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.

At this point in the diagram, we only distinguish between plasma and interstitial fluid, not urine, lymph, sweat, and other fluids. It is practical to device by compartments.