Body Fluid Compartments

Body water is not a single pool but is partitioned into compartments separated by cell membranes and capillary walls. The two primary compartments are the intracellular fluid inside cells and the extracellular fluid outside them, the latter further divided into interstitial fluid and blood plasma. The composition and volume of each compartment are tightly governed, and water moves between them according to osmotic gradients.

Definition

Body fluid compartments are the functionally distinct volumes into which total body water is divided — chiefly the intracellular fluid and the extracellular fluid (interstitial fluid plus plasma) — separated by semipermeable membranes across which water distributes according to osmotic equilibrium.

Scope

This topic covers how total body water is distributed across the intracellular and extracellular compartments, the ionic composition that distinguishes them, and the osmotic principles that determine how water shifts between compartments. It treats the compartments as a structural framework for understanding water and electrolyte physiology; it is not a clinical estimation or fluid-management guide.

Core questions

• How is total body water divided among the intracellular and extracellular compartments?
• What distinguishes the ionic composition of intracellular from extracellular fluid?
• How do osmotic gradients determine water movement between compartments?
• Why does adding or removing water or solute change compartment volumes differently?

Key concepts

• Total body water
• Intracellular fluid
• Extracellular fluid
• Interstitial fluid and plasma
• Osmotic equilibrium across cell membranes
• Effective osmoles and water shifts
• Cell volume regulation

Mechanisms

Total body water is conventionally divided so that roughly two-thirds lies within cells (intracellular fluid) and one-third outside them (extracellular fluid), with the extracellular fraction split between interstitial fluid and a smaller plasma volume. The cell membrane is freely permeable to water but not to most solutes, so water distributes until osmolality is equal across it; potassium dominates intracellular fluid while sodium dominates extracellular fluid, a gradient maintained by active transport. Because water follows effective osmoles, a change in extracellular osmolality drives water into or out of cells until equilibrium is restored, which is why disturbances of osmolality alter cell volume (danziger-2015, boron-2017, guyton-hall-2020, rose-postchel-2001).

Clinical relevance

The compartment model explains why changes in plasma sodium concentration affect cell volume, including in the brain, and why the type of fluid lost or gained matters for which compartment is affected. This entry presents the physiological framework that underlies such reasoning and is not a basis for individual fluid assessment or therapy.

Evidence & guidelines

Compartment volumes and composition are described in standard physiology and electrolyte-physiology texts and in reviews of osmotic homeostasis (danziger-2015, boron-2017, guyton-hall-2020, rose-postchel-2001). The conventional fractions are textbook approximations that vary with age, sex, and body composition.

History

The partition of body water into intracellular and extracellular compartments and the measurement of their volumes by dilution methods were established through twentieth-century physiological investigation, providing the quantitative basis for later work on osmoregulation (boron-2017).

Key figures

• Burton Rose
• Walter Boron
• Arthur Guyton

Seminal works

• danziger-2015

Frequently asked questions

How is total body water split between compartments?

As a textbook approximation, about two-thirds of total body water is intracellular and about one-third is extracellular; of the extracellular portion, most is interstitial fluid and a smaller part is blood plasma. These fractions vary with age, sex, and body composition.

Why does water move between compartments?

Cell membranes let water cross freely but restrict most solutes, so water moves until the osmolality on both sides is equal; a change in extracellular osmolality therefore shifts water into or out of cells until a new equilibrium is reached.