Fluid, Electrolyte, and Acid-Base
Homeostasis
A. Fluid compartments and fluid balance
B. Water
1. Regulation of fluid intake (gain)
2. Regulation of fluid output (loss)
C. Electrolytes
1. Distribution
D. Movement of body fluids
1. Exchange between plasma and interstitial fluid
2. Exchange between interstitial and intracellular fluids
E. Acid-base balance
1. Buffer systems
2. Exhalation of carbon dioxide
3. Kidney excretion of H+
Body Fluid Compartments
1.
intracellular fluid (67%)
2. extracellular fluid (33%
a. interstitial fluid (80%)
b. plasma (20%)
c. other fluids
Selectively permeable cell membranes
separate body fluids into the fluid
compartments. Which cells do this?
venular
end
plasma
arteriolar
end
endothelium
reabsorption
interstitial fluid
filtration
osmosis
tissue cells
Fluids are in constant motion between the three compartments.
Water is the main component of
all body fluids.
1. What does the term fluid balance mean?
2. What is the primary mechanism by which
water moves from one compartment to the
next?
3. What controls osmosis and therefore fluid
balance?
4. Fluid balance, then, means water balance,
but it implies electrolyte balance as well; the
two are inseparable in this regard.
Water makes up 45 - 75% of total body
weight. The exact percentage is
dependent upon:
Age- newborn = 75%, lean adult man = 60%,
old age = 45%
Body Size- obese = as little as 45%, very
lean as much as 75%
Gender- lean adult man = 60%,
lean adult women = 50%
Water gain and loss
(2,500 ml/day)
1. water gain
a. preformed vs
b. metabolic
2. water loss
sensible vs. insensible
Dehydration, Thirst, and Rehydration
Regulation of fluid intake (Thirst)
decreased
salivation
decreased blood
volume
increased blood osmotic
pressure
dry mouth and
throat
decreased blood
pressure
stimulation of
hypothalamic
osmoreceptors
stimulation of
tactile receptors
increased
angiotensin
II
stimulation of
hypothalamic thirst center
conscious awareness of
thirst
increased water intake
Regulation of Fluid Output
(adjusted by 3 hormones)
ANTIDIURETIC HORMONE
ALDOSTERONE
ATRIAL NATRIURETIC PEPTIDE
dehydration
dehydration
increased blood volume
increased blood
osmolarity
increased
angiotensin II
stretch of right atrium
stimulation of
hypothalamic
osmoreceptors
increased
aldosterone
secretion of ANP
secretion of ADH
from posterior
pituitary gland
increased Na+
reabsorption
decreased Na+
reabsorption
increased water
reabsorption
decreased water
reabsorption
rehydration
decreased blood volume
increased thirst
increased water
reabsorption
rehydration
Electrolytes versus Non-electrolytes
Electrolytes have a greater effect on osmosis than do
nonelectrolytes. Consider the following:
C6H12O6 in water = C6H12O6
NaCl in water = Na+ and ClCaCl2 in water = Ca++ and Cl- and ClWhich of these three compounds will exert the greater
effect on osmosis?
Just as important, once the electrolyte dissociates, its
ions can attract other ions of the opposite charge,
creating an electrochemical gradient.
Electrolyte Distribution
Electrolytes serve four
general functions:
1.
2.
3.
4.
essential minerals
exert a greater effect on osmosis
help maintain acid-base balance
carry electrical current
Exchange of body fluids between plasma and
interstitial fluid occurs across capillary
membranes in what three ways?
1. diffusion
2. vesicular transport
3. bulk flow
Bulk flow is dependent on what four pressures that
determine the net filtration pressure?
NFP = (BHP + IFOP) - (BCOP + IFHP)
= +10 mm Hg (net filtration)
= - 9 mm Hg (net reabsorption)
Exchange between ICF and
interstitial fluid
1.
2.
3.
4.
5.
based on osmosis
different ionic compositions
net flow = 0
Na+ and K+ most important
example -- decreased [Na+]
Relationship Between Sodium
Balance and Water Balance
RELATIONSHIP BETWEEN SODIUM BALANCE AND WATER BALANCE
excessive sweating, vomiting, diarrhea
decreased interstitial sodium
+
intake of plain water
decreased interstitial osmotic pressure
net flow of water into cells from interstitial space
increased filtration from blood capillaries
overhydration of cells
decreased blood volume
convulsions  coma
decreased blood pressure
circulatory shock
DEATH
Acid-base balance
Normal body fluid pH is 7.35 - 7.45.
This range is maintained by three major
mechanisms:
1. buffer systems
2. exhalation of carbon dioxide
(respiratory compensation)
3. kidney excretion of H+
(renal compensation)
Metabolic acidosis vs. respiratory acidosis
Metabolic alkalosis vs. respiratory alkalosis
In a buffer system, a strong acid is converted
to a weaker one, using a weak base, or, a
strong base is converted to a weaker one using
a weak acid.
The carbonic acid-bicarbonate system:
H2CO3
HCO3- + H+
HCO3- (bicarbonate ion acts as a weak base.)
H2CO3 (carbonic acid acts as a weak acid.)
H+ + HCO3H2CO3
(Used if there is an excess of H+.)
H2CO3
H+ + HCO3(Used if there is a shortage of H+)
The phosphate buffer system is an
important regulator of pH in the
intracellular fluid.
H2PO4-
HPO42- + H+
H2PO4- (dihydrogen phosphate acts as a weak acid)
HPO42- (monohydrogen phosphate acts as a weak base)
OH- + H2PO4H2O + HPO42(used as a weak acid to buffer strong bases)
H+ + HPO42H2PO4(used as a weak base to buffer a strong acid)
The protein buffer system is the most
abundant buffer in cells and plasma. Proteins
act as both acidic and basic buffers because
they have a free carboxyl group and a free
amine group.
R
NH2
R
C
COOH
NH2
C
H+
H
H
R
R
COOH
COO- +
C
H
NH2 + H+
COOH
C
H
NH3+
Inside red blood cells, hemoglobin is an especially
good buffer. In the tissues, the Bohr effect ensures
that oxygen is delivered to the tissues as hydrogen
ions are buffered.
H2O +
H2CO3
H+
+
HCO3+
O2
HbO
Hb- +
H+
HbH
CO2
Exhalation of Carbon Dioxide
What would be the net effect of hyperventilation?
CO2 + H2O
H2CO3
H+ + HCO3-
reaction shifts to the left
increased H+ used to reform carbonic acid used to reform CO2
pH increases
increased CO2 lost from the body
What would be the net effect of hypoventilation?
CO2 + H2O
H2CO3
H+ + HCO3-
reaction shifts to the right as CO2 accumulates in the body
H+ accumulate in the body
pH decreases
Kidney excretion of hydrogen ions
pH is adjusted by changing the rate of
H+ secretion by the renal tubules.
Kidneys slow to respond to imbalances
Imbalances need to last for a few days
or longer
More permanent
Respiratory Acidosis/Alkalosis versus
Metabolic acidosis/alkalosis