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Body Fluid Compartments

For medical students2 min readUpdated 2026-10-10

Virtually any pathology in the human body inevitably leads to shifts in water and electrolyte balance. Therefore, understanding how body water is distributed is critical for evaluating the clinical picture of various diseases. All fluid in our body is strictly divided into two key compartments: intracellular and extracellular. Although active exchange occurs between these spaces every second, their chemical compositions are not identical.

Intracellular fluid (ICF)~31% of body weight (averaging about 24 liters of fluid)

Extracellular fluid (ECF)~22% of body weight (approximately 15 liters)

Blood plasma~4% of body weight (2 to 2.5 liters)

DyshydrationAny water balance disorder (hypo- or hyperhydration)

Intracellular Fluid Compartment

Intracellular fluid represents the largest fluid reservoir in the body, accounting for an average of 31% of total body weight, which corresponds to approximately 24 liters in absolute terms. An important clinical feature of this compartment is its inertia: during pathological processes, the volume of fluid inside cells begins to change significantly later and more slowly than in the surrounding extracellular environment.

Intracellular water is heterogeneous and exists in three distinct physicochemical states:

Bound water — firmly bound to various hydrophilic organic and inorganic molecules.
Adhered ("attracted") water — accumulated and retained on the surface of large colloidal molecules.
Free (mobile) water — this fraction is the most dynamic. Its volume changes most significantly and rapidly during any fluctuations in cellular activity, both under normal physiological conditions and in pathology.

Extracellular Fluid Compartment

The extracellular compartment accounts for an average of 22% of body weight (about 15 liters). It is not uniform, but structurally divided into three functionally distinct fluids: blood, interstitial fluid, and transcellular fluid.

Blood Plasma (Intravascular Fluid) Located within the vascular bed, it occupies approximately 4% of body weight (2 to 2.5 liters). Plasma is a complex solution composed of:

About 90% water.
9% various organic substances, with the lion's share (about 6%) being proteins.
1% inorganic substances.

Interstitial Fluid This is the largest pool of the extracellular sector, accounting for 18% of body weight (about 12 liters). Interstitial fluid directly bathes tissue cells. Its chemical composition is extremely close to that of blood plasma, and a constant, free exchange of components occurs between these two environments.

Transcellular Fluid Occupies the smallest volume — only about 1.5% of body weight. This fluid is strictly localized within specialized spaces and body cavities. The transcellular pool traditionally includes:

Cerebrospinal fluid (CSF).
Synovial fluid lubricating joints and tendons.
Digestive secretions (gastric and intestinal juices).
Primary urine (fluid in Bowman's capsule and renal tubules).
Contents of serous cavities (pleural, peritoneal, and pericardial cavities).
Aqueous humor of the eye.

Water Balance Disorders (Dyshydration)

Normally, the body maintains a strict water balance. If fluid accumulates in excess, it is referred to as a positive balance. If a fluid deficit occurs, the balance becomes negative.

In pathophysiology, all water balance disorders are united under the general term dyshydration. Broadly, they are divided into two main groups:

Hypohydration — a pathological decrease in fluid volume below physiological norms, the extreme manifestation of which is severe dehydration.
Hyperhydration — excessive accumulation of fluid in the body. In clinical practice, the most frequent and noticeable form of hyperhydration is edema.

For precise diagnosis, dyshydrations are classified according to two main criteria.

1. By Extracellular Fluid Osmolality:

Hypo-osmolar form: plasma osmolality falls below 280 mOsm/kg H₂O.
Iso-osmolar form: plasma osmolality remains within normal limits.
Hyperosmolar form: plasma osmolality rises above 300 mOsm/kg H₂O.

2. By Predominant Localization (Affected Sector):

Cellular dyshydration.
Extracellular dyshydration.
Mixed form (when both compartments are affected).

Mnemonic

To quickly remember ECF volumes in descending order, use the acronym IPT: Interstitial (18%) → Plasma (4%) → Transcellular (1.5%). Together they make up the 22% of the extracellular sector.

Frequently asked questions

How does the chemical composition of interstitial fluid differ from blood plasma?

The chemical composition of interstitial fluid and blood plasma is similar, but the key difference is the protein concentration. In blood plasma, it is significantly higher, at approximately 70 g/L.

What are the main pathogenetic factors in the development of edema?

The main pathogenetic factors of edema include:

Hydrodynamic/hemodynamic (hydrostatic): disruption of Starling forces; elevated hydrostatic capillary or venous pressure increases fluid filtration into the interstitium and inhibits reabsorption.
Oncotic (hypoproteinemic): decrease in plasma oncotic pressure and/or increase in interstitial oncotic pressure; the primary mechanism is a reduction in the effective oncotic reabsorption force of blood plasma.
Osmotic: in hypo-osmolar hyperhydration, interstitial osmotic pressure is lower than intracellular pressure, causing water to shift from the interstitium into the cytoplasm, leading to cellular edema.
Membranogenic: increased vascular-tissue permeability facilitates the escape of water and plasma proteins into the interstitium.
Lymphatic: impaired lymphatic drainage hinders the removal of excess filtrate and protein from the interstitium.

What organic substances are present in blood plasma besides proteins?

Blood plasma contains about 90% water and 10% dissolved substances (dry residue); organic substances account for 9%, and inorganic substances 1%. Proteins comprise about 2/3 of the solid residue, while low-molecular-weight substances and electrolytes comprise about 1/3.

Which fluid compartment reacts to the development of a disease first?

The extracellular fluid always reacts first. The intracellular sector is more stable, so the volume of fluid inside the cells themselves begins to change much later and much slower.

Which fraction of intracellular water is used up during changes in cellular activity?

The free (or mobile) water is the most dynamic. Its volume undergoes the most significant changes during any shifts in cellular activity, whereas bound and adhered water remain relatively stable.

What is transcellular fluid and what is its volume?

These are highly specialized fluids with a total volume of about 1.5% of body weight. This group includes CSF, synovial fluid, gastric and intestinal juices, primary urine, aqueous humor, and fluids of the serous cavities (pleura, pericardium, peritoneum).

How are dyshydrations diagnosed based on plasma osmolality?

Physicians evaluate the concentration of osmotically active substances. If the value drops below 280 mOsm/kg H₂O, hypo-osmolar dyshydration is diagnosed; if it exceeds 300 mOsm/kg H₂O, hyperosmolar dyshydration. Normal values indicate an iso-osmolar disturbance.

Go deeper

Mechanisms of fluid exchange between blood plasma and the interstitial space.
Pathogenesis of edema formation in various types of hyperhydration.
Clinical consequences of cellular and extracellular hypohydration (dehydration).
The role of hydrophilic molecules and colloids in binding intracellular water.
Causes of plasma osmolality shifts outside the 280–300 mOsm/kg H₂O range.