There is a primary division between two body fluid compartments – the intracellular fluid (ICF) and the extracellular fluid (ECF). The ICF represents all the fluid in the cells, whereas the ECF represents everything outside the cells. They are separated by a semi-permeable plasma membrane, meaning that nothing can diffuse between them. The ICF contains a high concentration of potassium ions and a low concentration of sodium and chloride ions compared the the ECF. This concentration is maintained by sodium/potassium pumps in the plasma membrane which actively transports sodium ions out of the cell and potassium ions into the cell.