At the end of the session the students should be able to a) Describe the basic physiological mechanism of thirst. b) Discuss body’s response to thirst. c) Explain the renal mechanism for control of blood volume and Extra Cellular Fluid (ECF) volume.

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THIRST AND WATER BALANCE
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2. • At the end of the session the students should be
able to
a) Describe the basic physiological mechanism of
thirst.
b) Discuss body’s response to thirst.
c) Explain the renal mechanism for control of
blood volume and Extra Cellular Fluid (ECF)
volume.
Guyton and Hall “Textbook of Medical Physiology”12th ed. (p
357-360,716)

3. Water Balance
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4. Water Balance
Water Input Water Output
Controlled mainly by “KIDNEY”Controlled mainly by “THIRST”
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5. Thirst
 the conscious desire for water
Thirst Center
Site
 along the anteroventral wall of the third ventricle (site for
osmoreceptors) (responds mainly to ↑ osmoalrity of ECF)
 in the preoptic nucleus
 organum vasculosum of the lamina terminalis (responds
mainly to ↑ osmoalrity of CSF)
Definition:

6. Stimuli of thirst center
1- increased extracellular fluid osmolarity (most important):
 When the sodium concentration increases only about 2 mEq/L
above normal, the thirst mechanism is activated, causing a desire
to drink water. This is called the threshold for drinking.
 This causes dehydration of thirst center -----> stimulation
2- Decreases in extracellular fluid volume and arterial pressure (as in hemorrhage):
 This causes stimulation of arterial baroreceptors ------>
stimulation of thirst center.
3- Angiotensin II
 ↓ renal perfusion ------> ++ juxtaglomerular apparatus ----->
release of renin ------> that ends by formation of angiotensin II ---->
directly acts on organum vasculosum ------> thirst sensation.

7. Decreased renal perfusion Renin A II
CORRECT
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8. 4- Dryness of the mouth and mucous membranes of the esophagus
 ++ thirst sensation that is relieved immediately by drinking
water even the water in not yet absorbed from GIT and the
dehydration is not corrected.
 Evidence:
Drinking water by animals in which the esophageal contents are
brought out releive thirst.
 Significance:
This meter (regulate) the volume of water drunk -----> protect
against overhydration.
 Fullness of GIT can relieve thirst temporarily

9. Renal regulation of water output
Water handling by the kidney:
60 – 70 % by
passive osmosis
15 % by passive
osmosis (DLH is
only water
permeable
Totally water
impermeable
Hardly water
permeable (very small
fraction)
15 % in the
presence of
ADH
1 ml/min. in urine

10.  Under normal conditions:
The amount of water reabsorption by the renal tubules is 99.1%
& the remaining 0.9% is excreted as urine in a rate of 1.1
ml/min OR 1.5 L/day.
 In overhydration:
ADH secretion would  ed. 87.5% of the filtered load of water is
obligatory reabsorbed & the remaining 12.5% of the filtered
load of water is excreted as urine at a rate of 16 ml/min. OR
27.5 L/day.
 In dehydration:
ADH secretion is maximum. The fraction of water reabsorbed is
99.8% & only 0.2% of water is excreted as urine at a rate of
0.25 ml/min or 400 ml/day.

12. Renal handling of water
A) In proximal tubules:
* 65 – 70% of GFR is reabsorbed by osmosis.
* The osmolarity of the lateral intercellular spaces is
ed by:
 1ry active Na+ reabsorption.
 Secondary Na+ reabsorption at the apical border.
* Route of water reabsorption:
 Transcellular.
 Paracellular.

13. Water
Reabsorption in
the Proximal
Tubules

14. B) From Loop of Henle:
* In DLH: 15% of the filtered load of water is
reabsorbed passively by osmosis.
* The highest osmolarity of medullary interstitium
helps the reabsorption of water. Some type of water
channels could exist in the epithelium of DLH, not
controlled by ADH as that of collecting duct.

15. C) From Distal tubules & Collecting duct:
* ALH is totally impermeable to water.
* Early distal tubules is hardly permeable to water.
* Late distal tubules & collecting duct are permeable to water
in the presence of ADH.
- The fraction of water reabsorbed in the cortex by late
segment of the nephron is about 2/3 of the amount delivered to
ALH (2/3 15-17% of the filtered load of water).
- So, the medullary CD, receive 5% of GFR, all is
reabsorbed except 0.5 – 1 ml/min. which form the urine.

16. ADH
A
A
A
A
Peritubular
capillaries
V2
receptors
Adenyl Cyclase
ATP
cAMP Aquaporins
Water

17. When ADH binds to its specific receptors (V2
receptors) at the capillary border of the principal
cells -----> activation of adenyl cyclase enzyme
------>  cAMP ------>  protein kinase ---->
opening of water channels , called " aquaporins
2, 3, 4” that are present in the principal cells of
collecting duct ------>  water reabsorption.

18. Stimuli of ADH secretion
1- increased extracellular fluid osmolarity (most important):
2- Decreases in extracellular fluid volume and arterial pressure (as in hemorrhage):
3- Angiotensin II
SEE
BEFORE
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