weight of sodium is 23 milligrams. Therefore, 23 milligrams of sodium is meas-
ured as 1 mmol of sodium.
An electrolyte is stored either intracellularly (inside the cell) or extracellu-
larly (outside the cell).
Intracellular electrolytes are mainly potassium, magnesium and some calcium.
Extracellular electrolytes are mainly sodium and some calcium.
Fluid Concentration
Electrolytes move between compartments based on the concentration of elec-
trolytes, the gradients of the concentration, and the electrical charge. For exam-
ple, there is a higher concentration of sodium outside the cell than inside the cell.
Therefore, the gradient is towards the inside of the cell.
Fluids move through the body continuously. The heart pumps the blood, pres-
sure is exerted on the vessels from outside the body, and muscles relax and con-
tract to help the heart move the fluid through the vascular system. Fluid moves
into and out of the cells and the extracellular spaces by osmotic pressure. This is
the pressure exerted by the flow of water through a semipermeable membrane
separating two solutions with different concentrations of solute. Osmotic pres-
sure is determined by the concentration of the electrolytes and other solutes
in water and is expressed as osmolarity or osmolality. However, the terms are
used interchangeably.
Osmolality is the concentration of body fluids. Tonicity is the effect of fluid
on cellular volume. Serum osmolality is a better indicator of the concentration
of solutes in body fluids than tonicity; tonicity is primarily used as a measure-
ment of the concentration of intravenous solutions.
There are three types of fluid concentration:
- Iso-osmolar.This is a fluid that has the same concentration of particles of
solute as water. - Hypo-osmolar.This is a fluid that has a lower concentration of particles of
solute than water. - Hyper-osmolar.This is a fluid that has a higher concentration of particles
of solute than water.
Normal serum has an osmolality of between 275 and 295 mOsm/dg. Less
than 275 mOsm/dg is hypo-osmolar and greater than 295 mOsm/dg is hyper-
CHAPTER 10 Fluid and Electrolyte Therapy^159