SODIUM (Na*)
Type: Cation (positive ion) Normal ECF concentration: 135-145 mEq/L Primary location: Extracellular fluid (major cation) Functions: • Maintains extracellular osmotic pressure and volume • Essential for nerve impulse transmission and muscle contraction • Closely linked with water balance and chloride levels Clinical notes: • 1 (Hyponatremia): fluid overload, diuretics, Addison’s disease • T (Hypernatremia): dehydration, excessive salt intake, diabetes insipidus
POTASSIUM (K*)
Type: Cation
Normal ECF concentration: 3.5-5.5 mEq/L
Primary location: Intracellular fluid (major cation), small but vital extracellular amount
Functions:
• Maintains resting membrane potential
• Crucial for cardiac rhythm, muscle function, and nerve signaling
• Inverse relationship with hydrogen ions (acid-base balance)
Clinical notes:
• 1 (Hypokalemia): vomiting, diuretics, alkalosis
• 1 (Hyperkalemia): renal failure, acidosis, ACE inhibitors
CALCIUM (Ca?*)
Lype: Cation
Normal ECF concentration: 4.5-5.5 mEg/L (= 2.2-2.6 mmol/L)
Primary location: Mostly in bone; small fraction in plasma and interstitia
Functions:
• Muscle contraction, neurotransmitter release, blood clotting
• Stabilizes cell membranes
Clinical notes:
• 1 (Hypocalcemia): hypoparathyroidism, vitamin D deficiency
1 (Hypercalcemia): hyperparathyroidism, malignancy
CHLORIDE (CI)
Type: Anton (negative ion) Normal ECF concentration: 95-105 mEg/L Primary location: Main extracellular anion Functions: • Maintains osmotic pressure, electrical neutrality (balances Na* • Involved in acid-base balance (with HCO,~ exchange) Clinical notes: • 1 (Hypochloremia): vomiting, metabolic alkalosis • † (Hyperchloremia): dehydration, metabolic acidosis
MAGNESIUM (Mg²⁺)
Type: Cation
Normal ECF concentration: 1.5–2.5 mEq/L
Primary location: Mainly intracellular; small ECF fraction
Functions:
Enzyme cofactor (especially for ATP-related reactions)
Affects neuromuscular excitability and cardiac conduction
Clinical notes:
↓ (Hypomagnesemia): malnutrition, alcoholism, diuretics
↑ (Hypermagnesemia): renal failure, excess supplementation
BICARBONATE (HCO₃⁻)
Type: Anion
Normal ECF concentration: 22–28 mEq/L
Primary location: Extracellular fluid, blood plasma
Functions:
Major buffer maintaining blood pH (~7.4)
Works with carbonic acid (H₂CO₃) system
Clinical notes:
↓ (Metabolic acidosis): diarrhea, renal failure
↑ (Metabolic alkalosis): vomiting, excess antacids
PHOSPHATE (HPO₄²⁻ / PO₄³⁻)
Type: Anion
Normal ECF concentration: 1.8–2.6 mEq/L
Primary location: Mostly intracellular; small extracellular portion
Functions:
Component of ATP, DNA, RNA, and phospholipids
Buffers acids in urine and intracellular fluid
Works with calcium in bone structure
Clinical notes:
↓ (Hypophosphatemia): malnutrition, alcoholism, DKA treatment
↑ (Hyperphosphatemia): renal failure, tissue breakdown
Core functions of electrolytes
Fluid Balance (Osmotic Regulation)
Sodium (Na⁺) and chloride (Cl⁻) control how water moves in and out of cells.
Keeps blood volume and blood pressure stable.
Electrical Activity (Nerve & Muscle Function)
Potassium (K⁺), sodium (Na⁺), and calcium (Ca²⁺) create the electrical impulses needed for nerve transmission and muscle contraction, including the heart.
Acid–Base (pH) Balance
Bicarbonate (HCO₃⁻) and phosphate (HPO₄²⁻) buffer acids and maintain blood pH around 7.4.
Enzyme and Metabolic Support
Magnesium (Mg²⁺), calcium (Ca²⁺), and phosphate (PO₄³⁻) act as cofactors for enzymes in metabolism, ATP production, and cell signaling.
Cell Integrity and Transport
The sodium–potassium pump (Na⁺/K⁺ ATPase) maintains concentration gradients that are essential for nutrient uptake and waste removal.
