(^420) Medical-Surgical Nursing Demystified
How Fluids and Electrolytes Work
Fluids in the body are found in three basic places: within the cells (intracellular),
outside the cells (extracellular), and within the tissue spaces (interstitial space or
third space). A balance should be maintained to keep concentrations of both fluids
and electrolytes in the proper areas for normal function. The cell walls are semi-
permeable to allow for movement (diffusion) of molecules. This helps to maintain
osmotic pressure.
Edema occurs when too much fluid enters the interstitial space. Peripheral
edema usually collects in subcutaneous areas. The higher hydrostatic pressure in
the vessel causes fluids to move into the interstitial areas which have lower pres-
sure, allowing the fluid to build up.
Normal osmolarity of plasma is 270 to 300 mOsm/L. Isotonic or normotonic flu-
ids have similar concentrations. This prevents fluids from shifting into spaces they
do not belong. Hypertonic solutions have a concentration greater than 300 mOsm/L
and exert a greater pressure, which pulls water from the isotonic area to the hyper-
tonic solution in an attempt to equalize the osmolarity. Hypotonic solutionshave a
Key Terms
Acid base balance
Aldosterone
Angiotensin
Antidiuretic hormone
(ADH)
Diffusion
Edema
Extracellular
Hypertonic
Hypotonic
Interstitial space
Intracellular
Isotonic
Natriuretic peptides
Osmolarity
pH
Renin
Third space
Learning Objectives
Hyponatremia
Hypernatremia
Hypocalcemia
Hypercalcemia
Hypokalemia
Hyperkalemia
Hypomagnesemia
Hypermagnesemia
Metabolic acidosis
Metabolic alkalosis
Hypophosphatemia
Hyperphosphatemia
Dehydration