Fundamentals of Fluid, Electrolyte and Acid Base Balance
Total Body Fluid
Approximately 67% of the body weight is water. However, it varies among men,
women and children—children may be as much as 77%water. The more fat a person has,
the less water they have. This is important in medicine—chemicals such as vitamins and
medication may preferentially dissolve in the fat or the water compartment—thus,
variations actually affect the dose and accumulation of such substances.
Given a % body water and a person’s weight, their total water content can be
determined:
Pounds x % water = pounds of water.
Then pounds of water X 1 Liter/2.2 pounds = liters of water
On the average, a person gains about 2500 mL per 24 hours and loses the same amount,
thus keeping the total water content stable. ADH and thirst respond to decreasing water
(increasing osmotic pressure), and a drop in ADH as well as increased filtration in the
kidney respond to increasing water and blood volume.
Fluid Compartments
The total water is distributed into three major compartments which are stable but
involved in dynamic fluid balance considerations. There are several transcellular
compartments which are much more stable (such as synovial fluid, cerebrospinal fluid,
aqueous humor, vitreous humor, perilymph and endolymph) and are not considered here.
The intracellular fluid (ICF) has about 67% of the total fluid and the boundary of this
compartment is all of the body’s cell membranes. The major anions in this compartment
are proteins and phosphate and the major cation is K+.
The interstitial fluid (IF) and the intravascular fluid (IV) are the two extracellular
fluid compartments (ECF). The major anions in these compartments are HCO3- and Cland the major cation is Na+. Though the ECF has little protein, the amount present is
plasma is more and is significant.
Movement between Compartments
Between ICF and IF the boundary is all of the cell membranes. The balance of
hydrostatic and osmotic pressures is such that movement across this boundary is
determined mainly by ECF [Na+] (sodium ion concentration). High [Na+] in the ECF
means water is pulled from the cells—this happens for example in dehydration, or if
excess salt (NaCl) is taken in without water(not usual). If excess salt is taken in,
generally the thirst mechanism causes the person to drink a compensating amount of
water. Then, the total ECF volume increases. The excess blood volume is hypervolemia
and the excess IF volume is edema.
If excess water is taken in (for example, sweat out salt and water by exercise or sports,
drink only water to replace (as used to happen in sports before the Univ. of Florida -the
‘Gators- introduced its “Gatorade”, or, as has happened tragically a few times, parents
punishing their children by forcing large volumes of water on them or in university
hazing, forcing large volumes of water on students) then the ECF is hypotonic and water
rushes into cells—causing damage and malfunction of neurons. This leads to a
disoriented state called “water intoxication” and can be lethal.
Between the IF and IV compartments, the plasma protein concentration and the blood
pressure are the two most important factors. Low plasma protein (as for example in
starvation) causes edema, excess fluid in the IF compartment.
Other conditions, such as parasites blocking lymph return (elephantiasis) or surgical
procedures blocking lymph return, or heart failure etc. can also cause edema.
Electrolytes. Na+ ion changes (hypo and hypernatremia) usually affect water
movement while K+ (hypo and hyperkalemia) and Ca++ ion changes (hypo and
hypercalcemia) have a major effect on neuromuscular irritability. Na+ and K+ are
regulated by aldosterone and Ca++ is regulated by PTH and calcitonin.
Acids/Bases/Buffers.
The body’s average ECF pH is 7.4. Deoxygenated blood is about pH 7.36 and
oxygenated blood is about pH 7.46. If this pH drops too low, the condition is acidosis
and could lead to decreased neuromuscular irritability and coma. If this pH is too high,
the condition is alkalosis and could lead to increased neuromuscular irritability and
muscle spasms and convulsions.
Buffers are chemicals that resist pH change if acids or bases become too high. A buffer
is made of a weak acid (releases protons) and its conjugate base (accepts protons).
Each buffer is effective in a range determined by how easily the acid ionizes (pKa).
The pH of the pure acid of the buffer is the lowest pH a given buffer can have, and the
pH of the pure conjugate base of the buffer is the highest pH a buffer can have.
Buffers work by combining with acids or bases that would change the pH—the basic part
of the buffer reacts with acids (producing more of the buffer’s acid) and the acid part of
the buffer reacts with bases (producing more of buffer’s base).
