The sum of fluids within all compartments constitute total body water (TBW) which is about 60% of the body weight. This is converted to volume by multiplying the TBW (kg) by 1 liter. This gives the volume of TBW in liters. In milliliters it is body weight times 1000 ml.


Body fluids are distributed among functional compartments. These compartments are classified as intracellular (ICF) and extracellular fluid (ECF).


Comprises all the fluid within cells



Extracellular Fluid

All the fluid outside the cells.

Interstial - The space between cells and outside the blood vessels

Intravascular - Blood plasma


Transcellular fluids such as synovial, intestinal, cerebrospinal fluid, sweat, urine and pleural, peritoneal, pericardial, and intraocular

Although the amount of fluid within the various compartments is relatively constant, solutes and water are exchanged between compartments to maintain their unique compositions.  The percentage of TBW varies with the amount of body fat and age.  Because fat is water repelling (hydrophobic), very little water is contained in adipose cells.  Individuals with more body fat have proportionately less TBW and tend to be more susceptible to fluid imbalances that cause dehydration.

Age and the distribution of body fluids

The distribution and the amount of TBW change with age.  Although daily fluid intake may fluctuate widely, the body regulates water volume within a relatively narrow range


Newborn Infants

At birth, TBW represents about 75% to 80% of body weight and decreases to about 67% during the first year of life.  Fish and logical loss of phi water amounting to 5% of body weight occurs as an infant adjusts to a new environment.  Infants are particularly susceptible to significant changes in TBW because of a high metabolic rate and a greater surface area.  Renal mechanisms of fluid and electrolyte conservation may not be mature enough to counter losses, allowing the hydration to occur.

Children and Adolescents

TBW slowly decreases to 60%-65% of body weight.  At adolescence, the percentage ofTBW approaches adult levels, and differences according to gender appear.  Males have a greater percentage of body water the cause of increased muscle mass, and females have more body fat recalls of the influence of estrogen and thus less water.

Elderly and the Distribution of Body Fluids

The further decline in the percentage of TBW in the elderly is in part the result of an increased amount of fact and decreased muscle, as well as reduced ability to regulate sodium and water balance.  The kidneys are less efficient in producing concentrated urine, and sodium conserving responses are sluggish.  With stress, when diseases present, this normal decrease in TBW can become life threatening.


Osmotic Equilibria Between Intercellular and Extracellular Fluids

Intercellular and extracellular fluids are separated by the membranes of the body's cells.


Aside: Blood cell membranes generally are not completely permeable. That is, they do not allow the unrestricted passage of any molecule back and forth across the membrane.


The construction of the membranes makes them very permeable to the passage of water, but more selective in the other types of molecules and electrolytes (e.g. ions) that can move through the membrane. This difference in permeability results in a different chemical composition for intercellular and extracellular fluids.

Role of Electrolytes


Sodium is primarily an extracellular ion

potassium occurs primarily intracellularly

calcium performs a variety of
functions. The important role of calcium in this discussion is its control of membrane
permeability. Being a positive ion, calcium that associates with the plasma membrane and serves to repel other positive ions (like charge repulsions) to help control the
membrane’s permeability to positive ions.

Role in Action Potential

Recall from your background study of excitable cells, such as nerve and muscle,
that sodium and potassium are essential for the action potentials conducted along the
surface membranes of these cell types. Sodium, an extracellular ion, enters an excitable
cell during the depolarization phase of the action potential. Whereas potassium, an
intracellular ion, leaves an excitable cell during the repolarization phase of the action
potential. All cells, including excitable cells in the “resting” state (not conducting an
action potential), are polarized such that the interior is negative relative to the exterior.
This difference in net charge across a cell’s membrane is due in part to the intracellular
proteins and body pH. Since normal body pH is slightly alkaline (7.4), proteins lose
hydrogen ions (behave like acids) and show a net negative charge. Since most proteins
are intracellular, when body pH is normal the positively charged intracellular potassium
ions are offset by the negatively charged intracellular proteins to give the cell’s interior a
net negative charge compared to the outside where proteins are deficient.

Sodium is responsible for the ECF osmotic balance, and potassium maintains the ICF osmotic a balance.  The osmotic force of ICF proteins and other non diffusible substances is balanced by the active transport of ions out of the cell.  Normally the ICF is not subject to rapid changes in osmolality, but when ECF osmolality changes water moves from one compartment to another until osmotic equilibrium is established.

The different ionic concentrations on one side of a membrane versus the other side can result in osmotic flow, the flow of water from a less concentrated region to a more concentrated region (e.g., against a particular molecular or ionic concentration gradient), in an effort to balance the ionic concentrations. In the body, even though the ionic concentrations of the intercellular and extracellular fluids can be quite different, the two fluid compartments are always in osmotic equilibrium. This is accomplished by the movement of water across the cell membrane.

Maintenance of osmotic equilibrium via water flow depends on the flexibility of the cell membrane. As water moves into a cell, for example, the membrane expands, allowing the cell to swell. Thus, more water is able to be present, which dilutes the concentration of the ion inside the cell. Conversely, as water moves out of a cellinto the surrounding extracellular fluid, the membrane can accommodate the shrinkage of the cell. If a cell membrane were not flexible, osmotic equilibrium could not be achieved.

