How is total body water divided into compartments?
Humans are mostly water: about 75% of body mass in infants and 50 to 60% in adults, falling to about 45% in old age. Water content differs between tissues — as little as 8% in teeth and as much as 85% in the brain. StatPearls uses a factor of 0.6 for men and 0.5 for women when calculating water deficits, reflecting the lower water fraction in women.
Body water sits in two main compartments separated by cell membranes. The intracellular fluid (ICF) is all fluid enclosed within cells. The extracellular fluid (ECF) surrounds cells and has two main parts: plasma, the fluid of the blood, and interstitial fluid (ISF), which bathes the cells and is where materials pass between capillaries and cells. Other water-based ECF — CSF, lymph, synovial, pleural, pericardial and peritoneal fluids and aqueous humour — are called transcellular fluids.
What are the normal volumes of each compartment?
| Compartment | % of body weight | Fraction of total water | Volume (approx.) |
|---|---|---|---|
| Total body water (TBW) | 60% | — | 42 L |
| Intracellular fluid (ICF) | 40% | two-thirds | 28 L |
| Extracellular fluid (ECF) | 20% | one-third | 14 L |
| — Interstitial fluid (ISF) | 15% | three-quarters of the ECF | about 11 L |
| — Plasma | 5% | one-quarter of the ECF | about 3 L |
In an average adult male, OpenStax gives about 25 litres of ICF (roughly 60% of body water), which is consistent with the 28 L worked above for a 70-kg man. The ICF volume is remarkably stable because the water content of cells is tightly regulated. If cells lose water, the cytosol becomes too concentrated for normal chemistry; if cells swell, they risk bursting.
How does the composition of ICF, ISF and plasma differ?
The ECF is rich in sodium, chloride and bicarbonate; the ICF is rich in potassium, magnesium, phosphate and protein. The interstitial fluid is similar to plasma but contains very little protein. Every compartment is electrically neutral — cations balance anions — and the osmolarity of plasma is about 286 mOsm/L (many question stems use 290).
| Feature | Plasma | Interstitial fluid | ICF |
|---|---|---|---|
| Main cation | Sodium | Sodium | Potassium |
| Main anions | Chloride, bicarbonate, protein | Chloride, bicarbonate | Phosphate and protein |
| Protein | High (albumin) | Very low | High |
| Other ions that are low | Potassium, magnesium, phosphate | Potassium, magnesium, phosphate | Sodium, chloride, bicarbonate |
How are body fluid volumes measured — which marker for which compartment?
Compartment volumes are measured by the indicator (dilution) principle: a known amount of a marker is injected, allowed to distribute and equilibrate in its compartment, and the plasma concentration is measured. A good marker distributes only in the compartment being measured and is not metabolised or excreted (or the loss is measured).
Volume = (amount injected − amount lost) ÷ concentration at equilibrium
Units must match: mg ÷ (mg/dL) gives dL; divide by 10 for litres.
| Compartment | Markers | How it is obtained |
|---|---|---|
| Total body water | Deuterium oxide (D2O, heavy water); oxygen-18 labelled water | Isotope dilution — the reference method; distributes through all body water |
| Extracellular fluid | Inulin, mannitol, bromide, radiosulfate | Dilution — these cross capillary walls but do not enter cells |
| Plasma volume | Evans blue dye, radio-iodinated serum albumin (RISA) | Dilution — bound to albumin, so confined to the plasma |
| Intracellular fluid | No direct marker | TBW − ECF (calculated) |
| Interstitial fluid | No direct marker | ECF − plasma volume (calculated) |
| Blood volume | RISA or Evans blue with the haematocrit | Plasma volume ÷ (1 − haematocrit) |
Bromide dilution has long been the reference method for extracellular water in non-pregnant adults, while deuterium oxide remains the reference for total body water because it is accurate, safe at tracer doses and measurable in saliva, breath, urine or blood. Because each marker measures only one space, the other volumes are always calculated by subtraction.
What forces move water between plasma, interstitium and cells?
Water crosses semipermeable membranes passively, following the gradient of osmotically active solutes — it moves towards the hyperosmotic compartment. Steady-state ionic gradients between ICF and ECF are maintained by active transport (the Na+/K+ pump). Across capillary walls, hydrostatic pressure also drives fluid out, and the balance is described by the Starling equation.
Jv = Kfc × [(Pc − Pi) − σ(πp − πi)]
Jv = net fluid movement; Pc, Pi = capillary and interstitial hydrostatic pressure; πp, πi = plasma and interstitial oncotic pressure; σ = reflection coefficient; Kfc = filtration coefficient.
| Force | Direction | Effect |
|---|---|---|
| Capillary hydrostatic pressure (Pc) | Out of the capillary | Filtration — highest at the arteriolar end |
| Plasma oncotic pressure (πp, mostly albumin) | Into the capillary | Reabsorption — opposes filtration |
| Interstitial hydrostatic pressure (Pi) | Into the capillary | Small opposing force |
| Interstitial oncotic pressure (πi) | Out of the capillary | Small — interstitial protein is low |
Oedema is fluid overload in the interstitium. It follows raised capillary hydrostatic pressure (cardiac failure, which causes pulmonary oedema by back-pressure in the pulmonary veins; renal failure, by failing to remove water and solute) or reduced plasma oncotic pressure (liver disease, where albumin synthesis falls). Treatment targets the cause and may include diuretics and albumin.
