the ascending limb and both
the interstitial fluid and the descending limb at each horizontal
level (step 3 ).
Note, however, that the concentration of tubular fluid is
progressively increasing in the descending limb and
progressively decreasing in the ascending limb.

17. Step-4, 5
 As the tubular fluid is advanced still farther (step 4 ), the 200
mOsm/L concentration gradient is disrupted again at all
horizontal levels.
Again, active extrusion of NaCl from the ascending limb, coupled
with the net diffusion of H2O out of the descending limb,
reestablishes the 200 mOsm/L gradient at each horizontal level.

20. Step-6
 As the fluid flows slightly forward again and this stepwise
process continues (step 6 ), the fluid in the descending limb
becomes progressively more hypertonic until it reaches a
maximum concentration of 1200 mOsm/L at the bottom of the
loop, four times the normal concentration of body fluids.
 The tubular fluid even becomes hypotonic before leaving the ascending limb
to enter the distal tubule at a concentration of 100 mOsm/L, one-third the
normal concentration of body fluids.

21. Step-6
 Note that although a gradient of only 200 mOsm/L exists between
the ascending limb and the surrounding fluids at each medullary
horizontal level, a larger vertical gradient exists from the top to
the bottom of the medulla.
Even though the ascending limb pump can generate a gradient of
only 200 mOsm/L, this effect is multiplied into a large vertical
gradient because of the countercurrent flow within the loop.
Thus, this concentrating mechanism accomplished by the loop of
Henle is known as countercurrent multiplication

23. Benefits of counter current multiplier mechanism
 The isotonic fluid that enters the loop becomes progressively
more concentrated as it flows down the descending limb,
achieving a maximum concentration of 1200 mOsm/L, only to
become progressively more diluted as it flows up the ascending
limb, finally leaving the loop at a minimum concentration of 100
mOsm/L.

24. Benefits of counter current multiplier mechanism
 Such a mechanism offers two benefits.
First, 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 urine is more
concentrated than normal body fluids can be excreted.
Second, because the fluid is hypotonic as it enters the distal
parts of the tubule, the kidneys can excrete urine more dilute than
normal body fluids.

25. Vasopressin
 After obligatory H2O reabsorption from the proximal tubule (65%
of the filtered H2O) and loop of Henle (15% of the filtered H2O),
20% of the filtered H2O remains in the lumen to enter the distal
and collect tubules for variable reabsorption under hormonal
control.
This is still a large volume of filtered H2O subject to regulated reabsorption;
20% x GFR (180 L/day) =36 L/day to be reabsorbed to varying extents,
depending on the body’s state of hydration.

26. Role of vasopressin
 For H2O absorption to occur across a segment of the tubule,
two criteria must be met:
(1) an osmotic gradient must exist across the tubule, and
 (2) the tubular segment must be permeable to H2O.
 The distal and collecting tubules are impermeable to H2O except
in the presence of vasopressin, also known as antidiuretic
hormone (antidiuretic means “against increased urine
output”),which increases their permeability to H2O.

27. Role of vasopressin
 Vasopressin is produced by several specific neuronal cell bodies in
the hypothalamus and then stored in the posterior pituitary gland,
which is attached to the hypothalamus by a thin stalk.
 The hypothalamus controls the release of vasopressin from the
posterior pituitary into the blood.
In a negative-feedback fashion, vasopressin secretion is stimulated by
a H2O deficit when the ECF is too concentrated (that is, hypertonic)
and H2O must be conserved for the body, and it is inhibited by an H2O
excess when the ECF is too dilute (that is, hypotonic) and surplus H2O
must be eliminated in urine.

28. Role of vasopressin
 Vasopressin reaches the basolateral membrane of the principal
tubular cells lining the distal and collecting tubules through the
circulatory system.
Here, it binds with V2 receptors specific for it.
Vasopressin binds with different V1 receptors on vascular
smooth muscle to exert its vasoconstrictor effects.

29. Role of vasopressin
 Binding of vasopressin with its V2 receptors, which are G-
protein-coupled receptors
 Activates the cyclic AMP (cAMP) second messenger system
within these tubular cells
This binding ultimately increases the permeability of the opposite
luminal membrane to H2O by promoting the insertion of
aquaporins (specifically, AQP-2) in this membrane by means of
exocytosis.

30. Role of vasopressin
 Without these aquaporins, the luminal membrane is
impermeable to H2O.
Once H2O enters the tubular cells from the filtrate through these
vasopressin-regulated luminal water channels, it passively leaves
the cells down the osmotic gradient across the cells’ basolateral
membrane to enter the interstitial fluid.

