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Fluid and Electrolyte Balance Overview

Jun 26, 2025

Overview

This lecture covers the fundamentals of fluid and electrolyte balance, including relevant anatomy and physiology, fluid compartments, movement of fluids, major electrolytes and their imbalances, and related nursing interventions.

Fluid & Electrolyte Basics

  • A solvent is the water part of body fluids; solutes are dissolved particles.
  • Electrolytes (ions) are solutes with an electrical charge; can be positive (cations) or negative (anions).
  • Body fluids maintain volume and proper electrolyte composition in fluid compartments for optimal function.
  • Fluid/electrolyte balance involves regulation via filtration, diffusion, osmosis, and selective excretion.
  • Extracellular fluid (ECF) = 1/3 of body water; includes interstitial, blood, lymph, transcellular fluids.
  • Intracellular fluid (ICF) = 2/3 of body water; fluid inside cells.

Movement of Fluids

  • Filtration: movement of water due to hydrostatic pressure (e.g., blood pressure).
  • Diffusion: particles move from high to low concentration across a permeable membrane.
  • Osmosis: water moves through a semi-permeable membrane to balance solute concentrations.
  • Isotonic fluids: osmolality 270-300 mOsm/L; hypertonic: >300; hypotonic: <270.

Fluid & Electrolyte Regulation

  • Electrolytes are balanced so total positive = total negative charges.
  • Three hormones regulate balance: aldosterone (retains water/Na+), ADH (retains water), natriuretic peptides (promote excretion).
  • Kidneys monitor blood pressure, volume, oxygen, sodium/osmolality and secrete renin if perfusion is low.
  • Age, gender, fat amount affect fluid distribution; older/obese adults have higher risk of imbalance.

Major Electrolytes & Imbalances

Sodium (Na+)

  • Normal plasma: 136-145 mEq/L.
  • Controls water movement/retention/excretion in ECF.

Potassium (K+)

  • Plasma: 3.5-5 mEq/L; ICF: ~150 mEq/L.
  • Essential for nerve/muscle function and action potentials.

Calcium (Ca2+)

  • 9-10.5 mg/dL.
  • Vital for bones, muscle contraction, nerve transmission, clotting.

Magnesium (Mg2+)

  • 1.3-2.1 mEq/L.
  • Important inside cells for energy, muscle contraction, vitamin activation.

Common Electrolyte Imbalances

Hyponatremia (Na+ < 136)

  • Causes: diuretic use, excess water, sodium loss.
  • Symptoms: cerebral changes, confusion, seizures, muscle weakness, GI upset, cardiovascular changes.
  • Treatment: correct slowly, adjust meds, IV saline if fluid deficit, restrict fluids if excess.

Hypernatremia (Na+ > 145)

  • Causes: fluid loss, kidney issues.
  • Symptoms: dehydration, neurological changes, muscle twitching, cardiac changes, increased thirst.
  • Treatment: restore fluid, give sodium-losing diuretics, sodium restriction.

Hypokalemia (K+ < 3.5)

  • Causes: diuretics, insufficient intake, GI losses.
  • Symptoms: muscle weakness, cardiac arrhythmias, ileus, confusion.
  • Treatment: replace K+, monitor IV site, prevent falls, never give K+ IM/subcutaneously.

Hyperkalemia (K+ > 5)

  • Causes: renal failure, meds.
  • Symptoms: cardiac dysrhythmias, muscle twitching/weakness, paresthesia, increased GI motility.
  • Treatment: promote K+ excretion, insulin + glucose, cardiac monitoring, dietary restriction.

Hypocalcemia (Ca2+ < 9)

  • Causes: poor intake, hypoparathyroidism.
  • Symptoms: tetany, paresthesia, muscle cramps, arrhythmias, osteoporosis.
  • Treatment: calcium/vitamin D replacement, safety precautions, reduce stimulation.

Hypercalcemia (Ca2+ > 10.5)

  • Causes: cancer, hyperparathyroidism.
  • Symptoms: muscle weakness, lethargy, constipation, arrhythmias, kidney stones.
  • Treatment: rehydration, stop calcium-increasing meds, diuretics, cardiac monitoring.

Hypomagnesemia (Mg2+ < 1.3)

  • Causes: alcoholism, diuretics.
  • Symptoms: arrhythmias, hyperactive reflexes, muscle cramps, decreased GI motility.
  • Treatment: IV magnesium sulfate, stop magnesium-losing drugs, monitor reflexes.

Hypermagnesemia (Mg2+ > 2.1)

  • Causes: excessive intake, renal failure.
  • Symptoms: bradycardia, hypotension, decreased reflexes, lethargy, possible respiratory failure.
  • Treatment: stop magnesium, IV fluids, loop diuretics, calcium for severe cardiac symptoms.

Key Terms & Definitions

  • Solvent — water part of fluids.
  • Solute — dissolved particles.
  • Electrolyte (ion) — charged solute particle.
  • Filtration — movement of fluid due to pressure differences.
  • Diffusion — movement of particles from high to low concentration.
  • Osmosis — water movement to balance solute concentrations.
  • Isotonic — equal osmolality (270-300 mOsm/L).
  • Cation — positively charged electrolyte.
  • Anion — negatively charged electrolyte.
  • Hyponatremia — low sodium.
  • Hypernatremia — high sodium.
  • Hypokalemia — low potassium.
  • Hyperkalemia — high potassium.
  • Hypocalcemia — low calcium.
  • Hypercalcemia — high calcium.
  • Hypomagnesemia — low magnesium.
  • Hypermagnesemia — high magnesium.

Action Items / Next Steps

  • Review the lab profiles for fluid and electrolyte assessment.
  • Prepare drug cards for vasopressin receptor antagonists (conivaptan/tolvaptan) and diuretics (furosemide).
  • Study the common causes and signs of electrolyte imbalances for exam preparation.

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