Human fluid metabolism
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. 
In virtually all fluids, not just the concentration, but the ratios of four principal ions are critical:
- 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|
|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.