1. The document discusses the kidney's role in maintaining water balance and osmolality through thirst, ADH secretion, and kidney function. It specifically focuses on the mechanisms of sodium transport and water reabsorption in the nephron that allow the kidney to regulate water. 2. Free water clearance is defined as the rate at which plasma is cleared of solute-free water. Positive values indicate water loss in urine, negative values indicate water conservation by producing concentrated urine under ADH influence. 3. A case example is given of a patient with DI who is severely dehydrated with dilute urine, highlighting the kidney's inability to concentrate urine in DI.

2. Hyper Na in DI
By:
y.madihi MD
Ped.nephrologist
Isfahan university of
medical sciences

3. The body strives to maintain
water balance such that
serum osmolality remains
within a normal range of
280-285 mOsm/L. this
requires normal functioning:
1. Thirst mechanism
2. ADH secretion
3. Kidney

5. 1. Na+ is transported out (Na+/K+/2Cl- pump) and
water remains behind resulting in a hypotonic fluid in
the tubular lumen (remember, ADH enhances this
effect).
2. H2O moves out of distal tubule due to ADH. The
osmolality of the cortical interstitium is 280
mOsm/L. The amount of H2O reabsorbed from the
cortical segment is determined by the osmolar
gradient; therefore, the more dilute the tubular fluid
at the start of the distal tubule, the greater the amount
of H2O that can be reabsorbed. Urea stays behind,
increasing the urea concentration in the distal tubule.
3. H2O moves into the juxtamedullary area because
of sodium accumulation in the interstitium due to the
action of the Na+/K+/2Cl- pumps in the TAL.
4. If ADH is present, urea enters the medullary
interstitium.
5. H20 moves out of descending limb into the
interstitium, made hypertonic by urea.
6. NaCl moves out of ascending limb, down its
concentration gradient

6. Non-osmotic / non-volume
related changes
ADH secretion can be
stimulated by:
pain, nausea, vomiting,
anaesthesia, drugs, tumours,
or CNS/lung diseases.
Alcohol and drugs can cause
decreased ADH secretion.

8. free water
clearance(CH2O)
the kidney produces urine
composed of solute and pure
(solute-free) water. The rate at
which plasma is cleared of
solute is the osmolal clearance;
the rate at which plasma is
cleared of solute-free water is
the free water clearance.

9. If an individual is
producing urine more
dilute than the plasma,
there is a positive value
for free water clearance,
meaning pure water is lost
in the urine in addition to
a theoretical isoosmotic
filtrate.

10. If the urine is more
concentrated than the
plasma, then free water is
being extracted from the
urine, giving a negative
value for free water
clearance.

11. free water
clearance(CH2O)
is the volume of blood plasma
that is cleared of solute-free
water per unit time.

12. V=Cosm+CH2o
CH2o=V_Cosm
CH2o=V_(Uosm/Posm).V
=(1_Uosm/Posm)V

13. For example,
for an individual with a
urine osmolality of 160
mOsm/L(SG:1.004),
bplasma osmolality of 320
mOsm/L, and a urine
production of 2 ml/min,
the free water clearance
is:

14. CH2o=
(1_Uosm/Posm)V
=(1_160/320)2
=1mL/min
=1.44Lit/day

15. Free water clearance
can be used as an
indicator of how the
body is regulating
water.

16. A free water
clearance of zero
means the kidney is
producing urine
isosmotic with
respect to the
plasma.

17. Values greater than
zero imply that the
kidney is producing
dilute urine through
the excretion of
solute-free water.

18. Values less than zero
imply that the kidney
is conserving water
(likely under the
influence of ADH),
resulting in the
production of
concentrated urine.

19. A known case of DI
Wt:50kg
Severe
dehydration
Urine SG:1.005