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SEPT 10 Built from the handwritten pages you uploaded on 10 September 2026.

Sodium & fluid compartments

The 42-litre man, the water deficit calculation, and SIADH versus cerebral salt wasting.

Body fluid compartments — the 70 kg adult

TOTAL BODY WATER ≈ 42 L (60% of body weight) INTRACELLULAR ≈ 28 L (⅔) EXTRACELLULAR ≈ 14 L (⅓) PLASMA 3 L (¼) EXTRAVASCULAR 11 L (¾) Interstitial ≈ 10.5 L · Transcellular ≈ 0.5 L The rule of thirds and quarters TBW is ⅔ intracellular. Of the ⅓ that is extracellular, ¼ is plasma and ¾ is outside the vessels. Blood volume ≈ 70 ml/kg (men) · 65 (women) · 80–90 (neonates). TBW is ~75% of body weight in neonates, and lower in the elderly and the obese.

Where infused fluid goes

  • 5% dextrose — the glucose is metabolised, leaving free water that distributes across all 42 L. Only about 1/14 stays intravascular. Useless for resuscitation.
  • 0.9% saline / Hartmann's — distributes through the extracellular 14 L, so about a quarter stays intravascular.
  • Colloid — in theory stays intravascular, but a damaged glycocalyx in sepsis lets it leak.

Water deficit

  • Water deficit (L) = 0.6 × weight (kg) × [ (measured Na⁺ / normal Na⁺) − 1 ]
  • Use 0.6 for men, 0.5 for women and the elderly, 0.45 for elderly women. Normal Na⁺ is taken as 140.
  • Worked example: a 70 kg man with Na⁺ 160 → 0.6 × 70 × (160/140 − 1) = 42 × 0.143 = 6 L deficit.
  • Replace slowly. Correct hypernatraemia at no more than about 10 mmol/L per 24 hours, or the brain — which has adapted by generating idiogenic osmoles — will swell.
  • Remember this is the free water deficit only; add ongoing losses and any separate volume deficit on top.

SIADH vs cerebral salt wasting

Both present after neurosurgery, subarachnoid haemorrhage or head injury with hyponatraemia, urine osmolality > 100 and serum osmolality < 280. The biochemistry alone will not separate them.

SIADHCerebral salt wasting
Volume statusNormovolaemicDehydrated / hypovolaemic
Urine sodium> 30 mmol/L>> 30 mmol/L — markedly raised
Urine outputLow or normalHIGH
Urine osmolality> serum osmolality> serum osmolality
24 h urinary Na⁺NormalRaised
CVP / haematocrit / ureaNormal or lowLow CVP, raised haematocrit and urea
Primary problemWater retentionSalt loss (natriuretic peptides)
TreatmentFluid restriction · demeclocycline · vaptansSalt and volume replacement · fludrocortisone

Why the distinction matters so much

  • The treatments are opposites. Fluid restricting a salt-wasting patient after subarachnoid haemorrhage worsens hypovolaemia and precipitates cerebral vasospasm and infarction.
  • Volume status is the discriminator — assess it clinically and with CVP, haematocrit, urea and urine output, not from the sodium alone.
  • Urine output is the most useful single bedside clue: high in CSW, low or normal in SIADH.

Treating symptomatic hyponatraemia

  • Seizures or a falling GCS demand hypertonic saline, whatever the cause. 3% NaCl, aiming to raise sodium by 4–6 mmol/L to stop the seizure — not to normalise it.
  • 3% NaCl at about 100 ml/h, or a correction of 6–8 mmol/L in 24 hours where symptoms are milder.
  • Do not exceed 8–10 mmol/L per 24 hours in chronic hyponatraemia — osmotic demyelination (central pontine myelinolysis) is the consequence, and it is irreversible.
  • Acute hyponatraemia (under 48 hours, as in TURP syndrome or exercise-associated) tolerates faster correction, because the brain has not yet adapted.

Quick recall

42 L = 28 ICF + 14 ECF
ECF = 11 interstitial + 3 plasma
0.6 × wt × (Na/140 − 1)
SIADH euvolaemic · restrict
CSW dry · high UO · give salt
<8–10/day or demyelination

Built from your body fluid compartments and SIADH/CSWS pages. See also Renal physiology.