Fluid and Electrolyte Physiology

Quick Answer

Total body water (TBW) is approximately 60% body weight in males, 50% in females, 65-70% in infants, 50% in elderly. Distribution: 2/3 intracellular (ICF), 1/3 extracellular (ECF). ECF divided into interstitial fluid (ISF, 3/4) and plasma (1/4). Osmolality: 285-295 mOsm/kg (regulated by ADH, thirst). Sodium: Primary extracellular cation (135-145 mmol/L), maintains ECF osmolality and volume. Potassium: Primary intracellular cation (3.5-5.0 mmol/L), critical for resting membrane potential. Calcium: 2.2-2.6 mmol/L total (50% free/ionized), essential for neuromuscular function, coagulation, cardiac contractility. Magnesium: 0.75-0.95 mmol/L, cofactor for ATPases. Chloride: 95-105 mmol/L, main extracellular anion. Fluid shifts: Follow osmotic gradients (water moves from low to high osmolality). Starling forces: Determine transcapillary fluid exchange (hydrostatic vs oncotic pressure). Anaesthetic implications: Preoperative fasting deficits, third space losses, blood loss replacement, glucose management, fluid overload prevention. [1-10]

Body Water Distribution

Total Body Water (TBW)

Percentage of Body Weight:

• Adult males: 60% (60 kg in 70 kg male)
• Adult females: 50-55% (35-38 L in 70 kg female)
  • Lower due to higher body fat percentage
• Infants: 65-70% (higher water content)
• Elderly: 50% (decreased muscle mass)
• Obesity: Lower percentage (fat has low water content)

Compartments:

1. Intracellular Fluid (ICF):

• Volume: 40% body weight (2/3 of TBW)
• Location: Inside all cells
• Composition:
  • High potassium (140 mmol/L)
  • Low sodium (10-15 mmol/L)
  • High magnesium (40 mmol/L)
  • High phosphate and protein anions
  • Osmolality: 285-295 mOsm/kg (same as ECF)

2. Extracellular Fluid (ECF):

• Volume: 20% body weight (1/3 of TBW)
• Subcompartments:
  • Interstitial fluid (ISF): 15% body weight (75% of ECF)
    • Between cells, outside vessels
  • Plasma: 5% body weight (25% of ECF)
    • Intravascular fluid
  • Transcellular fluid: Small volume (CSF, synovial, pleural, peritoneal)

Summary Table:

Gibbs-Donnan Effect:

• Principle: Non-diffusible anions (proteins) in plasma create unequal distribution of diffusible ions
• Effect: Slightly higher chloride and bicarbonate in ISF vs plasma
• Result: Plasma has slightly higher osmotic pressure than ISF (oncotic pressure)

Electrolyte Physiology

Sodium (Na⁺)

Physiological Role:

• Primary extracellular cation: 135-145 mmol/L (plasma)
• Osmolality maintenance: Primary determinant of ECF osmolality (Posm ≈ 2[Na⁺] + glucose/18 + BUN/2.8)
• ECF volume: Determines ECF volume (water follows osmotic gradients)
• Resting membrane potential: Contributes (less than K⁺)
• Acid-base balance: Na⁺ reabsorption linked to H⁺ secretion

Daily Requirements:

• Intake: 1-2 mmol/kg/day (70-140 mmol for 70 kg adult)
• Distribution: 98% extracellular
• Regulation:
  • Aldosterone: Na⁺ retention (distal tubule)
  • ADH: Water retention (collecting duct)
  • ANP: Na⁺ excretion (atria)

Clinical Disorders:

Hyponatremia (<135 mmol/L):

• Symptoms: Headache, nausea, confusion, seizures, coma (if <120 mmol/L or rapid decline)
• Classification by volume status:
  • Hypovolemic: Loss of Na⁺ and water (diuretics, vomiting, Addison's)
  • Euvolemic: Water excess (SIADH, hypothyroidism, cortisol deficiency)
  • Hypervolemic: Edema states (heart failure, cirrhosis, nephrotic syndrome)
• Treatment: Depends on cause and severity
  • Acute symptomatic: 3% NaCl (100 mL bolus)
  • Chronic: Slow correction (max 8-12 mmol/L/day to avoid osmotic demyelination)

Hypernatremia (>145 mmol/L):

