Body Fluids
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Direct answer
A 70-kilogram adult carries about 42 litres of water — roughly 60 per cent of body weight — divided into 28 litres inside cells (the ICF, two-thirds) and 14 litres outside (the ECF, one-third), the ECF splitting again into roughly 10.5 litres of interstitial fluid and 3 litres of plasma. Because adipose tissue holds little water, the percentage falls in women and in obesity and is highest in neonates. Each compartment is measured by the dilution principle — volume equals the amount of marker retained divided by its concentration at equilibrium — using tritiated water or antipyrine for total body water, inulin for the ECF and Evans blue for plasma. FMGE stems test marker-to-compartment matching and the direction water shifts when one compartment's tonicity changes.
What you must remember
- The 60-40-20 rule for a standard adult male: total body water 60 per cent of weight, ICF 40 per cent, ECF 20 per cent; neonates reach 75-80 per cent and the elderly may fall to about 50 per cent.
- Volumes to quote: TBW 42 L, ICF 28 L, ECF 14 L, interstitial 10.5 L, plasma 3 L; blood volume about 5 L, near 70 mL per kilogram.
- Marker matching: D2O or antipyrine for TBW; inulin, sucrose, mannitol or sulfate for ECF; Evans blue (T-1824, albumin-bound) for plasma; blood volume from labelled red cells or haematocrit.
- Composition contrast: ECF is a sodium (142 mEq/L), chloride and bicarbonate fluid; ICF is a potassium (about 140 mEq/L), magnesium and phosphate fluid — the Na-K ATPase maintains the divide.
- Plasma proteins of 6-8 g/dL exert an oncotic pressure near 25 mmHg, mostly through albumin, which is why albumin loss pulls water out of vessels.
- Water moves to the hypertonic side: hypernatraemia shrinks cells, hypotonic ECF (water intoxication, SIADH) swells them, and isotonic saline stays entirely extracellular.
- Bedside convention worth one mark: a 1 kilogram change in body weight approximates a 1 litre change in fluid balance.
Working through a fluid-shift problem
When a question adds fluid or removes it, track tonicity, not volume labels. Infuse 1 litre of 0.9 per cent saline: it is isotonic, so it stays in the ECF — plasma and interstitial fluid share it roughly one part to three, cells are untouched, and the haematocrit falls only by dilution. Drink 1 litre of plain water: absorbed into the ECF it makes it momentarily hypotonic, water flows into cells until ICF and ECF are again iso-osmotic, so about two-thirds ends up intracellular — and the healthy kidney excretes it within hours. Infuse 1 litre of 3 per cent saline: the hypertonic ECF drags water out of cells, so the ECF expands by more than the infused volume and cells shrink, which is exactly why hypertonic saline treats symptomatic hyponatraemia with cerebral oedema.
Now reverse the direction. A man sweating heavily loses hypotonic fluid: the remaining ECF becomes hypertonic, thirst is intense, and water shifts out of cells. A patient losing pure plasma in burns loses protein as well, so interstitial oncotic pressure patterns change and massive oedema follows. Every such question resolves the same way — decide which compartment changed tonicity, then let water move toward the higher osmolarity until you can explain every clinical clue in the stem.
High-yield viva angles
Examiners here ask why, not what. Why does Evans blue measure only plasma volume? Because albumin cannot cross the capillary wall, the dye stays intravascular, and interstitial volume is then derived as ECF minus plasma. Why is plasma slightly more osmolar than interstitial fluid? The Gibbs-Donnan effect: negatively charged plasma proteins hold a small excess of cations, adding roughly 1 mOsm and a small hydrostatic offset at the capillary. Why does inulin fail to measure total body water? It crosses capillaries but not cell membranes, so it distributes only in the ECF. Third-spacing is a favourite: fluid sequestered in burns, ascites or post-operative ileus is ECF that has left the functional circulation without leaving the body, which is why weighing the patient beats charting intake-output.
Frequently asked questions
Which marker measures total body water and by what principle?
Tritiated water or antipyrine by dilution: volume equals marker injected minus marker lost, divided by concentration at equilibrium.
Why does Evans blue measure plasma rather than extracellular volume?
It binds albumin, which stays within the capillary; interstitial fluid is calculated separately as ECF volume minus plasma volume.
In hypernatraemia, which way does water move?
Out of cells: the hypertonic ECF pulls intracellular water out, producing cellular dehydration, irritability and intense thirst.
What is the 60-40-20 rule?
Total body water is about 60 per cent of body weight, ICF two-thirds of it (40 per cent of weight) and ECF one-third (20 per cent) in a standard adult male.
How do body water proportions differ at the extremes of age?
Neonates carry up to 75-80 per cent body water with a larger ECF share, while elderly adults fall towards 50 per cent — both groups dehydrate fast.