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Fluid, Electrolyte
& pH Balance
Cell function depends not only on continuous nutrient supply / waste removal,
but also on the physical / chemical homeostasis of surrounding fluids
Body Fluids:
1) Water: (universal solvent)
Fluid / Electrolyte / Acid-Base Balance
Body water varies based on of age, sex, mass, and body composition
H
2
O ~ 73 % body weight
Low body fat
Low bone mass H
2
O (♂) ~ 60 % body weight
H
2
O (♀) ~ 50 % body weight
♀ = body fat / muscle mass
H
2
O ~ 45 % body weight
Intracellular Fluid (ICF)
Volume = 25 L
(40% body weight)
Extracellular Fluid (ECF)
Total Body Water
Interstitial
Fluid
Plasma
Volume = 12 L
Volume = 3
L
Volume = 15 L
(20% body weight)
Volume = 40 L
(60% body weight)
Body Fluids:
Cell function depends not only on continuous nutrient supply / waste removal,
but also on the physical / chemical homeostasis of surrounding fluids
1) Water: (universal solvent)
Clinical Application:
The volumes of the body fluid compartments are measured
by the dilution method
Fluid / Electrolyte / Acid-Base Balance
Step 1:
Identify appropriate marker
substance
A marker is placed in
the system that is distributed
wherever water is found
Marker: D 2
O
Extracellular Fluid Volume:
A marker is placed in
the system that can not cross
cell membranes
Marker: Mannitol
Total Body Water:
Step 2:
Inject known volume of
marker into individual
Step 3:
Let marker equilibrate and
measure marker
Step 4:
Calculate volume of body
fluid compartment
Volume =
Amount
Concentration (L)
Amount:
Amount of marker injected (mg)
Concentration:
Concentration in plasma (mg / L)
Note:
ICF Volume = TBW – ECF Volume
(mg)
Ingested liquids (60%)
Solid foods (30%)
Metabolism (10%)
Water Intake
2500 ml/day = 0
Urine (60%)
Skin / lungs (30%)
Sweat (8%)
Water Output
2500 ml/day
Feces (2%)
0
< 0
Osmolarity rises:
Osmolarity lowers:
ICF functions as
a reservoir
Water Balance:
For proper hydration: Water
intake
= Water
output
volume /
osmolarity of
extracellular fluid
saliva
secretion
Dry mouth
Osmoreceptors
stimulated
(Hypothalamus)
Sensation
of thirst
osmolarity /
volume
of extra. fluid
Drink
Fluid / Electrolyte / Acid-Base Balance
Water Balance:
Thirst Mechanism:
Intracellular
Fluid
Extracellular
Fluid
300 mOsm
300 mOsm
Pathophysiology:
Disturbances that alter solute or water balance in the body can
cause a shift of water between fluid compartments
Volume Contraction: A decrease in ECF volume
Isomotic Contraction
Diarrhea
300 mOsm
Intracellular
Fluid
Extracellular
Fluid
300 mOsm
300 mOsm
Intracellular
Fluid
Extracellular
Fluid
< 300 mOsm
300 mOsm
Intracellular
Fluid
Extracellular
Fluid
300 mOsm
ECF osmolarity = No change
Hyperosmotic Contraction
ECF volume = Decrease
Water
deficiency
< 300 mOsm
ECF volume = Decrease
Aldosterone
insufficiency
NaCl not reabsorbed
from kidney filtrate
Hyposmotic Contraction
300 mOsm < 300 mOsm
ICF volume = No change
ICF osmolarity = No change
ECF volume = Decrease
ICF osmolarity = Increase
ICF volume = Decrease
ECF osmolarity = Increase ECF osmolarity = Decrease
ICF volume = Increase
ICF osmolarity = Decrease
Intracellular
Fluid
Extracellular
Fluid
300 mOsm
300 mOsm
Pathophysiology:
Disturbances that alter solute or water balance in the body can
cause a shift of water between fluid compartments
Volume Expansion: An increase in ECF volume
Isomotic Expansion
Osmotic IV
300 mOsm
Intracellular
Fluid
Extracellular
Fluid
300 mOsm
300 mOsm
Intracellular
Fluid
Extracellular
Fluid
< 300 mOsm
< 300 mOsm
Intracellular
Fluid
Extracellular
Fluid
300 mOsm
Hyperosmotic Expansion
Yang’s lunch SIADH
Greater than normal
water reabsorption
Hyposmotic Expansion
IV bag
300 300 mOsmmOsm
NaCl
Water
intoxication
IV bags of varying solutes
allow for manipulation of
ECF / ICF levels…
Fluid / Electrolyte / Acid-Base Balance
ECF osmolarity = No change
ECF volume = Increase ECF volume = Increase
ICF volume = No change
ICF osmolarity = No change
ECF volume = Increase
ICF osmolarity = Increase
ICF volume = Decrease
ECF osmolarity = Increase ECF osmolarity = Decrease
ICF volume = Increase
ICF osmolarity = Decrease
300 mOsm
Chemical Buffer:
A mixture of a weak acid and its conjugate base or a weak base and
its conjugate acid that resist a change in pH
Acid-Base Balance:
H
Gain:
- Across digestive epithelium
- Cell metabolic activities
H
Loss:
- Release at lungs
- Secretion into urine
Distant from
one another
Brønsted-Lowry Nomenclature:
Weak acid:
Acid form = HA = H
donor
Base form = A
= H
acceptor
Weak base:
Acid form = BH
= H
donor
Base form = B = H
acceptor
11.4 L
11.4 L
150 mEq H
150 mEq H +
Robert Pitt:
The Henderson-Hasselbalch equation is used to calculate
the pH of a buffered solution
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
Henderson-Hasselbalch Equation:
pH = pK + log
[A
]
[HA]
pH = - log 10
[H
]
pK = - log 10
K (equilibrium constant)
[A
- ] = Concentration of base form of buffer (mEq / L)
[HA] = Concentration of acid form of buffer (mEq / L)
pK is a characteristic value for a buffer pair
(strong acid = pK; weak acid = pK)
Costanzo (Physiology, 4
th ed.) – Figure 7.
