Medical-surgical Nursing Demystified

(^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

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