Picture a sealed bag divided into two rooms. The large room is the inside of the cells. The other room is the space outside the cells, and it has two parts: the fluid that bathes the cells, and a thin channel that is the blood inside the vessels. Sodium is the main guest outside the cells and potassium is the main guest inside them. Water goes where the guests are, so the outer space fills when sodium is added and empties when sodium leaves. Everything below hangs on this picture.
In a healthy adult man weighing 70 kg, water is about 60% of body weight, roughly 42 litres.
How much of the body is water?
The 60% figure is an average, not a constant. It falls with age. It also differs between the sexes.
Where does that water sit?
The split matters because the two compartments hold different solutes and respond differently when fluid is lost.
The intracellular compartment holds about two thirds of total body water, 28 litres.
The extracellular compartment holds the smaller share, about one third of total body water, 14 litres. Roughly three quarters, about 10.5 litres, is interstitial fluid. The remaining quarter, about 3.5 litres, is plasma inside blood vessels.
Why does body fat change the picture?
Muscle stores more water than fat does. Muscle is water-rich, while fat tissue holds little water. As fat tissue increases, water percentage falls. Obese individuals sit below the average water percentage.
Which charged particles sit where?
Sodium is the main cation of extracellular fluid, and potassium is the main cation inside cells. Outside the cells the main anions are chloride and bicarbonate. Inside the cells they are phosphate and protein.
Why does sodium decide where water goes?
Sodium is confined almost entirely to extracellular fluid. Extracellular fluid volume follows sodium. Water follows sodium too, filling or emptying the extracellular space with it.
Plasma is part of extracellular fluid, so a plasma sodium measurement reports the concentration of that compartment. Why this sodium concentration sets tonicity is explained in Osmolality, Osmolarity and Tonicity.