Body fluid compartments
Body fluid compartments: volumes, markers, osmolality and fluid shifts
Total body water is 60% of body weight: two-thirds inside cells, one-third outside. Learn the markers for each compartment, plasma osmolality, and how sweating, vomiting, diarrhoea and IV fluids shift water between ECF and ICF.
The core idea
Water crosses cell membranes freely and moves towards higher osmolality, so ICF and ECF always end up at the same osmolality (about 285–295 mOsm/kg). Sodium is the main extracellular solute and potassium the main intracellular one. Therefore total body sodium determines ECF volume, while water balance determines the plasma sodium concentration. Any change in ECF osmolality moves water into or out of cells.
How the water is divided (70 kg adult male)
| Compartment | Fraction | Approximate volume |
|---|---|---|
| Total body water (TBW) | 60% of body weight | 42 L |
| Intracellular fluid (ICF) | ⅔ of TBW (40% of weight) | 28 L |
| Extracellular fluid (ECF) | ⅓ of TBW (20% of weight) | 14 L |
| Interstitial fluid (with lymph) | ¾ of ECF (about 15% of weight) | 10.5 L |
| Plasma | ¼ of ECF (about 4–5% of weight) | 3.5 L |
| Transcellular fluid (CSF, synovial, pleural, eye, gut secretions) | Small | About 1–2 L |
| Blood volume | About 7% of weight (70–80 mL/kg) | 5 L |
- The water fraction is lower in women, the elderly and the obese (fat holds little water), and higher in infants (about 75–80%).
- Cerebrospinal fluid (about 150 mL, turned over about three times a day, roughly 500 mL/day) is a transcellular fluid. Most is produced by the choroid plexus of the lateral, third and fourth ventricles and absorbed through arachnoid granulations into the dural venous sinuses.
- Of the blood volume, the veins hold about 60–65%, so a slow loss of a small volume of blood comes mainly from the venous reservoir, with little fall in arterial pressure.
Measuring compartments
Indicator dilution: volume = amount given ÷ concentration after mixing (minus any amount excreted or metabolised). For example, 140 mg of an ECF marker that reaches 10 mg/L after mixing gives an ECF of 14 L. Keep the units consistent: mg ÷ mg/L = L.
| Compartment | Marker |
|---|---|
| Total body water | Deuterium oxide (heavy water), tritiated water, antipyrine |
| ECF | Inulin, mannitol, sucrose, radioactive sodium, sulphate or thiosulphate |
| Plasma volume | Radio-iodinated albumin, Evans blue (T-1824) dye |
| Red cell volume | ⁵¹Cr-labelled red cells |
| ICF | TBW − ECF (calculated) |
| Interstitial fluid | ECF − plasma volume (calculated) |
| Blood volume | Plasma volume ÷ (1 − haematocrit) |
Composition
| Ion | ECF (plasma) | ICF |
|---|---|---|
| Na⁺ | About 140 mmol/L | About 10–15 |
| K⁺ | About 4 | About 140–150 |
| Mg²⁺ | About 1 | High (main intracellular divalent cation) |
| Cl⁻ | About 100 | Low |
| HCO₃⁻ | About 24 | Low |
| Main anions inside cells | Phosphates and proteins |
- The Na⁺/K⁺-ATPase maintains these gradients. Because the resting membrane potential depends on the K⁺ gradient, a change in extracellular K⁺ has the greatest effect on the heart.
- Plasma differs from interstitial fluid mainly in its higher protein content (Gibbs–Donnan effect gives slightly more cations in plasma).
Osmolality and tonicity
- Calculated plasma osmolality ≈ 2 × Na⁺ + glucose + urea (all in mmol/L). With Na⁺ 140, glucose 30 and urea 10: 280 + 30 + 10 = 320 mOsm/kg. (Glucose 30 mmol/L is about 540 mg/dL; urea 10 mmol/L is BUN about 28 mg/dL.)
- Urea crosses cell membranes freely, so it raises osmolality but does not move water: it is an ineffective osmole. Effective osmolality (tonicity) = 2 Na⁺ + glucose. Sodium, glucose (without insulin) and mannitol are effective osmoles.
- An osmolar gap (measured minus calculated) above about 10 points to an unmeasured osmole: ethanol, methanol, ethylene glycol, mannitol.
- Hypernatraemia (for example sodium 162 mmol/L after days of poor intake) draws water out of cells, so brain cells shrink; over 1–2 days they generate idiogenic osmoles to recover volume. Hyponatraemia swells brain cells. Correcting either too fast is dangerous: rapid correction of chronic hyponatraemia causes osmotic demyelination; rapid correction of hypernatraemia causes cerebral oedema.
