Fluid and Electrolyte Balance and Imbalance

(High-Yield Notes for Competitive Nursing Exams)

A) Water Content of the Body, Electrolytes, and Acid-Base Balance

1. Water Content of the Body:

• Total Body Water (TBW): ~60% of body weight in adults.
  • Intracellular Fluid (ICF): 2/3 of TBW (inside cells).
  • Extracellular Fluid (ECF): 1/3 of TBW (plasma, interstitial fluid).
• Functions: Maintains temperature, transports nutrients, removes waste.

2. Major Electrolytes:

• Cations (positive):
  • Sodium (Na⁺): Main ECF ion (135–145 mEq/L) – controls fluid balance.
  • Potassium (K⁺): Main ICF ion (3.5–5.0 mEq/L) – nerve and muscle function.
  • Calcium (Ca²⁺): Muscle contraction, blood clotting (8.5–10.5 mg/dL).
  • Magnesium (Mg²⁺): Enzyme activity (1.5–2.5 mEq/L).
• Anions (negative):
  • Chloride (Cl⁻): Fluid balance (95–105 mEq/L).
  • Bicarbonate (HCO₃⁻): Acid-base buffer (22–26 mEq/L).
  • Phosphate (PO₄³⁻): Energy metabolism (2.5–4.5 mg/dL).

3. Acid-Base Balance:

• Normal pH: 7.35–7.45
• Buffer Systems:
  • Bicarbonate buffer (HCO₃⁻/H₂CO₃) – primary buffer.
  • Respiratory system: Regulates CO₂ (fast response).
  • Renal system: Regulates H⁺ and HCO₃⁻ (slow response).

B) Homeostasis

• Definition: The body’s ability to maintain a stable internal environment despite external changes.
• Key Regulators:
  • Kidneys: Regulate electrolytes, water balance, acid-base status.
  • Lungs: Control CO₂ removal.
  • Hormones:
    • ADH (Antidiuretic Hormone): Controls water reabsorption.
    • Aldosterone: Promotes Na⁺ retention, K⁺ excretion.
    • Renin-Angiotensin-Aldosterone System (RAAS): Regulates blood pressure and fluid balance.

C) Review of Mechanisms Regulating Fluid and Electrolyte Movement

1. Processes:

• Diffusion: Movement of solutes from high to low concentration.
• Osmosis: Movement of water across a membrane from low to high solute concentration.
• Active Transport: Requires energy (ATP) to move ions against a gradient (e.g., Na⁺/K⁺ pump).
• Filtration: Movement of fluid due to hydrostatic pressure (e.g., in capillaries).

2. Hormonal Regulation:

• ADH: Increases water reabsorption → reduces urine output.
• Aldosterone: Increases Na⁺ reabsorption → water retention.
• Natriuretic Peptides (ANP, BNP): Promote Na⁺ and water excretion.

D) Fluid, Electrolyte, and Acid-Base Imbalances & Their Management

1. Fluid Imbalances:

• Dehydration (Hypovolemia):
  • Causes: Vomiting, diarrhea, diuretics, burns.
  • Symptoms: Dry mucous membranes, hypotension, tachycardia, decreased urine output.
  • Management: Oral/IV fluids (isotonic solutions like 0.9% NaCl).
• Fluid Overload (Hypervolemia):
  • Causes: Heart failure, kidney failure, excess IV fluids.
  • Symptoms: Edema, hypertension, crackles in lungs, weight gain.
  • Management: Diuretics (e.g., furosemide), fluid restriction, oxygen if needed.

2. Electrolyte Imbalances:

• Hyponatremia (Na⁺ < 135 mEq/L):
  • Causes: Excess water intake, SIADH.
  • Symptoms: Confusion, seizures, muscle weakness.
  • Management: Fluid restriction, hypertonic saline if severe.
• Hypernatremia (Na⁺ > 145 mEq/L):
  • Causes: Dehydration, diabetes insipidus.
  • Symptoms: Thirst, dry mouth, agitation.
  • Management: Oral/IV hypotonic fluids.
• Hypokalemia (K⁺ < 3.5 mEq/L):
  • Causes: Diuretics, vomiting, diarrhea.
  • Symptoms: Muscle weakness, arrhythmias, cramps.
  • Management: K⁺ supplementation (oral/IV).
• Hyperkalemia (K⁺ > 5.0 mEq/L):
  • Causes: Kidney failure, acidosis, potassium-sparing diuretics.
  • Symptoms: Muscle weakness, ECG changes (peaked T waves), arrhythmias.
  • Management: Calcium gluconate (cardiac protection), insulin + glucose, diuretics.
• Hypocalcemia (Ca²⁺ < 8.5 mg/dL):
  • Symptoms: Chvostek’s & Trousseau’s signs, tetany, seizures.
  • Management: Calcium supplements, IV calcium gluconate if severe.
• Hypercalcemia (Ca²⁺ > 10.5 mg/dL):
  • Symptoms: Weakness, constipation, kidney stones.
  • Management: IV fluids, diuretics, bisphosphonates.

3. Acid-Base Imbalances:

• Respiratory Acidosis (↑CO₂, pH < 7.35):
  • Causes: COPD, respiratory depression.
  • Symptoms: Confusion, drowsiness.
  • Management: Improve ventilation, oxygen therapy.
• Respiratory Alkalosis (↓CO₂, pH > 7.45):
  • Causes: Hyperventilation, anxiety.
  • Symptoms: Dizziness, tingling.
  • Management: Slow breathing techniques, rebreathing bag.
• Metabolic Acidosis (↓HCO₃⁻, pH < 7.35):
  • Causes: Diabetic ketoacidosis, renal failure.
  • Symptoms: Kussmaul respirations, confusion.
  • Management: Treat the cause, IV bicarbonate if severe.
• Metabolic Alkalosis (↑HCO₃⁻, pH > 7.45):
  • Causes: Vomiting, diuretics.
  • Symptoms: Muscle cramps, weakness.
  • Management: Correct electrolytes, IV fluids.

🚨 Quick Revision Points (High-Yield):

• Normal pH: 7.35–7.45
• Hypokalemia → Arrhythmias; watch ECG.
• Hyponatremia → Seizure risk; correct slowly.
• Hyperkalemia → Peaked T waves on ECG → Treat with calcium gluconate.
• Metabolic Acidosis → Kussmaul respirations.
• ADH → Water retention | Aldosterone → Na⁺ retention.
• Respiratory = CO₂ problem; Metabolic = HCO₃⁻ problem.