• Symptoms: Thirst, confusion, seizures, coma (if >160 mmol/L)
• Mechanism: Water deficit or Na⁺ excess
• Treatment: Free water replacement (oral or IV D5W)

Potassium (K⁺)

Physiological Role:

• Primary intracellular cation: 140 mmol/L (ICF), 3.5-5.0 mmol/L (ECF)
• Resting membrane potential: Primary determinant (Nernst equation)
  • Hyperkalemia: Depolarization (excitable tissues)
  • Hypokalemia: Hyperpolarization (weakness)
• Cardiac conduction: Critical (ECG changes with K⁺ disturbances)
• Acid-base: K⁺ shifts with H⁺ (acidosis → hyperkalemia, alkalosis → hypokalemia)

Daily Requirements:

• Intake: 1 mmol/kg/day (70 mmol for 70 kg adult)
• Regulation:
  • Insulin: Drives K⁺ into cells
  • Catecholamines (β-2): Drive K⁺ into cells
  • Aldosterone: K⁺ excretion (distal tubule)
  • Acid-base status: Affects distribution

Clinical Disorders:

Hypokalemia (<3.5 mmol/L):

• Causes: Diuretics, vomiting, diarrhea, alkalosis, insulin, β-agonists
• ECG: Flattened T waves, ST depression, U waves, prolonged QT
• Effects: Muscle weakness, ileus, arrhythmias (especially with digoxin)
• Treatment: Oral preferred, IV if severe (max 20 mmol/hour via central line)

Hyperkalemia (>5.0 mmol/L):

• Causes: Renal failure, acidosis, cell lysis, K⁺ sparing diuretics, ACE inhibitors
• ECG: Peaked T waves, widened QRS, sine wave, VF/asystole
• Emergency treatment:
  1. Calcium (stabilize membrane)
  2. Insulin/glucose (shift K⁺ intracellular)
  3. Bicarbonate (if acidotic)
  4. β-agonists (salbutamol)
  5. Dialysis (if refractory)

Calcium (Ca²⁺)

Physiological Role:

• Total calcium: 2.2-2.6 mmol/L (plasma)
  • Ionized (free): 50% (1.1-1.3 mmol/L) - physiologically active
  • Protein-bound: 40% (mostly albumin)
  • Complexed: 10% (with phosphate, citrate, bicarbonate)
• Albumin correction: [Ca²⁺] decreases 0.02 mmol/L per 1 g/L decrease in albumin (below 40 g/L)

Functions:

• Neuromuscular: Neurotransmitter release, muscle contraction
• Cardiac: Contractility, conduction
• Coagulation: Factor IV (required for multiple steps)
• Bone: Structural (hydroxyapatite)
• Intracellular signaling: Second messenger

Regulation:

• Parathyroid hormone (PTH): Increases Ca²⁺ (bone resorption, renal reabsorption, intestinal absorption via vitamin D)
• Vitamin D: Increases Ca²⁺ absorption
• Calcitonin: Decreases Ca²⁺ (bone uptake)

Clinical Disorders:

Hypocalcemia (<2.2 mmol/L total, <1.1 mmol/L ionized):

• Causes: Hypoparathyroidism, vitamin D deficiency, hypomagnesemia, citrate (massive transfusion), alkalosis (↓ionized)
• Symptoms: Tetany, Chvostek's sign, Trousseau's sign, seizures, hypotension
• ECG: Prolonged QT interval
• Treatment: Calcium chloride or gluconate IV, magnesium if low

Hypercalcemia (>2.6 mmol/L):

• Causes: Malignancy, hyperparathyroidism, sarcoidosis, immobilization
• Symptoms: "Bones, stones, groans, moans, psychic overtones"
• Treatment: Hydration, bisphosphonates, calcitonin, steroids

Magnesium (Mg²⁺)

Physiological Role:

• Concentration: 0.75-0.95 mmol/L (plasma), 15 mmol/L (ICF)
• Cofactor: ATPases (Na⁺/K⁺-ATPase, Ca²⁺-ATPase)
• Enzyme activation: >300 enzymes
• Neuromuscular: Required for PTH release, modulates NMDA receptors
• Cardiac: Antiarrhythmic (class effect)

Clinical Disorders:

Hypomagnesemia (<0.75 mmol/L):

• Causes: Diuretics, alcoholism, malabsorption, proton pump inhibitors
• Effects: Refractory hypokalemia and hypocalcemia, tremor, seizures, arrhythmias (torsades de pointes)
• Treatment: Magnesium sulfate IV (1-2 g over 15 min, then infusion)