What happens with gains and losses (Darrow–Yannet logic)
Decide first what was lost or gained relative to plasma: isotonic, hypotonic (more water than salt) or hypertonic.
| Event | ECF volume | ICF volume | ECF osmolality |
|---|---|---|---|
| Isotonic loss (diarrhoea, haemorrhage, burns exudate) | ↓ | No change | No change |
| Hypotonic loss: sweating, diabetes insipidus, poor intake | ↓ | ↓ | ↑ |
| Sweat loss replaced with plain water | Slightly ↓ | ↑ | ↓ |
| Salt excess (hypertonic saline, sea water) | ↑ | ↓ | ↑ |
| Water excess (SIADH, hypotonic fluids) | ↑ | ↑ | ↓ |
| Isotonic gain (0.9% saline) | ↑ | No change | No change |
| Adrenal insufficiency (salt loss) | ↓ | ↑ | ↓ |
- A runner who loses 2 L of sweat (hypotonic) and drinks nothing: osmolality rises and both ECF and ICF shrink, ICF supplying about two-thirds of the water.
- If he drinks 2 L of plain water, the volume is replaced but not the salt: ECF osmolality falls and water moves into cells, so ICF volume rises.
Electrolyte losses from the gut
| Loss | Main electrolyte effect | Why |
|---|---|---|
| Vomiting (pyloric stenosis: 6-week-old boy, projectile non-bilious vomiting, olive mass) | Hypochloraemic, hypokalaemic metabolic alkalosis | Loss of HCl; volume depletion drives aldosterone, which wastes K⁺ and H⁺ (paradoxical aciduria) |
| Diarrhoea (cholera) | Hypokalaemia with normal anion gap metabolic acidosis | Stool is rich in HCO₃⁻ and K⁺; isotonic volume loss |
- Persistent hypokalaemia despite replacement: check and correct magnesium. Low intracellular Mg²⁺ releases the block on renal outer medullary K⁺ channels, so K⁺ keeps being lost. Low Mg²⁺ (and low K⁺) also increase digoxin toxicity.
Where IV fluids go
| Fluid | Distribution | Stays in plasma |
|---|---|---|
| 0.9% saline (Na⁺ 154 mmol/L) | ECF only | About ¼ |
| 5% dextrose | Glucose is metabolised, leaving free water spread through all TBW | About 1/12; useless for volume |
| Colloids (albumin, gelatins) | Mostly plasma at first | Most |
| Hypertonic saline | ECF; draws water from cells | Raises ECF and TBW, lowers ICF |
Daily water balance (adult)
Intake about 2.5 L (drinks about 1.5 L, food 0.8 L, metabolic water 0.3 L). Losses: urine about 1.5 L, insensible losses from skin and lungs about 0.8 L, faeces 0.1–0.2 L. Insensible losses rise with fever (roughly 10–15% per °C) and hyperventilation. Maintenance is about 25–30 mL/kg/day for adults, with about 1 mmol/kg/day each of Na⁺ and K⁺.
Remember 60–40–20: total, intracellular, extracellular water as a percentage of body weight; plasma is a quarter of the ECF.
Exam trap Inulin measures ECF (and GFR); Evans blue measures plasma volume; deuterium oxide measures total body water; ICF and interstitial fluid are calculated by subtraction.
Exam trap Sweat is hypotonic: losing it shrinks both compartments; replacing it with water alone lowers osmolality and swells cells.
How FCPS asks about this
- TBW, ICF, ECF in a 70 kg man: 42 L, 28 L, 14 L; interstitial fluid about 10.5 L, plasma 3.5 L.
- Marker for total body water: deuterium oxide or tritiated water; plasma volume: Evans blue or labelled albumin.
- Calculated osmolality with Na 140, glucose 30, urea 10: 320 mOsm/kg; urea is an ineffective osmole.
- Brain cells in hypernatraemia: shrink as water moves out; correct slowly.
- Pyloric stenosis bloods: hypochloraemic hypokalaemic metabolic alkalosis; cholera gives hypokalaemic acidosis.
- Refractory hypokalaemia: correct magnesium.
- CSF: made mainly by choroid plexus, absorbed by arachnoid granulations, about 500 mL/day.
Test yourself on this topic
5 of the 29 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Why is 5% dextrose a poor choice for restoring blood volume in haemorrhage?
Answer: It acts as free water
Fluid distribution follows its effective tonicity. Colloids stay mainly in the plasma. Isotonic saline or Hartmann's distributes through the extracellular fluid, so about a quarter remains in plasma (plasma is about a quarter of ECF). 5% dextrose is effectively free water once the glucose is metabolised; it spreads through total body water (two-thirds into cells), so only about 1/12 remains intravascular. Dextrose is used for maintenance water, not resuscitation, and large volumes cause hyponatraemia.
Where fluid goes
- Colloid: mostly plasma
- Saline / Hartmann's: ECF, ~1/4 stays in plasma
- 5% dextrose: total body water, ~1/12 in plasma
Remember Dextrose is water in disguise.
Why the other options are wrong
- It contains too much sodium It contains no sodium.