How does this apply to dehydration, IV fluids and burns?
- Dehydration — an adult has an obligatory intake requirement of about 1600 mL per day; losses occur through urine, sweat, respiration and stool, plus diarrhoea, vomiting, fever and osmotic diuresis. Features: reduced urine output, dizziness, tachycardia, and confusion if severe. Oral replacement is preferred when possible.
- Sodium-based water deficit — Deficit = 0.6 × weight (kg) × [1 − (140 ÷ measured Na)] in men and 0.5 × weight × [...] in women; StatPearls cautions that it can underestimate losses by more than 40%.
- Maintenance fluid (4-2-1 rule) — 4 mL/kg/h for the first 10 kg + 2 mL/kg/h for the next 10 kg + 1 mL/kg/h for each kg above 20. A 55-kg patient needs 40 + 20 + 35 = 95 mL/h.
- Children — fluid deficit is estimated from weight loss: 1 kg of lost weight is about 1 L of free water. A bolus of 30 mL/kg is used in emergencies.
- Burns — modified Parkland: 4 mL/kg per % body surface area burned; modified Brooke: 2 mL/kg per %. Half is given in the first 8 hours of the first 24; urine output guides the final volume.
- Crystalloid side effects — large volumes of normal saline can cause non-gap hyperchloraemic metabolic acidosis; half-normal saline can cause hyponatraemia if not monitored.
- Correction of sodium — rapid correction of chronic hyponatraemia can cause central pontine myelinolysis; rapid correction of hypernatraemia risks cerebral oedema.
How do osmolarity changes shift water between compartments?
Osmolarity is the number of osmotically active particles per litre of fluid. Plasma osmolarity is about 286 mOsm/L (below this is hypo-osmotic, above it hyperosmotic). Besides ions, albumin and glucose contribute to osmolarity. Because water follows solute, any change in the osmolarity of one compartment moves water until the compartments equilibrate — ICF and ECF osmolarity are equal at steady state, but their volumes change.
| Situation | What happens to water | Consequence |
|---|---|---|
| Diabetic ketoacidosis / hyperosmolar state | Glucose is osmotically active and is excreted in urine, carrying water with it | Marked fluid loss and dehydration; adults often need 6 to 9 L of normal saline |
| Too-rapid correction of hypernatraemia | Rapid fall in sodium raises intracellular and extracellular fluid in the brain | Cerebral oedema; correct slowly |
| Too-rapid correction of chronic hyponatraemia | Brain cells that had lost organic osmoles shrink and the blood-brain barrier is disrupted | Central pontine myelinolysis (osmotic demyelination) |
| Low plasma albumin (liver disease) | Plasma oncotic pressure falls, fluid shifts to the interstitium | Oedema and ascites |
| Large-volume normal saline | Chloride load expands the ECF | Non-gap hyperchloraemic metabolic acidosis |
What is homeostasis, and how do body fluids fit in?
Homeostasis is the maintenance of a relatively constant, narrow range of internal conditions (a dynamic equilibrium) despite changes in the external environment. The term was coined by Walter Cannon in 1926 and builds on Claude Bernard's *milieu intérieur* (1865). External changes alter the composition of the extracellular fluid that bathes the cells, so it must be kept within narrow limits to prevent cell, tissue and organ death.
- Setpoint — the desired value (or range) the control system works toward.
- Detection, effectors and feedback — thousands of control systems detect a disturbance and use effectors to correct it.
- Local control (paracrine or autocrine) and reflex control (nervous and endocrine systems).
- Allostasis — stability through change; it accepts normal daily variation, whereas homeostasis aims to reduce variability.
Cells defend their own volume too: in hyperosmolar or hypo-osmolar surroundings they transport molecules to return to normal volume, which links directly to the osmolarity and water-shift rules above.
How are body fluid compartments tested in NEET PG and INI-CET?
- Marker-matching — total body water → deuterium oxide; ECF → inulin; plasma → Evans blue or RISA; ICF and ISF → calculated, never measured directly.
- Percentage questions — 60% water, ICF 40%, ECF 20%, plasma 5%, ISF 15%; infants have a higher water fraction than adults.
- Arithmetic — volume = amount injected (minus any excreted) divided by concentration; blood volume from plasma volume and haematocrit.
- Composition — which ion is highest intracellularly (potassium), which differs most between plasma and ISF (protein).
- Clinical — Starling forces and the causes of oedema, the 4-2-1 maintenance rule, Parkland and Brooke formulae, saline-induced hyperchloraemic acidosis.