Most effective
buffering
For the human body, the
most effective physiologic buffers
will have a pK at 7.4 1.
1) HCO
3
/ CO
2
Buffer:
Utilized as first line of defense when H
enters / lost from system:
Acid-Base Balance:
The major buffers of the ECF are bicarbonate and phosphate
H
CO
HCO
CO
O
(HA) (A
(1) Concentration of HCO
3
normally high at 24 mEq / L
(2) The pK of HCO
3
/ CO
2
buffer is 6.1 (near pH of ECF)
(3) CO
2
is volatile; it can be expired by the lungs
The pH of arterial blood can
be calculated with the
Henderson-Hasselbalch equation
pH = pK + log
HCO
3
0.03 x P
CO
pH = 6.1 + log
0.03 x 40
pK = 6.
[HCO 3
Solubility = 0.03 mmol / L / mm Hg
P CO
= 40 mm Hg
pH = 7.
1) HCO
3
/ CO
2
Buffer:
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
The major buffers of the ECF are bicarbonate and phosphate
Costanzo (Physiology, 4
th ed.) – Figure 7.
Acid-base map:
Shows relationship between the
acid and base forms of a buffer
and the pH of the solution
Isohydric line
(‘same pH’)
Ellipse:
Normal values for
arterial blood
Note:
Abnormal combinations of PCO
and HCO 3
yield normal values of pH
(compensatory mechanisms)
1) Respiratory Regulation:
If H
begins to rise in
system, respiratory
centers excited
H 2
CO 3
carbonic
acid
CO 2
O HCO 3
CO
2
leaves
system
H 2
CO 3
carbonic
acid
CO 2
O HCO 3
If H
begins to fall in
system, respiratory
centers depressed
CO
2
leaves
system
Doubling / halving of areolar ventilation
can raise / lower blood pH by 0.2 pH units
( H; homeostasis
restored)
( H; homeostasis
restored)
Buffers are a short-term fix to the problem; in the long term,
H
must be removed from the system…
Maintenance of Acid-Base Balance:
2) Renal Regulation:
A) HCO
3
reabsorption:
Process continues
until 99.9% of
HCO
3
reabsorbed
Buffers are a short-term fix to the problem; in the long term,
H
must be removed from the system…
Fluid / Electrolyte / Acid-Base Balance
Maintenance of Acid-Base Balance:
Kidneys are ultimate acid-base
regulatory organs
- Reabsorb HCO 3 -
- Secrete fixed H
Costanzo (Physiology, 4
th ed.) – Figure 7.
(brush
border)
(intracellular)
Net secretion of
H
does not occur
Net reabsorption of
HCO 3
THUS
pH of filtrate
does not significantly
change
If HCO 3
transport maximized and
HCO
3
2) Renal Regulation:
B) Secretion of fixed H
Buffers are a short-term fix to the problem; in the long term,
H
must be removed from the system…
Maintenance of Acid-Base Balance:
Kidneys are ultimate acid-base
regulatory organs
- Reabsorb HCO 3 -
- Secrete fixed H
Excretion of H
as titratable acid
Costanzo (Physiology, 4
th ed.) – Figure 7.
Titratable Acid:
H
excreted with buffer
Recall:
Only 85% of phosphate
reabsorbed from filtrate
CO 2
New HCO 3
HCO
3
CO
2
exits blood
Minimum urinary
pH is 4.
(Maximum H
gradient
H
ATPase can
work against)
(Removes 40% of fixed acids – 20 mEq / day)
2) Renal Regulation:
B) Secretion of fixed H
Buffers are a short-term fix to the problem; in the long term,
H
must be removed from the system…
Fluid / Electrolyte / Acid-Base Balance
Maintenance of Acid-Base Balance:
Kidneys are ultimate acid-base
regulatory organs
- Reabsorb HCO 3 -
- Secrete fixed H
Excretion of H
as NH
4
Costanzo (Physiology, 4
th ed.) – Figure 7.
(Removes 60% of fixed acids – 30 mEq / day)
Diffusional trapping:
Lipid soluble NH 3
is able to diffuse
into tubule but once combined with
NH 4
it is unable to leave
