In multicellular organism, transport functions are conceptualized on the tissue level as well as the cellular level. The fluid inside individual cells is called the intracellular fluid. That outside is the extracellular fluid. These are the two main fluid compartments. Concentrations of many substances such as electrolytes, sugars, and proteins depend on the fluid compartment. For example, the potassium concentration is 100 times greater in the intracellular fluid compartment in humans than in the extracellular compartment, due to the activity of sodium potassium pumps (the converse is true for sodium, which is more concentrated outside the cell).