Hypermagnesemia (>1.1 mmol/L):

• Causes: Renal failure, magnesium administration (tocolysis, preeclampsia)
• Effects: Neuromuscular blockade, hypotension, bradycardia, respiratory depression
• Treatment: Calcium antagonist, dialysis if severe

Chloride (Cl⁻)

Physiological Role:

• Concentration: 95-105 mmol/L (plasma), major extracellular anion
• Acid-base balance:
  • Hypochloremic alkalosis: With vomiting (lose HCl), diuretics
  • Hyperchloremic acidosis: With saline administration (dilutional)
• Strong ion difference: Part of Stewart approach to acid-base

Phosphate (PO₄³⁻)

Physiological Role:

• Concentration: 0.8-1.4 mmol/L (plasma)
• Functions: ATP, 2,3-DPG, bone mineralization, cellular signaling
• Clinical: Hypophosphatemia with refeeding syndrome, hyperphosphatemia in renal failure

Fluid Physiology

Osmolality and Tonicity

Osmolality:

• Definition: Number of osmotically active particles per kg solvent (mOsm/kg)
• Normal plasma osmolality: 285-295 mOsm/kg
• Calculation: 2[Na⁺] + [glucose]/18 + [urea]/2.8 (normal glucose and urea contribute ~10 mOsm/kg)
• ICF and ECF osmolality: Equal (water moves freely across cell membranes)

Tonicity (Effective Osmolality):

• Definition: Osmolality that causes water movement (excludes urea which crosses membranes freely)
• Effective osmoles: Na⁺, glucose, mannitol (do not cross membranes easily)
• Ineffective osmoles: Urea, ethanol (cross freely, equilibrate)
• Clinical importance: Tonicity determines cell volume (brain cells swell in hypotonic states)

Regulation of Fluid Balance

Thirst Mechanism:

• Stimulus: ↑plasma osmolality (>295 mOsm/kg), ↓ECF volume
• Response: Water intake
• ADH release: Simultaneous with thirst

Antidiuretic Hormone (ADH/Vasopressin):

• Source: Posterior pituitary (synthesized in hypothalamus)
• Stimuli:
  • Osmotic: ↑plasma osmolality (very sensitive, 1% change triggers)
  • Volume: ↓ECF volume >10% (less sensitive)
• Action: Aquaporin-2 insertion in collecting duct → water reabsorption
• Result: Concentrated urine, water retention

Aldosterone:

• Source: Adrenal cortex (zona glomerulosa)
• Stimuli: ↓Na⁺, ↓ECF volume, ↑K⁺, angiotensin II
• Action: Na⁺ reabsorption, K⁺ excretion (distal tubule)
• Result: Volume expansion

Natriuretic Peptides (ANP, BNP):

• Source: Atria (ANP) and ventricles (BNP)
• Stimuli: Atrial stretch (volume overload)
• Action: Na⁺ and water excretion, vasodilation
• Result: Volume reduction

Capillary Fluid Exchange

Starling Forces:

Forces moving fluid OUT of capillary (filtration):

• Capillary hydrostatic pressure (Pc): 30-35 mmHg (arterial end), 10-15 mmHg (venous end)
• Interstitial oncotic pressure (πi): 5 mmHg (minimal protein in ISF)

Forces moving fluid INTO capillary (reabsorption):

• Interstitial hydrostatic pressure (Pi): -2 to -5 mmHg (negative due to lymphatic drainage)
• Capillary oncotic pressure (πc): 25-28 mmHg (albumin)

Net filtration pressure (arterial end): = (Pc + πi) - (Pi + πc) = (30 + 5) - (-3 + 25) = 35 - 22 = +13 mmHg (filtration)

Net reabsorption pressure (venous end): = (15 + 5) - (-3 + 25) = 20 - 22 = -2 mmHg (reabsorption)

Clinical Implications:

• Increased Pc (venous congestion): Edema (heart failure, fluid overload)
• Decreased πc (hypoalbuminemia): Edema (liver disease, nephrotic syndrome)
• Increased capillary permeability: Edema (sepsis, burns, inflammation)
• Lymphatic obstruction: Edema (filariasis, malignancy)

Anaesthetic Implications

Preoperative Fluid Management

Fasting Deficit:

• Standard calculation: Hourly maintenance × hours fasting
• Maintenance (4-2-1 rule):
  • First 10 kg: 4 mL/kg/hour
  • Next 10 kg: 2 mL/kg/hour
  • Remaining weight: 1 mL/kg/hour
• Example: 70 kg patient NPO 8 hours
  • Hourly: 40 + 20 + 50 = 110 mL/hour
  • Deficit: 110 × 8 = 880 mL
  • Replace 50% in first hour, 25% in next 2 hours

Fluid Choice for Deficit:

• Isotonic crystalloid: 0.9% saline, Hartmann's, Plasma-Lyte
• Avoid hypotonic: D5W, 0.45% saline (post-neonatal period)

Intraoperative Fluid Therapy

Goals:

1. Replace fasting deficit
2. Replace ongoing losses (blood, third space, evaporation)
3. Maintain perfusion (BP, urine output, lactate)
4. Avoid fluid overload

Types of Fluid Loss:

1. Sensible Losses:

• Blood loss: Measure (suction, sponges), replace
  • <500 mL: Crystalloid (3:1 ratio)

  • 500 mL: Consider colloid or blood

• Urine output: Monitor, maintain >0.5 mL/kg/hour
• Evaporation: From surgical field (variable)

2. Insensible Losses:

• Respiratory: Humidified gases reduce loss
• Cutaneous: Open abdomen increases loss significantly

3. Third Space Losses (Redistribution):

• Definition: Fluid shifts from intravascular to interstitial (non-functional)
• Sites: Surgical trauma, inflammation, bowel obstruction
• Estimates:
  • Minor surgery: 0-2 mL/kg/hour
  • Moderate surgery: 2-4 mL/kg/hour
  • Major surgery: 4-8 mL/kg/hour
  • Open abdomen: Up to 10-15 mL/kg/hour
• Replacement: Isotonic crystalloid

Fluid Choice:

Crystalloids:

• 0.9% Sodium Chloride (Normal Saline):
  • 154 mmol/L Na⁺, 154 mmol/L Cl⁻
  • Pros: Cheap, compatible with blood, isotonic
  • Cons: Hyperchloremic metabolic acidosis (dilutional), renal vasoconstriction
• Hartmann's (Lactated Ringer's):
  • 131 Na⁺, 111 Cl⁻, 5 K⁺, 2 Ca²⁺, 29 lactate (converts to bicarbonate)
  • Pros: Balanced, less acidosis
  • Cons: Slightly hypotonic, calcium precipitates with citrate (blood)
• Plasma-Lyte 148:
  • 140 Na⁺, 98 Cl⁻, 5 K⁺, 1.5 Mg²⁺, 23 gluconate, 27 acetate
  • Pros: Balanced, no calcium, acetate/gluconate metabolized to bicarbonate
  • Cons: More expensive

Colloids:

• Albumin (4%, 20%):
  • Pros: Expands intravascular volume effectively
  • Cons: Expensive, blood product (theoretical risks), no survival benefit shown
• Hydroxyethyl Starch (HES):
  • Avoid: Nephrotoxic, coagulopathy, mortality concern (6S, CHEST trials)
• Gelatins (Gelofusine):
  • Pros: Cheap, effective short-term plasma expansion
  • Cons: Allergic reactions, short duration

Blood Products:

• Indications:
  • Hb <70 g/L (or <80-90 g/L if cardiovascular disease)
  • Ongoing bleeding
  • Symptomatic anemia
• Transfusion ratio (massive hemorrhage): 1:1:1 (PRBC:FFP:Platelets)

Glucose Management

Perioperative Considerations:

• Stress response: Hyperglycemia (catecholamines, cortisol, insulin resistance)
• Risk of hypoglycemia: If fasting on insulin/oral hypoglycemics
• Target: 6-10 mmol/L (avoid hypoglycemia and severe hyperglycemia)

Diabetes Management:

• Type 1: Must continue basal insulin (risk of DKA)
• Type 2: Stop metformin 48 hours pre-op if using contrast or if eGFR <60
• Monitoring: Regular BSLs, insulin sliding scale if needed

Postoperative Fluid Therapy

Goals:

• Maintain euvolemia
• Replace ongoing losses (NG tube, drains, fistulas)
• Restore electrolyte balance
• Transition to oral intake

Complications to Avoid:

• Fluid overload: Peripheral edema, pulmonary edema (especially in elderly, cardiac failure)
• Fluid restriction: Acute kidney injury, hypoperfusion
• Electrolyte disturbances: Monitor K⁺, Na⁺ daily, replace as needed

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Patients

Higher Risk:

• Higher rates: Renal disease (affects fluid/electrolyte handling), diabetes (glucose management)
• Remote access: Dialysis-dependent patients may be far from centers
• Hospital presentation: Often dehydrated or fluid overloaded

Cultural Considerations:

• Kidney health: High importance in Aboriginal communities (high ESKD rates)
• Medication adherence: Complex regimens (fluid/electrolyte management in dialysis)
• Communication: Explain fluid restrictions clearly

Māori Health Considerations

Health Disparities:

• Higher rates of diabetes and renal disease
• Access to nephrology and dialysis services

Cultural Safety:

• Whānau involvement: Family support for patients on dialysis (fluid/diet restrictions)
• Communication: Clear explanation of fluid balance concepts
• Dietary support: Low-sodium, fluid-restricted diets (culturally appropriate)

ANZCA Primary Exam Focus

Key Concepts

Body Water Distribution:

• TBW: 60% (males), 50% (females)
• ICF: 40% body weight (2/3 TBW)
• ECF: 20% body weight (1/3 TBW)
  • ISF: 15%
  • Plasma: 5%

Electrolytes:

• Na⁺: 135-145 mmol/L (ECF osmolality, volume)
• K⁺: 3.5-5.0 mmol/L (resting membrane potential)
• Ca²⁺: 2.2-2.6 mmol/L total (neuromuscular, cardiac)
• Mg²⁺: 0.75-0.95 mmol/L (enzyme cofactor)

Osmolality:

• Normal: 285-295 mOsm/kg
• Formula: 2[Na⁺] + glucose/18 + urea/2.8
• Tonicity: Effective osmoles (Na⁺, glucose) vs ineffective (urea)

Fluid Therapy:

• Maintenance: 4-2-1 rule
• Fasting deficit: Calculate and replace
• Third space: 0-8 mL/kg/hour depending on surgery
• Blood loss: 3:1 crystalloid or 1:1 colloid/blood

Common Exam Questions

"Calculate the fluid deficit for a 70 kg man fasting 10 hours."

• Hourly maintenance: (10×4) + (10×2) + (50×1) = 40 + 20 + 50 = 110 mL/hour
• Deficit: 110 × 10 = 1100 mL
• Replace: 550 mL in first hour, 275 mL in hours 2 and 3

"Why is normal saline not 'normal'?"

• Na⁺ 154 mmol/L (higher than plasma 140)
• Cl⁻ 154 mmol/L (higher than plasma 100)
• Results in hyperchloremic metabolic acidosis (dilutional)
• Hartmann's or Plasma-Lyte more "physiological"

"What are the Starling forces governing capillary fluid exchange?"

• Outward: Capillary hydrostatic pressure (Pc), interstitial oncotic pressure (πi)
• Inward: Interstitial hydrostatic pressure (Pi), capillary oncotic pressure (πc)
• Net filtration = (Pc + πi) - (Pi + πc)
• Arterial end: Net filtration (+13 mmHg)
• Venous end: Net reabsorption (-2 mmHg)
• Lymphatics return excess interstitial fluid

"How does acid-base status affect potassium?"

• Acidosis: H⁺ enters cells, K⁺ exits to maintain electroneutrality → hyperkalemia
• Alkalosis: H⁺ exits cells, K⁺ enters → hypokalemia
• Clinical: Correct acidosis gradually to avoid rapid K⁺ shifts

References

1. ANZCA. Primary Examination Syllabus. Physiology Section.
2. Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. McGraw-Hill; 2001.
3. Campbell D et al. Fluid management. BJA Educ. 2015;15(3):118-123.
4. Hahn RG. Fluid therapy in clinical medicine. BJA. 2021;126(2):417-424.
5. Lobo DN et al. Perioperative fluid management. BMJ. 2011;342:d1736.
6. Miller RD et al. Miller's Anesthesia. 9th ed. Elsevier; 2020:1755-1780.
7. Grocott MPW et al. Perioperative fluid management. N Engl J Med. 2005;353(11):1077-1080.
8. ATSI Health. Chronic kidney disease in Aboriginal and Torres Strait Islander peoples. Australian Institute of Health and Welfare; 2020.