Electrolytes: Sodium, Potassium, and Complete Panel – Physiology, Clinical Significance, and Laboratory Evaluation

Electrolytes are charged low-molecular-weight molecules present in plasma and cytosol. They are essential for maintaining osmotic pressure, acid-base balance, neuromuscular function, and cellular metabolism. In adults, approximately 60% of body weight is water, containing these electrolytes.

Major electrolytes include:

• Cations: Sodium (Na⁺), Potassium (K⁺), Calcium (Ca²⁺), Magnesium (Mg²⁺)
• Anions: Chloride (Cl⁻), Bicarbonate (HCO₃⁻), Phosphate (PO₄³⁻), Sulfate (SO₄²⁻)
• Organic anions: Lactate, trace elements

Electrolytes are divided into:

• Cations: positively charged, move toward the cathode
• Anions: negatively charged, move toward the anode

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Sample Collection and Handling

Blood: Venous blood is preferred; arterial blood may be used for certain parameters.
Plasma or Serum: Serum is commonly used; plasma is acceptable but may show slightly lower K⁺ values. Separate serum/plasma as soon as possible.
Urine: 24-hour urine collection without preservatives; fractional collection may be used.
Other Samples: Sweat, feces, gastrointestinal fluids.

Precautions:

• Avoid hemolysis, which can falsely elevate potassium.
• Avoid prolonged tourniquet use or repeated fist clenching.
• Lipemic serum must be ultracentrifuged before sodium measurement.
• EDTA tubes should not be used for potassium (contain K⁺).

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Potassium (K⁺)

Physiology

Potassium is the primary intracellular cation, with approximately 90% inside cells and <2% in extracellular fluid. This gradient is critical for membrane potential and neuromuscular conduction.

Daily intake: 40–150 meq/day (average 1.5 meq/kg body weight)
Concentrations:

• Intracellular: 150 meq/L
• Blood: ~4 meq/L

Renal Handling

• 80–90% excreted by kidneys (glomerular filtration, proximal and distal reabsorption)
• 10–20% excreted in sweat and stool
• Aldosterone stimulates renal K⁺ excretion

Functions

• Nerve conduction
• Skeletal and cardiac muscle contraction
• Acid-base balance
• Enzyme reactions in carbohydrate and protein metabolism

Pathophysiology

Hypokalemia:

• Causes: Diuretics, vomiting, diarrhea, hyperaldosteronism
• ECG changes: Flattened T-wave, prominent U-wave, ST depression

Hyperkalemia:

• Causes: Renal failure, tissue damage, hemolysis, metabolic acidosis
• ECG changes: Peaked T-wave, flattened P-wave, prolonged PR/QRS

Potassium Shifts:

• Acidemia: K⁺ moves out of cells → hyperkalemia
• Alkalemia: K⁺ moves into cells → hypokalemia

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Sodium (Na⁺)

Physiology

Sodium is the major extracellular cation, crucial for plasma osmolality, fluid balance, and electric neutrality.
Distribution:

• Extracellular: ~140 meq/L
• Intracellular: ~5 meq/L

Renal Handling

• 100% filtered at glomerulus
• 70–80% reabsorbed in proximal tubules with water and chloride
• 20–25% reabsorbed in loop of Henle
• Distal nephron and collecting ducts regulate Na⁺ based on aldosterone and ADH

Active Transport

• Na⁺/K⁺ ATPase pump: 3 Na⁺ out, 2 K⁺ in → maintains negative intracellular charge

Functions

• Maintains extracellular osmolality and volume
• Nerve impulse transmission
• Muscle contraction with K⁺ and Ca²⁺
• Acid-base balance via sodium bicarbonate and phosphate

Clinical Significance

Hyponatremia (<135 meq/L):

• Symptoms: Weakness, confusion, stupor, coma
• Causes: Low dietary intake, diuretics, renal insufficiency, excess water intake

Hypernatremia (>145 meq/L):

• Symptoms: Thirst, agitation, CNS disturbances, heart failure
• Causes: Dehydration, excessive Na⁺ intake, hormone imbalances (aldosterone, ADH)

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Electrolytes Panel

Definition: A set of tests measuring Na⁺, K⁺, Cl⁻, HCO₃⁻, CO₂, Ca²⁺, Mg²⁺, phosphate.

Functions:

• Maintain osmotic pressure
• Regulate heart rhythm and muscle contraction
• Support brain function and energy production
• Maintain acid-base balance and prevent dehydration

Critical Values (Examples):

Electrolyte	Low Value	High Value
K⁺	<2.5 meq/L → Ventricular fibrillation	>8.0 meq/L → Muscle & myocardial irritability
Na⁺	<125 meq/L → Neurological symptoms	>160 meq/L → Heart failure
Ca²⁺	<4.4 mg/dL → Tetany	>13 mg/dL → Cardiotoxicity, coma
Mg²⁺	<1.2 mg/dL → Tetany	30–40 mg/dL → Cardiac arrest
Cl⁻	<70 meq/L	>120 meq/L
PO₄³⁻	<1.0 mg/dL	–
CO₂ / HCO₃⁻	<10 meq/L	>40 meq/L

Lab Errors and Pseudodisorders:

• Pseudo-hyperkalemia: Hemolysis, thrombocytosis
• Pseudo-hyponatremia: Hyperlipidemia, hyperproteinemia

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Summary / Quick Reference

• Electrolytes are essential for cell function, nerve and muscle activity, cardiac rhythm, osmotic balance, and acid-base homeostasis.
• Sodium dominates the ECF, Potassium dominates the ICF.
• Electrolyte disturbances manifest as neurological, muscular, or cardiac complications.
• Regular monitoring via an electrolyte panel is crucial in critically ill patients, those on diuretics, or patients with renal or endocrine disorders.

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Possible References Used

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