Body Fluid Compartments
Total body water (TBW) ≈ 60% of adult body weight. Intracellular fluid (ICF) = 40% TBW; Extracellular fluid (ECF) = 20% TBW including intravascular and interstitial spaces.
Fluid Movement Mechanisms
Osmosis: water moves from low to high solute concentration. Diffusion: solutes move from high to low concentration. Filtration: driven by hydrostatic pressure. Active transport: requires ATP. Starling's forces govern capillary fluid exchange.
Sodium Imbalances
Normal serum Na+: 135–145 mEq/L. Hyponatremia (<135): SIADH, excessive water intake, HF. Symptoms: nausea, confusion, seizures. Hypernatremia (>145): dehydration, diabetes insipidus. Symptoms: thirst, dry membranes, neurological changes.
Potassium Imbalances
Normal serum K+: 3.5–5.0 mEq/L. Hypokalemia (<3.5): diuretics, vomiting, diarrhea. ECG: flattened T waves, U waves. Hyperkalemia (>5.0): renal failure, ACE inhibitors. ECG: peaked T waves, widened QRS.
Nursing Interventions
Monitor I&O meticulously — daily weights are most reliable. Review BMP/CMP, BUN/creatinine, urine specific gravity. IV fluid selection: isotonic (0.9% NS, LR) for volume replacement; hypotonic (0.45% NS) for cellular dehydration; hypertonic (3% NS) for severe hyponatremia (ICU only).
Important Notes
- Never correct hyponatremia faster than 8–10 mEq/L per 24 hours — risk of osmotic demyelination syndrome.
- Always verify potassium levels before administering IV K+ — never give IV push; always dilute and infuse slowly.
Clinical Tips
- A 1 kg weight gain = approximately 1 liter of fluid retained.
- Isotonic saline (0.9% NS) remains in the intravascular space — ideal for rapid volume replacement.
Summary
Fluid and electrolyte balance is maintained through complex homeostatic mechanisms. Nurses must accurately assess fluid status, interpret laboratory values, and implement appropriate interventions.
