General principles of electrolyte balance
Electrolytes are charged ions that provide osmotic pressure, acid-base balance, nerve conduction, muscle contraction, and enzyme and hormone activity. The constancy of their concentrations is termed isoinion and is maintained primarily by renal function, the gastrointestinal tract, and hormonal regulation (aldosterone, parathyroid hormone, vitamin D, insulin).
Sodium metabolism
Sodium determines extracellular fluid osmolality, nerve conduction, muscle contraction, and transmembrane transport. Reabsorption in the collecting ducts and the wall of the colon is regulated by aldosterone. Normal plasma level is 135–145 mmol/L; daily requirement is 1–1.5 mmol/kg.
- Hypernatremia (>145 mmol/L): relative sodium excess with water deficit, decreased GFR, secondary hyperaldosteronism. Clinical features: edema, increased blood volume, arterial hypertension, thirst, dry mucous membranes, hyperreflexia, altered consciousness, seizures, coma.
- Hyponatremia (<135 mmol/L): impaired reabsorption, diuretics, losses from diarrhea, sweating, peritonitis. Clinical features: headache, nausea, drowsiness, confusion, neuromuscular hyperexcitability, seizures, coma.
Potassium metabolism
Potassium maintains the membrane potential via Na⁺/K⁺-ATPase and selective potassium channels, ensuring nerve conduction and myocardial contraction. About 70% of filtered potassium is reabsorbed in the kidneys; insulin and adrenaline enhance potassium entry into muscle cells. Normal plasma level is 3.5–5.5 mmol/L; the bulk is intracellular. Sources: meat, fruits, vegetables.
- Hyperkalemia (>5 mmol/L): renal failure, hypoaldosteronism, hypocorticism, acidosis, insulin deficiency, excessive administration. Clinical features: muscle weakness, paresis, diarrhea, cramps. On ECG: tall peaked T waves, PQ prolongation, sinoatrial block, bradyarrhythmias. At K⁺ >7.5 mmol/L — ventricular tachycardia, fibrillation, asystole.
- Hypokalemia (<3.5 mmol/L): inadequate intake, hyperaldosteronism, glucocorticoids, vomiting, diarrhea, diuretics, alkalosis, insulin administration. Clinical features: fatigue, muscle weakness, hyporeflexia, constipation, paralytic ileus, increased myocardial sensitivity to digoxin. ECG: QT prolongation, T wave flattening, AV blocks, tachyarrhythmias. At K⁺ <1.5 mmol/L, cardiac arrest is possible.
Magnesium and chloride metabolism
Magnesium is an intracellular cofactor for numerous enzymes, participates in neuromuscular transmission, and acts similarly to calcium. It is absorbed in the intestine (up to 35%) and reabsorbed in the kidneys; absorption is enhanced by the active form of vitamin D, and insulin stimulates cellular uptake. Alcohol, mineralocorticoids, intravenous administration of calcium and glucose reduce reabsorption. 60% of magnesium is in the bones.
- Hypermagnesemia: vitamin D hypervitaminosis, renal and adrenal insufficiency, magnesium-containing drugs. Clinical features: decreased tendon reflexes, hypotonia, nausea, skin flushing, drowsiness; at >5 mmol/L — respiratory depression, arrhythmias; at >7.5 mmol/L — cardiac arrest.
- Hypomagnesemia (<0.7 mmol/L): protein-energy malnutrition, malabsorption, tubulopathies, diuretics, hyperaldosteronism, alcoholism. Clinical features: tremor, lower limb cramps, ataxia, insomnia, depression, hallucinations, tachycardia, extrasystoles — typical for chronic alcoholism.
Chloride is involved in the formation of gastric hydrochloric acid, maintenance of osmotic pressure, and acid-base balance. Normal plasma level is 99–105 mmol/L. Hypochloremia (vomiting, sweating, diuretics) manifests as muscle weakness and cramps; at <20 mmol/L — severe metabolic alkalosis. Hyperchloremia in dehydration, adrenal insufficiency, and diarrhea is accompanied by weakness, nausea, and metabolic acidosis.
Calcium metabolism
Calcium ensures muscle contraction, nerve conduction, acts as coagulation factor IV, and participates in apoptosis, calcification, and bone tissue formation. 99% is contained in bones and teeth. Normal plasma level is 2.2–2.5 mmol/L; the biologically active form is the ionized fraction (increases in acidosis, decreases in alkalosis). Regulators: parathyroid hormone, calcitriol (vitamin D), calcitonin.
- Hypocalcemia: hypoparathyroidism, active vitamin D deficiency, renal and hepatic failure, enteritis, pancreatitis, citrated blood transfusion, loop diuretics, glucocorticoids, alkalosis, hyperphosphatemia, hypomagnesemia. Clinical features: hyperreflexia, tetany, laryngo- and bronchospasm, paresthesias, arrhythmias; chronic form leads to osteomalacia, osteoporosis, fractures.
- Hypercalcemia: hyperparathyroidism, vitamin D hypervitaminosis, malignant neoplasms, prolonged immobilization, acidosis. Clinical features: nausea, vomiting, anorexia, constipation, muscle hypotonia, arrhythmias, depression. Development of nephropathy with decreased GFR, nephrocalcinosis, urolithiasis, vascular and corneal calcification; osteoclasts are paradoxically activated with a risk of fractures.
Phosphate metabolism
Phosphates are part of the structure of bones, nucleic acids, membrane phospholipids, and high-energy ATP bonds. 85% is in bones and teeth. Plasma inorganic phosphate concentration is 0.9–1.5 mmol/L, intracellular is 65–100 mmol/L. Regulation: vitamin D and to a lesser extent parathyroid hormone.
- Hypophosphatemia: chronic alcoholism, starvation, malabsorption, antacids, vitamin D deficiency, tubulopathies, hypophosphatemic rickets. Clinical features: muscle weakness, respiratory failure, hemolysis, decreased ATP synthesis, fingertips paresthesias, seizures, depression.
- Hyperphosphatemia: vitamin D intoxication, hypoparathyroidism, renal failure, acute intestinal obstruction, fractures, bone neoplasms. Clinical features: manifestations of renal failure, arrhythmias, corneal opacity, anorexia, nausea, tetany, hyperreflexia.
Key points
- Sodium determines extracellular fluid osmolality; its metabolism is regulated by aldosterone, normal level is 135–145 mmol/L.
- Potassium is predominantly an intracellular ion; extreme values are dangerous due to fatal arrhythmias up to fibrillation and asystole.
- ECG signs of hyperkalemia include tall T waves and PQ prolongation; of hypocalcemia/hypokalemia — QT prolongation and T wave flattening.
- Magnesium imbalance frequently accompanies chronic alcoholism and manifests as neuromuscular excitability.
- Only the ionized fraction of calcium is biologically active, and its level depends on blood pH.
- Hypercalcemia leads to nephropathy, nephrocalcinosis, and ectopic calcification of vessels and the cornea.
- Phosphates ensure energy metabolism via high-energy ATP bonds; their metabolism is regulated by vitamin D.
- Electrolyte disorders require a comprehensive assessment taking into account acid-base balance, renal function, and concomitant therapy (diuretics, insulin, glucocorticoids).