Body fluid compartments

Body fluid compartments describe how the body's water is distributed. Total body water makes up roughly 60% of body weight in an average adult male (less in women and the elderly, more in infants, because fat holds little water). That water divides into two major compartments: intracellular fluid (ICF), the water inside cells, and extracellular fluid (ECF), the water outside them.

The proportions follow the classic 60-40-20 rule: total body water is about 60% of body weight, intracellular fluid about 40%, and extracellular fluid about 20%. Within the ECF, roughly three-quarters is interstitial fluid bathing the tissues and one-quarter is plasma, the fluid portion of blood. A smaller transcellular compartment includes cerebrospinal, pleural, and other specialized fluids. For a 70 kg man, that works out to about 42 L of total body water — 28 L intracellular and 14 L extracellular.

The compartments differ in composition: potassium is the dominant intracellular cation, while sodium dominates the ECF. Water moves freely between compartments to equalize osmolality, so changes in sodium or water balance shift fluid between them in predictable ways — the basis for understanding dehydration, volume overload, and IV fluid therapy. Compartment volumes are measured with indicator dilution: tracers that stay in plasma measure plasma volume, those confined to the ECF measure extracellular volume, and ICF is calculated by subtraction.

The USMLE Step 1 tests body fluid compartments in renal physiology. Expect calculations using the 60-40-20 rule, tracer-based volume measurements, and Darrow-Yannet-style questions predicting how fluid gains or losses of different tonicities shift volume and osmolality between compartments.