[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Cell and membrane](https://durgical.com/en/physiology/#s-cell) › Body fluid compartments

[Physiology](https://durgical.com/en/physiology/) · [Cell and membrane](https://durgical.com/en/physiology/#s-cell)

# 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

## Key points

- 60-40-20 rule: total water ≈60% of weight, intracellular ≈40%, extracellular ≈20% (interstitial ≈15%, plasma ≈5%).
- Cell membrane: water crosses freely, Na⁺ does not; so Na⁺ sets extracellular volume and osmolality decides how water is shared.
- Capillary wall: water and ions cross, proteins do not; Starling forces share fluid between plasma and interstitium.
- Outside cells Na⁺ and Cl⁻ dominate; inside, K⁺, phosphates and proteins.
- Women, older adults and people with obesity have less water (≈50%; fat holds little water); newborns have more (≈75%).

## 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

**Indicator-dilution principle**V = amount injected / concentration at equilibrium

**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⁺](https://durgical.com/en/conditions/hyperkalaemia/) | ≈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.

## Memory aid

Six, four, two: six tenths water, four inside, two outside; and of those two, only half a part in the plasma.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/body-fluid-compartments/)

## Related topics

- [Osmosis, osmolarity and tonicity](https://durgical.com/en/physiology/osmosis-osmolarity-and-tonicity/)
- [Starling forces at the capillary](https://durgical.com/en/physiology/starling-forces-at-the-capillary/)
- [Fluid shifts between compartments](https://durgical.com/en/physiology/fluid-shifts-between-compartments/)
- [Dehydration and fluid overload](https://durgical.com/en/physiology/dehydration-and-fluid-overload/)
- [IV fluids: tonicity and where they go](https://durgical.com/en/physiology/iv-fluids-tonicity-and-where-they-go/)
- [How edema forms](https://durgical.com/en/physiology/how-edema-forms/)

## Related conditions

- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Hypernatraemia](https://durgical.com/en/conditions/hypernatraemia/)

## In Biochemistry

- [Serum osmolality and osmolar gap](https://durgical.com/en/biochemistry/topics/serum-osmolality-and-osmolar-gap/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-1-body-fluids-and-fluid-compartments) (OpenStax (Rice University), 2022); [26.2 Water Balance](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-2-water-balance) (OpenStax (Rice University), 2022); [Hypovolemia and Hypovolemic Shock](https://www.ncbi.nlm.nih.gov/books/NBK513297/) (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).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/physiology/body-fluid-compartments/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/body-fluid-compartments/