Human fluid metabolism

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Basic compartments

At the most basic, the physiology of human fluid metabolism splits into the extracellular fluid compartment and the intracellular fluid compartment. Even with that separation, there is a constant exchange of water, ions, and non-ionized substances between the compartments and subcompartments. [1]


In virtually all fluids, not just the concentration, but the ratios of four principal ions are critical:[2]

  • Sodium (Na+) and potassium (K+)
  • Chloride (Ca+2) and bicarbonate (HCO3+

Several other ions and molecules also are important, but sodium:potassium balance, for example, is fundamental to cell electrical activity.

Substance Extracellular volume Intracellular volume
Sodium 135-145 mEq/L 10-20 mEq/L
Potassium 3.5-5.0 mEq/L 130-140 mEq/L
Chloride 95-105 mEq/L
Bicarbonate 22-26 mEq/L
Glucose 90-120 mg/dL
Calcium 8.5-10 mg/dL
Magnesium 1.4-2.1 mg/dL 20-30 mEqL
Urea nitrogen 10-20 mg/dL 10-20 mg/dL

At this point in the diagram, we only distinguish between plasma and interstitial fluid, not urine, lymph, sweat, and other fluids within the extracellular compartment.

Blood versus fluid

Again as a basic idea, blood is plasma that carries blood cells and additional circulating chemicals. Many clinical measurements involving blood chemistry are made on the easier-to-collect blood serum, which is the fluid remaining after blood clots. Serum does not circulate in the body, although it can accumulate near blood clots.


  1. Arthur C. Guyton and John E. Hall, ed. (2000), Guyton and Hall Textbook of Medical Physiology, vol. Tenth Edition, W. B. Saunders, ISBN 072168677Xpp. 2-4
  2. Richard A. Preston (2002), Acid-Base, Fluids and Electrolytes Made Ridiculously Simple, McMaster, ISBN 0940780313, p. 5