- It stays in vessels, overloading It leaves the vessels quickly.
- It causes hyperkalaemia It does not contain potassium.
- It is hypertonic, dehydrating cells It is isotonic in the bag but behaves as free water.
A burn on the forearm is white and leathery, does not blanch and is painless to pinprick. What is its depth?
Answer: Full thickness
Superficial (epidermal) burns are red and painful without blisters. Superficial partial-thickness burns are pink, blistered, moist and very painful with brisk capillary refill. Deep partial-thickness burns are blotchy red, with sluggish or absent blanching and reduced sensation. Full-thickness burns destroy the whole dermis with its nerve endings: white, waxy, leathery or charred, non-blanching and painless. They need excision and grafting.
Burn depth
- Epidermal: red, painful
- Partial thickness: blisters, painful
- Deep dermal: blotchy, less sensation
- Full thickness: white, leathery, painless
- Full thickness needs grafting
Remember No pain, no blanching: full thickness.
Exam trap Pain means nerves survive; a painless burn is deeper, not milder.
Why the other options are wrong
- Superficial (epidermal) Red and painful, no blisters.
- Superficial partial thickness Pink, blistered and very painful.
- Deep partial thickness with blisters Blotchy, some sensation remains.
- Superficial dermal Painful with blisters.
An 86-year-old care home resident with dementia is drowsy with sodium 158 mmol/L after days of poor intake during a heatwave. What has happened?
Answer: Water loss exceeding salt
Hypernatraemia usually reflects water deficit relative to sodium. Older people with dementia have reduced thirst and cannot access water, while losses continue through skin, breathing and urine, especially in heat or fever. Treatment is careful water replacement (oral or IV 5% dextrose or hypotonic fluids), lowering sodium slowly (no more than about 10 mmol/L per day) to avoid cerebral oedema.
Hypernatraemia
- Usually water deficit
- Elderly: poor thirst, no access
- Replace water slowly
- Fall ≤ 10 mmol/L/day
Remember High sodium usually means not enough water.
Exam trap Rapid correction causes cerebral oedema and seizures.
Why the other options are wrong
- Excess salt intake Rare.
- SIADH Causes low sodium.
- Adrenal insufficiency Causes low sodium.
- Overuse of IV saline Not in this setting.
A 50 kg elderly woman with dementia has a sodium of 160 mmol/L after several days of poor intake. Taking total body water as 50% of body weight, what is her approximate free water deficit?
Answer: About 3.6 L
Free water deficit = total body water × (plasma sodium ÷ 140 − 1). Her total body water is 0.5 × 50 = 25 L, and 160 ÷ 140 − 1 is about 0.14, so the deficit is about 3.6 L. Forgetting to multiply by the water fraction gives 7.1 L, and 25 L is total body water itself. The deficit should be replaced slowly, lowering sodium by no more than about 10 mmol/L in 24 hours to avoid cerebral oedema, and ongoing losses must be added.
Free water deficit
- Deficit = TBW × (Na⁺/140 − 1)
- TBW ~50% of weight in older women
- Here 25 L × 0.14 ≈ 3.6 L
- Lower Na⁺ ≤ 10 mmol/L per day
Remember Calculate total body water first, then the deficit.
Exam trap Correcting hypernatraemia too fast can cause cerebral oedema.
Why the other options are wrong
- About 1.0 L Too small for this degree of hypernatraemia.
- About 7.1 L Uses body weight instead of total body water.
- About 25 L That is total body water.
- About 0.5 L Too small.
After major abdominal surgery, a patient's weight rises while urine output falls and the blood volume appears depleted. What explains this?
Answer: Fluid shifts into the third space
Surgical trauma and inflammation increase capillary permeability, and fluid moves from the intravascular space into the interstitium and gut wall or lumen ('third space'). This fluid is not available to the circulation, so the patient is intravascularly depleted despite weight gain and oedema. The stress response (ADH, aldosterone) also retains water and sodium. The fluid returns to the circulation over a few days, with a diuresis.
Third space losses
- ↑ Capillary permeability
- Fluid into interstitium, gut
- Intravascular depletion + oedema
- Mobilised after 2–3 days
Remember Wet tissues, dry vessels.
Exam trap Excess crystalloid worsens tissue oedema and ileus.
Why the other options are wrong
- The kidneys stop filtering completely Filtration continues.
- Fluid is lost through the skin as sweat Not the main shift.
- Intracellular fluid moves into the blood Opposite of what happens.
- Plasma proteins increase sharply Albumin usually falls.
Where this sits in the CPSP syllabus
- Paper 1: Renal physiology, body fluids and acid-base, Body fluid compartments and their measurement
- Surgery & Allied, Paper 2: Renal physiology, body fluids and acid-base, Body fluid compartments and their measurement
- Medicine & Allied, Paper 2: Renal physiology, body fluids and acid-base, Body fluid compartments and their measurement
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