Introduction
Electrolytes play a vital role in maintaining the body’s internal environment. They control water distribution, osmotic balance, acid–base equilibrium, and neuromuscular activity. Accurate assessment of electrolytes in body fluids is crucial for diagnosis and management of several clinical conditions.
1. Body Water and Fluid Compartments
Water constitutes about 60% of total body weight in an average adult (≈42 L in a 70 kg individual). The percentage varies depending on age, sex, and fat content — lean individuals have more water than obese persons.
Major Fluid Compartments
- Intracellular Fluid (ICF): Fluid within cells, about 40% of body weight or two-thirds of total body water.
- Extracellular Fluid (ECF): Fluid outside cells, about 20% of body weight. It includes:
- Plasma: About 5% of body weight (≈3.5 L in adults).
- Interstitial Fluid: About 15% of body weight (≈10.5 L in adults).
Other minor compartments include cerebrospinal, synovial, pleural, and peritoneal fluids.
2. Water Balance
Water balance is maintained when water intake equals water loss. In healthy adults, about 2500 mL of water enters and leaves the body each day.
| Route of Water Loss | Average Volume (mL/day) |
|---|---|
| Urine | 1500 |
| Sweat (skin) | 500 |
| Expired air | 350 |
| Feces | 150 |
| Total | ≈2500 mL |
Water balance is regulated through the thirst mechanism and the antidiuretic hormone (ADH), which adjusts water reabsorption by the kidneys. Losses exceeding intake cause dehydration, whereas excessive intake or retention leads to edema.
3. Electrolyte Distribution
Electrolytes are minerals in body fluids that carry electric charges and are essential for normal function of cells and organs.
Distribution of Major Electrolytes
| Ion | Extracellular Fluid (mmol/L) | Intracellular Fluid (mmol/L) |
|---|---|---|
| Sodium (Na⁺) | 154 | 15 |
| Potassium (K⁺) | 5 | 150 |
| Chloride (Cl⁻) | 111 | 7 |
| Bicarbonate (HCO₃⁻) | 29 | 10 |
| Phosphate (HPO₄²⁻) | 1 | 135 |
The unequal distribution of Na⁺ and K⁺ between compartments is maintained by the sodium–potassium pump (Na⁺/K⁺-ATPase), which actively exchanges Na⁺ for K⁺ across the cell membrane.
4. Sodium (Na⁺)
Sodium is the major extracellular cation and a key determinant of extracellular fluid volume and osmolality.
- Normal Serum Level: 136–145 mmol/L
- Functions:
- Maintains extracellular osmolality and fluid balance.
- Regulates blood pressure and nerve impulse transmission.
- Clinical Significance:
- Hypernatremia (↑ Na⁺): Due to dehydration, Cushing’s syndrome, diabetes insipidus, or excess sodium intake.
- Hyponatremia (↓ Na⁺): Caused by vomiting, diarrhea, renal failure, or Addison’s disease.
5. Potassium (K⁺)
Potassium is the main intracellular cation, essential for cell membrane potential, muscle contraction, and cardiac function.
- Normal Serum Level: 3.5–5.5 mmol/L
- Functions:
- Maintains intracellular osmolality.
- Regulates neuromuscular activity and acid–base balance.
- Clinical Significance:
- Hyperkalemia (↑ K⁺): Seen in renal failure, acidosis, and tissue breakdown.
- Hypokalemia (↓ K⁺): Occurs with vomiting, diarrhea, diuretics, or Cushing’s syndrome.
6. Chloride (Cl⁻)
Chloride is the chief extracellular anion and maintains electrical neutrality with sodium. It helps regulate osmotic pressure and acid–base balance.
- Normal Serum Level: 98–108 mmol/L
- Clinical Significance:
- Hyperchloremia: Occurs in dehydration, metabolic acidosis, or renal tubular acidosis.
- Hypochloremia: Seen in vomiting, Addison’s disease, and metabolic alkalosis.
Anion Gap: Anion Gap = Na⁺ – (Cl⁻ + HCO₃⁻) (Normal: 8–12 mmol/L). Increased values indicate metabolic acidosis due to accumulation of unmeasured anions (lactate, ketoacids, etc.).
7. Calcium (Ca²⁺)
Calcium is a divalent cation important for bone formation, neuromuscular transmission, and blood coagulation.
- Normal Serum Level: 8.5–10.5 mg/dL (total calcium)
- Functions:
- Structural component of bones and teeth.
- Activates enzymes in coagulation and muscle contraction.
- Clinical Significance:
- Hypercalcemia: Seen in hyperparathyroidism, malignancy, or vitamin D toxicity.
- Hypocalcemia: Caused by hypoparathyroidism, renal failure, or vitamin D deficiency.
8. Bicarbonate (HCO₃⁻)
Bicarbonate is a major plasma buffer that regulates blood pH along with carbonic acid. It represents the metabolic component of acid–base balance.
- Normal Serum Level: 22–28 mmol/L
- Functions:
- Maintains acid–base balance by buffering hydrogen ions.
- Part of the bicarbonate–carbonic acid buffer system.
- Clinical Significance:
- Metabolic Acidosis: Decrease in bicarbonate concentration.
- Metabolic Alkalosis: Increase in bicarbonate concentration.
9. Laboratory Determination of Electrolytes
(a) Ion-Selective Electrodes (ISE)
Modern analyzers use specific electrodes for each ion. The potential generated is proportional to the ion concentration.
- Na⁺: Glass membrane electrode
- K⁺: Valinomycin-coated membrane
- Cl⁻: Silver–silver chloride electrode
(b) Flame Photometry
Used for simultaneous estimation of sodium and potassium. The intensity of emitted light is directly proportional to ion concentration.
(c) Coulometric/Amperometric Method
Used for chloride estimation; silver ions are generated electrochemically to precipitate chloride as AgCl.
(d) Titrimetric or Enzymatic Methods
Bicarbonate and calcium can also be determined using enzymatic or complexometric titration (EDTA) methods.
Summary Table: Normal Electrolyte Values
| Electrolyte | Normal Range | Main Compartment | Physiological Role |
|---|---|---|---|
| Sodium (Na⁺) | 136–145 mmol/L | ECF | Regulates water balance, ECF osmolality |
| Potassium (K⁺) | 3.5–5.5 mmol/L | ICF | Maintains cell excitability, cardiac rhythm |
| Chloride (Cl⁻) | 98–108 mmol/L | ECF | Maintains acid–base balance |
| Bicarbonate (HCO₃⁻) | 22–28 mmol/L | ECF | Buffer in acid–base balance |
| Calcium (Ca²⁺) | 8.5–10.5 mg/dL | Both | Bone formation, muscle contraction |
Detailed Notes:
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