RENAL REGULATION OF
BODY FLUID
Dr. Eman El Eter
What is the impact of the following on your body fluid
volume and osmolarity?
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What happens when you are exposed to hot
weather?
What if you have a stomach bug and excessive
diarrhea?
What if you drink 2 liters of fluids?
ECF
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Constant concentration of electrolytes & solutes
They create the osmolarity of ECF
Na+, Cl- most abundant osmostic molecules.
Osmolality is determined by the amount of
extracellular NaCl and water which depends upon
balance between intake and excretion of these
substances.
Normal plasma Na+ = 140-145 mEq/L
Osmolarity = 300 mOsm/L
To stay in a state of fluid balance:
Input = output
Input= 1500 ml/day
H2O & fluid consumption
Output = 1500 ml/day
Urine
Respiration (water vapor)
Sweating.
Stool
Metabolism
Control of ECF volume &osmolarity
Regulation of input:
Osmoreceptors/Thirst mechanism
Regulation of output: Renal regulation
ADH.
Angiotensin/Aldosterone.
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Thirst mechanism
Role of thirst in controlling ECF osmolarity & fluid
volume
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To stay in balance water intake = water loss
Fluid intake is regulated by thirst mechanism
Stimuli of thirst mechanism:
A. Increase thirst:
Increased osmolarity ECF.
Decreased ECF volume.
Decreased blood pressure.
Angiotensin II.
Dryness of the mouth.
B. Decrease thirst?
Gastric distention decreases thirst.
Role of thirst in controlling ECF osmolarity & Na+ concentration, cont….
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The desire to thirst is completely satisfied when:
Plasma osmolarity,
Blood volume,
Or both return to normal
Osmoreceptor mechanism
Changes in plasma
osmolarity can lead to a
cascade of events to
return it back to normal.
What happens if ECF osmolarity increased?
ECF osmolarity
 (+) osmoreceptors in
Ant. Hypothalamus &
Send signals to
Supraoptic nucleus,
Then to posterior
Pituitary (+) ADH
To increase H2O absorption
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Where in the brain ADH is formed?
Main site for
ADH synthesis
ADH is stored
in posterior
pituitary
Osmotic vs non-osmotic stimuli; effect on ADH
Rapid
response
AVP=arginine
vasopressin=ADH
Summary of actions & stimuli of
ADH
Non-osmotic stimuli releasing ADH
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Arterial baroreceptor reflex
Chemoreceptor reflex
Whenever BP & blood volume reduced, ADH is
released
water retention by the kidney to
restore BP to normal.
Day-to-day regulation of ADH secretion is effected
mainly by changes in plasma osmolarity.
Role of Ang II & aldosterone
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They do not normally play a major role in
controlling ECF osmolarity and Na+ concentration.
Their major role is to absorb sodium through distal
convoluted tubules, leading to greater extracellular
fluid volume and sodium quantity.
Take home message
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ADH-thirst is the most powerful feedback system in
the body for controlling extracellular fluid
osmolarity and volume.