In the body, potassium ions often move back and forth across cellmembranes. Because ions carry electrical charges, the differing potassium concentration across the membrane results in anelectrical potential. Unless controlled, this potential could bedamaging to the cell. But, because of osmotic equilibrium, theelectrical potentials for potassium can be the same in the intercellular and extracellular fluids. An equation called the Nernst equation gives the potential difference across the membrane when ions are in equilibrium.

The ability of a cell to actively take up substances can be blocked. For example, if a membrane is permeable to some ions, such as potassium, sodium and chloride, but is not permeable to some other large negatively charged ion, then the concentration disparity of the large ion can become so great that movement of water into and out of the cell is stopped. This is also known as the Gibbs-Donnan equilibrium. A Gibbs-Donnan equilibrium exists between the interstitial fluid and blood plasma.



Normal Electrolyte Functions
By chemical definition, electrolytes are charged particles (ions). These particles
exist dissolved in the various fluid compartments of the body (intravascular, interstitial,
and intracellular) and perform a variety of functions in the total physiology of the human
body. The electrolytes of importance at this point in the course are: (1) Sodium; (2)
Potassium; (3) Calcium; (4) Hydrogen; and (5) Bicarbonate.


1. How do excitable cells differ from other cells of the body regarding their response
to stimulation?
2. Refamiliarize yourself with the four phases of an action potential: (1)
depolarization; (2) repolarization; (3) hyperpolarization; and (4) return to
resting state.
3. What are the ion movements associated with each phase of the action potential?
4. What would you predict might occur with an excitable cell if the normal locations
of sodium and potassium were to be reversed?
Electrolyte Disorders / Assessment
People who have disturbances in either sodium, potassium, or calcium are
probably going to show signs and symptoms of these disturbances in organ systems
whose normal functions depend upon action potentials, particularly neuromuscular
systems. Consequently, clients showing lethargy and muscle weakness or those with
increased irritability may have an electrolyte imbalance. The assessment problem then
becomes one of identifying the electrolyte(s) involved and whether they are abnormally
high (hyper-) or low (hypo-).
Electrolyte imbalances may be “primary” or “seconday” in origin. A primary
electrolyte imbalance usually affects only one electrolyte and typically involves an
abnormality in either the intake or output of the ion of interest. For example, a high salt
diet can result in hypernatremia while some diuretics “waste” potassium and can cause
hypokalemia. Since electrolytes are assayed in blood samples, you are seeing the intravascular concentrations expressed on a lab report. Because of capillary pore
permeability, these changes probably also appear in the interstitial fluid as well and can,
in some instances, affect intracellular concentrations. Consequently, you must know
where each of the three electrolytes occur normally so you can predict the causes and
effects of their changing concentrations. Since electrolytes are osmotically active, they
can cause fluid shifts as well.
1. Why would hyperkalemia cause cardiac arrest?
2. How does an EKG change with increasing potassium concentrations?
A seconday electrolyte imbalance is one resulting from an abnormality in some
other physiological function. Secondary imbalances usually affect more than one
electrolyte and are common with fluid imbalances since they occur as a result of
concentration or dilution of body fluids. For example, in renal failure the kidneys fail to
output urine and can result in increased concentrations of all electrolytes normally
excreted in the urine. Secondary imbalances are usually detected by looking first for
fluid imbalances. Skin turgor (“tenting” in dehydration and “pitting” in edema) is a good
fluid balance indicator as are sudden weight changes, blood pressure abnormalities, and
peripheral or pulmonary edema. Fluid imbalances often appear on lab reports as changes
in hematocrit (percentage of formed elements in blood). A high hematocrit suggests a
possible concentration of blood maybe due to dehydration while a low hematocrit can
appear with fluid overloads.
The table below covers electrolyte imbalances that involve Na, K, and Ca. Work
through the table as you did in previous exercises being certain that you can explain the
basic physiology underlying each cause and each clinical manifestation appearing in the
You should take each one of the imbalances and be sure that you can explain the
underlying physiological abnormality underlying each cause and each clinical
manifestation. This is an excellent review of basic physiology!!
Hyponatremia Decreased intake and adrenal
insufficiency (1
); inappropriate
ADH; diaphoresis with water
replacement; diuretic therapy
Cellular swelling with cerebral edema
leading to headache, stupor and coma;
muscle weakness; decreased thirst; edema
if secondary to hypervolemia;
Hypernatremia Increased intake or renal failure Cellular shrinking with increased CNS (1
); water deprivation; decreased
ADH secretion; increased
aldosterone; liver failure;
hypothalamic lesion
irritability; increased thirst; hypotension
with oliguria if secondary to hypovolemia
Hypokalemia Decreased intake, adrenal cortex
hyperfunction and diuretic
therapy (1
); alkalosis;
vomiting/gastric suction
Cardiac arhythmia (lower T and
appearance of U wave due to slow
repolarization) and muscle weakness;
Hyperkalemia Increased intake or renal failure
and hypoaldosteronism (1
acidosis; RBC hemolysis;
Cardiac depression (shallow, wide QRS
with elevated T due to exaggerated
repolarization); paresthesia and/or
Hypocalcemia Decreased intake (1
), vit. D
deficiency, hypoparathyroid;
hypoalbuminemia; alcohol abuse
or liver failure
Increased neuromuscular activity
(possible convulsions); skeletal muscle
Hypercalcemia Increased intake (1
); immobility;
hyperparathyroidism; bone
malignancies; renal failure
Decreased neuromuscular activity (stupor
to coma); renal calculi; increased fracture

1. Explain why hyponatremia could cause cerebral edema.
2. How could it be treated in order to get the most rapid results?


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