ECFs

ECFs, or extracellular fluids, are the body fluids located outside cells. In humans, the extracellular fluid compartment comprises plasma—the fluid portion of blood—interstitial fluid that bathes cells, and transcellular fluids such as cerebrospinal, pleural, peritoneal, synovial, and gastrointestinal secretions. The total volume of ECF is roughly one fifth of body weight in adults, about 14 liters in a 70-kilogram person, with interstitial fluid making up about three quarters of the ECF and plasma about one quarter. Transcellular fluids are a smaller, variable component.

Composition and osmolality: ECF contains high concentrations of sodium and chloride, with bicarbonate buffering and relatively low potassium compared with intracellular fluid. It carries nutrients, hormones, and waste products to and from cells. Osmolality is tightly regulated, typically around 275–295 mOsm/kg, to maintain cell volume and fluid distribution between compartments. Water movement between the ECF and intracellular fluid is governed by osmotic gradients.

Physiological role: The extracellular fluid compartment serves as the primary medium for the transport of oxygen, nutrients, and signaling molecules to cells, as well as the removal of metabolic wastes. It also participates in acid–base balance, temperature regulation, and immune defense.

Regulation: Fluid and electrolyte balance is controlled by thirst and vasopressin (antidiuretic hormone), the renin–angiotensin–aldosterone system, atrial natriuretic peptides, and renal handling of water and solutes. Adjustments to sodium and water reabsorption help maintain ECF volume and osmolarity.

Clinical relevance: Disruptions in ECF volume or composition can cause edema, dehydration, or electrolyte disturbances such as hyponatremia or hypernatremia, as well as acid–base disorders. Clinically, serum electrolytes are used as proxies for ECF composition, while specialized measurements quantify ECF volume as needed.