31. Role of vasopressin
 The aquaporins in the basolateral membrane of the distal and
collecting tubule (AQP-3 and AQP-4) are always present and
open, so this membrane is always permeable to H2O.
Vasopressin influences H2O permeability only in the distal and collecting
tubules. It has no influence over the 80% of the filtered H2O that is
obligatorily reabsorbed without control in the proximal tubule and
descending limb of the loop of Henle.
The ascending limb of Henle’s loop is always impermeable to H2O, even in
the presence of vasopressin.

32. Role of vasopressin
 The aquaporins in the basolateral membrane of the distal and
collecting tubule (AQP-3 and AQP-4) are always present and
open, so this membrane is always permeable to H2O.
Vasopressin influences H2O permeability only in the distal and collecting
tubules. It has no influence over the 80% of the filtered H2O that is
obligatorily reabsorbed without control in the proximal tubule and
descending limb of the loop of Henle.
The ascending limb of Henle’s loop is always impermeable to H2O, even in
the presence of vasopressin.

36. Counter current exchange mechanism
 The renal medulla must be supplied with blood to nourish the
tissues in this area and to transport water that is reabsorbed by
the loops of Henle and collecting ducts back to the general
circulation.
In doing so, however, it is critical that circulation of blood through the
medulla does not disturb the vertical gradient of hypertonicity established by
the loops of Henle.
Consider the situation if blood were to flow straight through from the cortex
to the inner medulla and then directly into the renal vein

37. Counter current exchange mechanism
 Because capillaries are freely permeable to NaCl and H2O, the
blood would progressively pick up salt and lose H2O through
passive fluxes down concentration and osmotic gradients as it
flowed through the depths of the medulla.
 Isotonic blood entering the medulla, on equilibrating with each
medullary level, would leave the medulla very hypertonic at 1200
mOsm/L.
It would be impossible to establish and maintain the medullary hypertonic
gradient.

39. Counter current exchange mechanism
 This dilemma is avoided by the hairpin construction of the vasa recta, which, by
looping back through the concentration gradient in reverse, allows the blood to
leave the medulla and enter the renal vein essentially isotonic to incoming arterial
blood.
As blood passes down the descending limb of the vasa recta, equilibrating with the progressively
increasing concentration of the surrounding interstitial fluid, it picks up salt and loses H2O until it is
very hypertonic by the bottom of the loop.
Then, as blood flows up the ascending limb, salt diffuses back out into the interstitial fluid, and
H2O reenters the vasa recta as progressively decreasing concentrations are encountered in the
surrounding interstitial fluid.
This passive exchange of solutes and H2O between the two limbs of the vasa recta and the
interstitial fluid is known as countercurrent exchange.

40. Counter current exchange mechanism
 Unlike countercurrent multiplication, it does not establish the
concentration gradient.
Rather, it preserves (prevents the dissolution of) the gradient.

41. Disorders of urinary concentrating ability
 Impairment in the ability of the kidneys to concentrate or dilute
the urine.
Inappropriate secretion of ADH- Either too much or too little ADH
secretion
Impairment of counter-current mechanism
Inability of DCT, CD to respond to the ADH

42. Central diabetes insipidus
 Head injury or infections or congenital
Loss of ability to produce or release ADH from posterior pituitary
DCT can not reabsorb water
Large volume of diluted urine
Urine volumes more than 15L/day
Central diabetes insipidus
As long as person drinks adequate water the body water will not
decrease
When water intake is restricted- dehydration occurs

43. Central diabetes insipidus
 Treatment
Administration of synthetic analog of ADH- Desmopressin
Selective action on V2 receptors
Increase water permeability in DCT
Can be given by injection, as a nasal spray, or orally

44. Nephrogenic diabetes insipidus
 Renal tubules can not respond to ADH
Large volume of diluted urine formed
Leads to dehydration unless fluid intake is increased
How to distinguish central and nephrogenic DI?

45. Central and Nephrogenic diabetes insipidus
 Administration of desmopressin
Lack of prompt decrease in urinary volume
Within 2 hours after injection of desmopressin
Strongly suggest nephrogenic DI
How to treat nephrogenic DI?

46. Nephrogenic diabetes insipidus
 To correct the underlying renal disorder if possible?
Hypernatremia is attenuated by a low sodium diet and
administration of diuretic that increases sodium excretion such
as thiazides.