Physiology · Renal, fluids and acid–base
Fluid shifts between compartments !
- Mechanism
- Exam priority: medium
Water moves between cells and the fluid around them toward wherever more particles are dissolved.
Updated 2026-11-01
Also called: Intracellular and extracellular compartments, Darrow–Yannet diagram
What it is
Body water is split between intracellular and extracellular fluid (interstitium and plasma). Across the cell membrane, water moves only by osmolality: it goes toward more effective solute. Between plasma and interstitium, Starling forces decide. Knowing what was lost or gained (water, salt or both) predicts which way water shifts and which compartment shrinks or swells.
Equations
- Na+
- mmol/L
- glucose, BUN
- mg/dL
The difference from measured osmolality is the osmolar gap (see Biochemistry).
Urea is left out because it moves freely in and out of cells.
Distribution in a 70 kg adult (approx.)
| Parameter | Value | Note |
|---|---|---|
| Total body water | ≈42 L | ≈60 % of weight; less in women and older adults, more in infants |
| Intracellular fluid | ≈28 L | |
| Interstitial fluid | ≈10–11 L | |
| Plasma | ≈3 L | |
| Plasma osmolality | ≈275–295 mOsm/kg |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Six disturbances and where the water goes
| Situation | Example | Extracellular | Intracellular | Osmolality |
|---|---|---|---|---|
| Isotonic gain | Intravenous isotonic saline | ↑ | = | = |
| Isotonic loss | Diarrhoea, haemorrhage | ↓ | = | = |
| Water loss (hypotonic) | Sweat, fever, diabetes insipidus | ↓ | ↓ | ↑ |
| Water gain | SIADH, excess water intake | ↑ | ↑ | ↓ |
| Salt gain (hypertonic) | Hypertonic saline, seawater | ↑ | ↓ | ↑ |
| Salt loss exceeding water loss | Adrenal insufficiency | ↓ | ↑ | ↓ |
Clinical notes
- Correcting chronic hyponatraemia too fast pulls water out of neurons that have already adapted: osmotic demyelination syndrome.
- In marked hyperglycaemia water leaves cells and dilutes sodium (hypertonic hyponatraemia); as glucose falls, sodium rises.
- Infants and older adults tolerate water loss poorly: more water per kilogram, or blunted thirst and weaker urine concentration.
Test yourself
Think of the answer, then tap the question to see it.
A patient with isotonic watery diarrhoea loses 3 L. Which compartment shrinks, and what happens to osmolality?
The extracellular compartment (plasma and interstitium) shrinks; because the loss is isotonic, osmolality is unchanged and so is intracellular volume.
Why does a high urea in kidney failure not dehydrate cells, while hyperglycaemia does?
Urea crosses membranes and equilibrates: it is an ineffective osmole. Glucose, without insulin, stays outside many cells and pulls water into the extracellular fluid.
In SIADH, what happens to intracellular volume, and why is there a neurological risk?
Water is retained without sodium: osmolality falls and water enters cells, including neurons, which swell inside the rigid skull (cerebral oedema).
Related topics
- Body fluid compartments
- Osmosis, osmolarity and tonicity
- Starling forces at the capillary
- IV fluids: tonicity and where they go
- Dehydration and fluid overload
- ADH, thirst and water balance
Related conditions
- Hyponatraemia
- Hypernatraemia
- Hypovolaemic shock
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Adrenal insufficiency
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 26.1 Body Fluids and Fluid Compartments (OpenStax (Rice University), 2022); 26.2 Water Balance (OpenStax (Rice University), 2022); Osmolality Tests: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 25). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.