Overview of Fluid and Electrolytes
Fluid and electrolyte balance is critical for muscle contraction, nerve impulses,
intracellular metabolic reactions, transport of gases, nutrients, and wastes, and
maintenance of cell shape.
Fluid imbalance manifests as dehydration, water intoxication,
hypovolemia, and hypervolemia.
An imbalance in sodium concentration creates
hypernatremia or hyponatremia.
Potassium imbalance produces hyperkalemia or
Fluid balance is maintained by an equilibrium between intake and output.
regulated by thirst and the hormone, arginine-vasopressin (aka anti-diuretic hormone).
is also largely regulated by the kidneys in tandem with sodium as “water follows salt”.
Our bodies are mostly water although we do dry out with age.
Water accounts for 80%
of a neonate’s body weight; 60% of an adult’s body weight, and 45% of an elderly
person’s body weight. Water exists in three compartments within the body:
Intracellular and 2) extracellular, which is subdivided into 2 compartments, vascular and
Since porous membranes (capillary endothelium and cellular membrane)
separate these compartments, water is able to move from one compartment to another.
Water moves via osmosis, and the direction of its movement is dependent upon the
osmotic pressures of adjacent water compartments.
Water moves from a hypoosmolal
region to a hyperosmolal one. .
Hypoosmolality is defined as an excess of water as
compared to solute, whereas hyperosmolality is defined as an excess of solute as
compared to water.
The osmolality of the three compartments is equal in a healthy
The volume of water in each compartment is dependent upon two factors:
amount of solute (electrolytes and proteins) and 2)
the osmolality of the compartment.
The major extracellular cation is sodium.
Therefore, the absolute amount of sodium is
the major determinant of the volume of extracellular fluid. When the extracellular fluid
volume increases, the normal response by the kidneys is to increase Na
order to prevent volume overload.
When the extracellular fluid volume decreases, the
normal response of the kidneys is to decrease Na
excretion in order to prevent volume
The amount of protein (especially albumin) in the blood along with sodium
determine the volume of the vascular compartment. The major intracellular cation is
Therefore, the absolute amount of potassium determines the volume of
Arginine-vasopressin (aka, ADH) regulates fluid (water) volume in response to
1) plasma osmolality (tonicity) and 2) effective arterial volume.
plasma osmolality (tonicity) takes precedence. Note that extracellular Na
is the main determinant of plasma tonicity. (Tonicity refers to the effect of solute
concentration on cell shape.
A hypertonic solution causes the cell to shrink, whereas a
hypotonic solution causes the cell to swell.) For example,
hypertonicity is due to an