Physiology · Cell and membrane
Body fluid compartments !
- Concept
- Exam priority: high
About two thirds of the body's water is inside cells and one third outside them, between cells and in the blood.
Updated 2026-11-01
Also called: Total body water, Intracellular and extracellular fluid
What it is
Total body water is about 60% of body weight in a young adult and splits into two main compartments: intracellular (≈2/3) and extracellular (≈1/3). The extracellular fluid divides into interstitial fluid (≈3/4) and plasma (≈1/4). The cell membrane separates compartments of very different ionic make-up, and the capillary wall separates plasma from interstitium; these barriers explain where an IV fluid goes and why oedema forms.
Key facts
- Measured by dilution of an indicator that stays in one compartment: heavy water for total water, inulin or mannitol for extracellular fluid, labelled albumin or Evans blue for plasma.
- Intracellular and interstitial volumes are found by subtraction: intracellular = total − extracellular; interstitial = extracellular − plasma.
- A 'third space' is trapped fluid (ascites, obstructed bowel, burns) that belongs to the extracellular fluid but does not circulate.
Equations
- V
- volume of the compartment the indicator spreads into
Subtract whatever was excreted or metabolised before equilibrium.
Young 70 kg adult (approximate)
| Parameter | Value | Note |
|---|---|---|
| Total body water | ≈42 L | ≈60% of weight |
| Intracellular fluid | ≈28 L | ≈40% of weight |
| Extracellular fluid | ≈14 L | ≈20% of weight |
| Interstitial fluid | ≈11 L | ≈3/4 of extracellular |
| Plasma | ≈3 L | ≈1/4 of extracellular |
| Blood volume | ≈5 L | ≈7% of weight; haematocrit ≈40–45% |
| Plasma osmolality | ≈285–295 mOsm/kg | equal inside and outside cells at equilibrium |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Approximate ionic composition (mEq/L)
| Solute | Extracellular | Intracellular | Why it matters |
|---|---|---|---|
| Na⁺ | ≈142 | ≈10–14 | Sets extracellular volume |
| K⁺ | ≈4 | ≈140 | Sets the resting potential; excess is arrhythmogenic |
| Cl⁻ | ≈103–108 | ≈4–10 | Follows Na⁺ outside the cell |
| HCO₃⁻ | ≈24–28 | ≈10 | Main extracellular buffer |
| Proteins | Plasma ≈7 g/dL; low in interstitium | Very high | Plasma oncotic pressure; intracellular anions |
| Phosphates | ≈2 | Very high | Intracellular buffer and energy |
Clinical notes
- Acute haemorrhage: whole blood is lost; haematocrit is unchanged at first and falls later as interstitial fluid refills the plasma.
- Dehydration from heat or diarrhoea: mainly extracellular loss; in Colombia, childhood diarrhoea is the classic cause of hypovolaemic shock.
- Older adults: less total water and weaker thirst; they dehydrate sooner and water-soluble drugs reach higher concentrations.
Test yourself
Think of the answer, then tap the question to see it.
1000 mL of isotonic saline is infused. How much stays in the plasma, and why?
About 250 mL. Na⁺ does not cross cell membranes, so the fluid stays extracellular and is shared like it: ≈1/4 plasma, ≈3/4 interstitium.
Why does an adult woman have a lower percentage of body water than a man of the same weight?
On average she has more adipose tissue, which holds little water, and less muscle, which is water-rich.
Which indicators would you use to find the interstitial volume?
It is not measured directly: subtract plasma volume (labelled albumin or Evans blue) from extracellular volume (inulin or mannitol).
Why can haematocrit be normal in the first hour of a major bleed?
Red cells and plasma are lost in the same proportion; haematocrit falls later, when interstitial fluid moves into the plasma and dilutes it.
Related topics
- Osmosis, osmolarity and tonicity
- Starling forces at the capillary
- Fluid shifts between compartments
- Dehydration and fluid overload
- IV fluids: tonicity and where they go
- How edema forms
Related conditions
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); Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 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.