Etiological factors of sodium deficiency
A decrease in body sodium levels occurs under the influence of three key mechanisms:
- Insufficient intake of the element (less than 6–8 g/day):
- Complete fasting (intentional or forced food restriction, weight loss).
- Partial deficiency occurring when following a strictly salt-free diet.
- Increased sodium loss from the body:
- Renal excretion pathways: aldosterone deficiency, diabetes mellitus, chronic forms of nephritis, renal failure, use of diuretics, as well as excessive production of atrial natriuretic peptide and prostaglandin $E$.
- Sweat glands: profuse and prolonged sweating under high ambient temperatures.
- GI losses: chronic diarrhea and repeated bouts of vomiting. When these conditions are combined, the body loses up to 10–15% of total sodium.
- Hemodilution (plasma dilution):
- Increased thirst (polydipsia), characteristic, for example, of diabetes mellitus.
- Excessive intravenous administration of sodium-free detoxification solutions.
- Impaired renal excretory function against the background of renal failure or hypersecretion of antidiuretic hormone (ADH).
- Fluid movement from the interstitial tissue into the vascular bed (during the elimination of hypoproteinemia).
Pathogenesis and clinical presentation
The triggering mechanism for all manifestations is plasma and extracellular fluid hypo-osmolality. Osmotic disorders form a complex of symptoms:
- Water-electrolyte disorders: Water rushes from the interstitial space into the cells along the osmotic pressure gradient. This causes cellular edema (cellular hyperhydration) against the background of extracellular hypohydration. Extracellular deficiency is manifested by dryness of the mucous membranes and skin, and a decrease in its elasticity and turgor.
- Neuromuscular disorders: Low extracellular $Na^+$ levels raise the cell excitation threshold. Clinically, this manifests as muscle hypotonia and a general decrease in excitability.
- Cardiovascular syndrome: Arterial hypotension and compensatory tachycardia develop. The mechanism includes a drop in vascular smooth muscle tone, weakened myocardial contractility, and reduced cardiac output.
- Higher nervous activity disorders: These arise due to ionic imbalance, brain tissue hypoxia, and neuronal hydration. They range from psycho-asthenia to depression and altered states of consciousness.
- Dyspeptic phenomena: Nausea, vomiting, and loss of appetite are caused by edema of the nerve center cells and metabolic disruption.
- General changes: Weight loss, and with prolonged pathology — delayed physical growth and development.
Adaptive bodily responses
To compensate for sodium deficiency, the body engages the following endocrine and renal mechanisms:
- Stimulation of the aldosterone system: Aldosterone secretion increases, leading to enhanced tubular reabsorption of sodium in the kidneys.
- Suppression of natriuretics: The production of and sensitivity to atriopeptin and renal prostaglandins, which normally block $Na^+$ reabsorption, are inhibited and decreased.
- Activation of ADH synthesis: With concomitant hypovolemia, vasopressin secretion is stimulated to retain water and preserve circulating blood volume.
Principles of therapeutic correction
Etiotropic therapy aimed at eliminating the underlying cause of hyponatremia is of primary importance.
To restore electrolyte balance, the following are used:
- Intravenous infusion: administration of a calculated volume of 1–2% sodium chloride solution.
- Parenteral administration of medications: infusions of blood plasma, plasma-substituting, and protein-containing solutions.
Briefly: answers to topical questions
How is hyponatremia classified by severity (mild, moderate, severe)?
Hyponatremia is classified by serum sodium levels as follows:
- Mild — Na 126–135 mmol/L.
- Moderate — Na 120–125 mmol/L.
- Severe — Na ≤120 mmol/L.
Remember the key chain of hyponatremia: «Low sodium → Water enters the cell → Neuronal edema and vascular hypotension».
Class and exam questions
What key pathogenetic link determines the symptoms of hyponatremia?
The primary factor is a decrease in the osmolality of blood and extracellular fluid (hypoosmia). This causes water to shift from the interstitium into the cells, resulting in dangerous cellular hyperhydration (cellular edema).
Why do blood pressure and muscle tone drop in sodium deficiency?
A decrease in extracellular $Na^+$ levels raises the myocyte excitation threshold (provoking muscle hypotonia), and also decreases vascular smooth muscle tone and myocardial contractility, leading to arterial hypotension.
How do the kidneys and endocrine glands compensate for hyponatremia?
The body activates aldosterone synthesis to retain sodium in the renal tubules, suppresses natriuretic factors (atriopeptin, prostaglandins), and engages ADH during hypovolemia to preserve circulating blood volume.
What else to study on this topic
- Mechanisms of cerebral cellular edema development in hypo-osmolality
- The role of atrial natriuretic peptide and prostaglandins in the regulation of natriuresis
- Pathophysiology of water-electrolyte disorders in absolute and relative hyperhydration
- Calculation of 1–2% NaCl solution volume during parenteral correction of hyponatremia
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