So, while HCL would drop the pH of water drastically (it’s a strong acid—releases lots of
H+), if it were added to water buffered with NaHCO3 (sodium bicarbonate-the buffer’s
base)/H2CO3 (carbonic acid-the buffer’s acid) it would react with the base producing
carbonic acid and NaCl-a neutral salt. Since carbonic acid is weak, the pH drops only a
little.
So: HCl in water  H+ and Cl- pH falls a lot
But HCl in water with NaHCO3  NaCl and H2CO3 and pH falls a little
The same is true in reverse for adding a strong base, such as NaOH. It reacts with the
carbonic acid, becoming water and sodium bicarbonate (a weak base) and the pH only
rises a little.
The more concentrated a buffer is, the more it has to react with acids or bases—it has
more buffering capacity.
In chemistry, since it’s not certain whether acids or bases will be added, buffers are used
that give a desired pH when the base/acid ratio is 1/1.
In the body, there is always an excess of acids from metabolism, so a buffer is used
where the desired pH is achieved with lots of extra base in the buffer. The buffer is the
NaHCO3 /H2CO3 in a 20/1 ratio—actually 26 meq of the base and 1.3 meq if the acid,
giving 26/1.3 = 20/1.
As long as this ratio is maintained, the pH will stay at 7.4.
Excess acid or low base will lower the pH.
Excess base or low acid will raise the pH.
The lungs can lower the acid part by hyperventilating and getting rid of CO2
(thus carbonic acid) and they can raise the acid by hypoventilating.
The kidney tubules can lower the base part by excreting NaHCO3 (keeping Clinstead) and can raise the base part by keeping the NaHCO3 (and losing Cl-)
The object is to keep the ratio 20/1---even if there is a wrong concentration, that
compromises buffering capacity only---but if there is a wrong ratio, the pH will be wrong
and enzymes don’t work at the wrong pH—this is life threatening.
There are four acid / base disturbances—if the acid part of the buffer is wrong, the
disturbance is called respiratory but if the base part of the buffer is wrong, the
disturbance is called metabolic. When one part of the buffer is disturbed, the other part
will have to make the adjustment to a 20/1 ratio.
Thus:
1) Low acid (e.g. by hyperventilation of hysteria) gives say 20/ 0.5 or 40/1 causing
respiratory alkalosis
The kidneys have to lower the base to adjust to the low acid –so may get 10/0.5 which =
20/1 and the proper pH
2) High acid ( e.g. by respiratory difficulties such as emphysema or asthma) gives say
20/2 or 10/1 causing respiratory acidosis
The kidneys have to keep base and raise the base to adjust to the high acid—so may get
40/2 which = 20/1 and the proper pH.
3) High base (e.g. from gastric vomiting or overmedication with antacids) gives say 40/1
causing metabolic alkalosis.
The body will hypoventilate, keeping acid to match the high base, giving say 40/2 which
is 20/1 and the proper pH.
4) Low base (e.g. from deep vomiting, or diarrhea, or loss of base used up in buffering
ketoacids of diabetes mellitus) gives say 10/1 causing metabolic acidosis.
The body will hyperventilate, losing acid to match the lower base, giving say 10/0.5
which = 20/1 and the proper pH.
Problems: (normal pH = 7.4, normal pCO2 = 40 and normal base bicarbonate = 26
I.
A persons blood gas analysis shows a pH of 7.3, a pCO2 of 43 and a base
bicarbonate of 28. The person is having problems with asthma.
Analysis: The pH is low—so acidosis.
The base is high—this cannot cause acidosis.
The CO2 is high—this must be the cause—so—respiratory acidosis.
Why is the base high? Trying to compensate—not fully compensated because the pH is
not normal.
Does this match the clinical picture? Yes—the person with asthma is having trouble
getting rid of CO2.
II.
A persons blood gas analysis shows a pH of 7.3, a pCO2 of 38 and a base
bicarbonate of 24. This person is a diabetic and has come into the hospital
hyperventilating.
Analysis: The pH is low, so acidosis.
The pCO2 is low—this cannot cause acidosis.
The base bicarbonate is low—this must be the cause—so—metabolic acidosis
Why is the pCO2 low? The body is hyperventilating –trying to get rid of acid to lower
the acid to match the low base—trying to compensate—not fully compensated because
the pH is still not normal.
Does this match the clinical picture? Yes—the person has not controlled their diabetes
mellitus well, and hyperventilation is a sign of acidosis.
Note the difference between cause/effect—hyperventilation from normal raises the
pH above normal causing alkalosis, but hyperventilation compensates for acidosis
by raising the pH up to normal.