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Water, Electrolyte, and
pH Balance
It’s all a balancing act
Where do we keep our
body fluids?

“Compartments”

– Intracellular fluid


Inside the cells
2/3 of body fluid
– Extracellular fluid


Interstitial fluid
Plasma


Intracellular fluid has an
exacting composition
Plasma has an exacting
composition
Interstitial fluid is the
“slosh” compartment
that lends to both as
needed
Premises of Fluid and
Electrolyte Regulation

Changes in extracellular fluid are monitored
not intracellular fluid
– Homeostatic response mechanisms are for the
big picture not the individual cell


Fluid and electrolyte balance is not directly
monitored, just hydrostatic and osmotic
pressures
Water cannot be moved by active transport
– Water can follow ions, remember the kidney

If gains exceed losses, you will gain water
weight and visa versa
Regulatory Hormones

Antidiuretic hormone
– Produced by
hypothalamus when it
detects increase in
osmolarity
– Causes kidneys to
concentrate urine
– Stimulates thirst and
desire for salty food

Aldosterone
– Released by adrenal
cortex when
juxtaglomerular
apparatus releases renin
or when decreased
levels of sodium and
potassium are detected
– Causes increased
reabsorption of water
Regulatory Hormones
cont.


Atrial naturitic peptide
Brain naturitic peptide
– Both released when increase in blood
pressure (stretch of right atrium or
vessels in brain) is noted
– Reduce thirst
– Block ADH and aldosterone production
Fluid and electrolyte
balance are linked


When water is up, electrolyte
concentrations are down
When water is down, electrolyte
concentrations are up
Water Movement

Water moves freely into and out of the ECF
compartment
– Peritoneal cavity
– Ventricles of brain
– Gains


Metabolic generation of water
Absorption from digestive tract
– Losses




Urine
Respiration
Feces
Sweat
Remember Hypertonic
and Hypotonic?

When the ECF gaines water it
becomes hypotonic to the ICF
– Which direction will the water flow?

When the ECF loses water it becomes
hypertonic to the ICF
– Which direction will the water flow?
Remember Hypertonic
and Hypotonic? Cont.




What can happen to ECF then ICF
volumes during dehydration?
What can happen to ECF then ICF
volumes during over hydration?
What does this mean for the burn
victim?
What does this mean for the dialysis
patient?
Remember –
Water follows sodium

Increase in sodium
1. Osmoreceptors
stimulated
2. Increases ADH
3. Increase thirst
4. Decrease urine
5. Increase ECF
6. Dilution of sodium
7. Homeostasis restored

Decrease in sodium
– Osmoreceptors
inhibited
– Decrease ADH
– Decrease thirst
– Increase urine
production
– Concentration of
sodium
– Homeostasis restored
Combined Water/Sodium
Regulation

ECF increase
– Inc blood volume
– Atrial stretch
– Release of atrial naturitic peptide
– Decrease ADH, aldosterone, thirst
– Decrease water intake, increase urination,
increase sodium loss
– Dec blood volume
Combined Water/Sodium
Regulation cont.

ECF decreased
– Dec blood volume/blood pressure
– Inc renin, angiontensin
– Inc aldosterone and ADH release
– Inc thirst
– Dec urine production
– Inc sodium retention
– Inc blood volume/blood pressure
Other Electrolytes

Potassium
– Kidneys regulate
– When pH falls so
does potassium
secretion
– When sodium
retained potassium
is secreted

Calcium
– Calcitonin
– Calcitriol (Vitamin D)
– Parathyroid hormone

Phosphate
– Calcitriol stimulates
reabsorption along
proximal convoluted
tubule
Maintaining Proper pH

What is an acid?
– H+ donor

What is a salt?
– An acid that has given up its H+ and taken up a
metal ion
– HCl  H+ + Cl– Cl- + Na+  NaCl

What is a buffer?
– A weak acid and its salt that give up or take up
H+ as needed and prevent wild swings in pH
Maintaining Proper pH
cont.

Types of acids
– Volatile


Can come out of solution and enter atmosphere
Carbonic – H2CO3
– Fixed


Stay in solution
Sulfuric and phosphoric
– Organic



Participate in or produced by glucose metabolism
Acetyl-CoA
Lactic acid
Maintaining Proper pH
cont.

Protein Buffer System
H
H
If pH drops, extra H+
can bind here
N
H
C
R
O
C
OH
If pH increases,
H+ can leave here
Maintaining Proper pH
cont.

Bicarbonate Buffer System
CO2 + H2O  H2CO3  H+ + HCO3H+ + HCO3- + NaCl  HCl + NaHCO3
Maintaining Proper pH
cont.

Phosphate Buffer System
H3PO4-  H+ + HPO42-
Maintaining Proper pH
cont.

Respiratory Compensation
– When pH drops respiration increases
– When pH rises respiration decreases

Renal compensation
– Regulation of H+ and HCO3- secretion and
reabsorption
Acid/Base Imbalance
Disorders

Acidosis
– Respiratory

Alveolar respiration can’t keep up
– Metabolic




Too much lactic acid
Ketone bodies from alcoholism
Too much aspirin
Alkalosis
– Respiratory

Hyperventilation
– Metabolic

Rare – overuse of bicarbonates
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