Body fluid compartments
Body fluid compartments describe how total body water is distributed between intracellular fluid and extracellular fluid. A body fluid compartments calculation is useful for estimating the major water spaces from body mass and for understanding how plasma and interstitial fluid fit inside the extracellular compartment.
The main quantity estimated first is total body water, then that value is divided into intracellular fluid, extracellular fluid, plasma, and interstitial fluid using standard teaching approximations.
Core definitions and formulas
Total body water is commonly estimated as a fraction of body mass. The intracellular fluid compartment is then taken as about two-thirds of total body water, while the extracellular fluid compartment is about one-third. Extracellular fluid can be subdivided into plasma and interstitial fluid.
\[
\begin{aligned}
TBW &= \text{body mass} \cdot f
\end{aligned}
\]
\[
\begin{aligned}
ICF &= \frac{2}{3} \cdot TBW \\
ECF &= \frac{1}{3} \cdot TBW
\end{aligned}
\]
\[
\begin{aligned}
Plasma &= \frac{1}{4} \cdot ECF \\
Interstitial &= \frac{3}{4} \cdot ECF
\end{aligned}
\]
Here, TBW is total body water, ICF is intracellular fluid, ECF is extracellular fluid, and f is the selected total body water fraction. In teaching models, liters are usually estimated from kilograms by treating 1 kg of water as approximately 1 L.
How to interpret results
A larger total body water estimate usually reflects either a higher body mass or a higher selected water fraction. A larger intracellular fluid value means more water is assigned to the cellular compartment, while larger extracellular values increase both interstitial fluid and plasma estimates.
Results are usually reported in liters, together with percentages of total body water and sometimes percentages of body mass. The most important interpretation is the distribution pattern: about two-thirds of total body water is intracellular, while about one-third is extracellular, with extracellular fluid further split mostly into interstitial fluid and partly into plasma.
- Do not treat these values as exact patient-specific measurements.
- Check the body weight unit carefully before calculating.
- A custom TBW percentage override changes every downstream compartment.
- Plasma is only a fraction of extracellular fluid, not of total body water directly.
Example: if body mass is 70 kg and the total body water fraction is 0.60, then total body water is \(70 \cdot 0.60 = 42\) L. From that estimate, intracellular fluid is about 28 L and extracellular fluid is about 14 L.
This tool is best used for foundational physiology, compartment visualization, and teaching approximations. It is not intended for pathology correction formulas, detailed IV fluid planning, or renal and acid-base calculations, which require more advanced clinical models.
