The extracellular fluid can be subdivided further into the interstitial fluid, which is the fluid that can be found surrounding the cells, and plasma, which is the aqueous portion of blood.

The major cation in extracellular fluid, both in the interstitial fluid and in the plasma, is sodium (Na+), and the major anions are chloride (Cl-) and bicarbonate (HCO3-). The plasma makes up about 55% of the blood, while the remaining 45% is mostly made of red blood cells, white blood cells, and platelets.

Learn about how these different body fluid compartments interact with each other on Osmosis: