Sodium Physiology

Related Subjects: |Water Physiology |Sodium Physiology |Potassium Physiology |Aldosterone Physiology |Atrial Natriuretic Peptide (ANP) |Brain Natriuretic Peptide (BNP)

🧂 Sodium (Na⁺) is the dominant extracellular cation and the single most important determinant of extracellular fluid (ECF) volume. A key “exam + clinical” principle is: total body sodium controls volume, while water balance controls sodium concentration. So most hyponatraemia is fundamentally a water excess (ADH problem), whereas most hypernatraemia is a water deficit.

📍 Distribution and why it matters

• Where sodium lives
  • ECF (plasma + interstitial fluid): ~135–145 mmol/L (normal lab reference range).
  • ICF: ~10–15 mmol/L.
  • This steep gradient is essential for membrane potentials, nerve conduction, and coupled transport.
• How the gradient is maintained: Na⁺/K⁺-ATPase
  • Pumps 3 Na⁺ out and 2 K⁺ in per ATP → maintains low intracellular Na⁺ and contributes to a negative resting membrane potential.
  • Creates the “stored energy” used by secondary active transport (e.g. glucose uptake in the gut, renal reabsorption).

🧠 Sodium, osmolality, and brain adaptation (high-yield)

• Tonicity vs osmolality
  • Osmolality = total dissolved particles per kg water (includes urea).
  • Tonicity (effective osmolality) = particles that do not freely cross cell membranes and therefore shift water (mainly Na⁺ and glucose).
  • That’s why hyperglycaemia can cause “dilutional” hyponatraemia (water shifts out of cells).
• Why rapid Na correction is dangerous
  • In chronic hyponatraemia, brain cells adapt by expelling osmoles to reduce cerebral oedema.
  • If you raise serum Na⁺ too fast, water rapidly leaves brain cells → risk of osmotic demyelination syndrome (ODS).
  • In acute hyponatraemia, there’s less time to adapt → cerebral oedema and seizures are the main threat.

⚙️ Key physiological functions of sodium

• ECF volume and blood pressure
  • Sodium content of ECF drives water retention → determines circulating volume and therefore perfusion pressure.
  • Chronic sodium excess contributes to hypertension via volume expansion and vascular remodelling.
• Electrical excitability (nerve and muscle)
  • Fast Na⁺ influx through voltage-gated channels generates the upstroke of action potentials in neurons and skeletal muscle.
  • In cardiac tissue, Na⁺ is crucial for phase 0 depolarisation (esp. atrial/ventricular myocytes and Purkinje fibres).
• Secondary active transport
  • Gut: SGLT1 co-transports Na⁺ + glucose (basis of oral rehydration therapy).
  • Kidney: Na⁺ gradients power multiple solute transporters along the nephron.
• Acid–base physiology (advanced but useful)
  • Na⁺ is the major “strong cation”; changes in accompanying anions (especially chloride) influence acid–base balance.
  • Large volumes of 0.9% saline (high chloride) can contribute to hyperchloraemic metabolic acidosis, hence interest in balanced crystalloids in some settings.

🧪 Renal handling of sodium (where it’s reabsorbed + key transporters)

• Filtered load
  • Na⁺ is freely filtered at the glomerulus; the kidney then “chooses” how much to reclaim.
  • Small changes in fractional excretion make big differences in volume status.
• Proximal tubule (~65% reabsorbed)
  • Major transporter: NHE3 (Na⁺/H⁺ exchanger) + Na⁺ co-transport with glucose, amino acids, phosphate.
  • “Iso-osmotic reabsorption”: water follows solute → preserves tonicity.
• Thick ascending limb (~25%)
  • Major transporter: NKCC2 (Na⁺-K⁺-2Cl⁻ cotransporter).
  • Water-impermeable → helps create the medullary gradient (countercurrent multiplication).
  • Loop diuretics act here.
• Distal convoluted tubule (~5%)
  • Major transporter: NCC (Na⁺-Cl⁻ cotransporter).
  • Thiazides act here and are a classic cause of hyponatraemia.
• Collecting duct (~2–3% but “fine-tuning”)
  • Major channel: ENaC (epithelial Na⁺ channel), stimulated by aldosterone.
  • Coupled to K⁺ and H⁺ secretion → explains hyperkalaemia and acidosis risk in hypoaldosteronism.
  • Amiloride blocks ENaC; spironolactone/eplerenone block mineralocorticoid receptor.

🧠 Hormonal control: sodium balance vs sodium concentration

• RAAS (renin–angiotensin–aldosterone system)
  • Triggered by low effective arterial blood volume (EABV): renal hypoperfusion, low NaCl delivery to macula densa, sympathetic activation.
  • Angiotensin II: vasoconstriction + increases proximal Na⁺ reabsorption + stimulates aldosterone and thirst.
  • Aldosterone: increases ENaC and Na⁺/K⁺-ATPase activity → Na⁺ retention (with K⁺/H⁺ loss).
• Natriuretic peptides (ANP/BNP)
  • Released with atrial/ventricular stretch → promote natriuresis and vasodilation, suppress renin/aldosterone.
• ADH (vasopressin): mainly water, not sodium
  • Increases water reabsorption in collecting duct via aquaporin-2 insertion.
  • Therefore ADH changes serum Na⁺ concentration by changing water balance.
• Thirst
  • Osmoreceptors in hypothalamus respond to small rises in tonicity → drives water intake.
  • In frail older adults, impaired thirst/access to water is a common hypernatraemia mechanism.

🏥 Clinical approach to abnormal sodium (the framework that wins exams)

1) Hyponatraemia (Na⁺ <135)

• Step 1: check osmolality
  • Hypotonic hyponatraemia (most common): true water excess.
  • Isotonic: pseudohyponatraemia (very high lipids/protein; lab artifact with some methods).
  • Hypertonic: translocational hyponatraemia (e.g., hyperglycaemia, mannitol).
• Step 2: assess volume status clinically
  • Hypovolaemic: GI losses, diuretics, adrenal insufficiency, renal salt wasting.
  • Euvolaemic: SIADH, hypothyroidism, glucocorticoid deficiency, primary polydipsia.
  • Hypervolaemic: heart failure, cirrhosis, nephrotic syndrome (low EABV → high ADH despite oedema).
• Step 3: use urine studies to “read ADH and aldosterone”
  • Urine osmolality:
    • <100 mOsm/kg suggests suppressed ADH (e.g., primary polydipsia, low solute intake).
    • >100 mOsm/kg suggests ADH is active (common in SIADH, hypovolaemia, heart failure).
  • Urine sodium (often with a threshold around 30 mmol/L):
    • Low urine Na⁺ suggests kidneys avidly retaining sodium (low EABV states).
    • Higher urine Na⁺ suggests renal sodium loss or SIADH pattern.
• Symptoms (brain swelling = the danger)
  • Mild: nausea, headache, gait instability.
  • Severe: confusion, seizures, coma (treat as emergency).

Emergency hyponatraemia (what you actually do)

• If seizures or severe neurological symptoms: give hypertonic saline in controlled boluses with frequent Na⁺ checks.
• Targets are modest early rises (e.g., ~4–6 mmol/L) to relieve cerebral oedema, then slow correction to avoid ODS.
• Do not “normalise” sodium quickly - you are treating brain oedema first, not the lab number.

2) Hypernatraemia (Na⁺ >145)

• Core concept: almost always water deficit relative to sodium.
• Common causes
  • Reduced intake: impaired thirst, reduced access to water, delirium.
  • Excess losses: diarrhoea, sweating, burns, osmotic diuresis (e.g., hyperglycaemia).
  • Diabetes insipidus: inadequate ADH (central) or renal resistance (nephrogenic).
• Treatment principle
  • If shocked/hypovolaemic: resuscitate first (isotonic fluid), then replace free water.
  • Correct gradually if chronicity uncertain to reduce cerebral oedema risk.

💉 Sodium content of common IV fluids (practical prescribing)

• 0.9% sodium chloride: ~154 mmol/L sodium (and 154 mmol/L chloride).
• 3% sodium chloride: very concentrated hypertonic saline (~513 mmol/L sodium).
• In hyponatraemia, giving isotonic saline may raise Na⁺ very little directly, but can stop hypovolaemia-driven ADH → brisk water diuresis and a faster-than-expected Na rise (watch for overcorrection).

📝 Summary

Sodium is the main extracellular cation and the key determinant of extracellular volume and perfusion. The kidneys fine-tune sodium via segment-specific transporters (PT → TAL → DCT → collecting duct) under RAAS and natriuretic peptide control, while ADH and thirst primarily regulate water and therefore sodium concentration. Clinically, think in frameworks: hyponatraemia = check tonicity → assess volume → urine osm/Na; hypernatraemia = water deficit. In emergencies, treat the brain first (seizures/cerebral oedema) and correct sodium slowly enough to avoid iatrogenic neurological injury.

🔗 Useful UK-friendly links: | NICE CKS: Hyponatraemia | NICE CG174: IV fluids in adults | Society for Endocrinology (2022): emergency hyponatraemia