Water makes up about 60% of an adult man's body weight and a little less in women and older adults. It sits in separate compartments divided by cell membranes and capillary walls.
Inside the cells
About two thirds of body water is intracellular fluid. It is rich in potassium and low in sodium.
Outside the cells
The remaining third is extracellular fluid, rich in sodium and chloride. About three quarters of it is interstitial fluid around the cells and one quarter is plasma in the blood vessels.
How water moves
Water follows solutes by osmosis. Sodium mostly stays outside cells, so sodium sets the volume of the extracellular space and pulls water in or out of cells.
Plasma and tissue
Blood pressure pushes fluid out of the capillaries, while albumin holds it in. High venous pressure or low albumin lets fluid build up in the tissues as edema.
| Compartment | Share of body water | In a 70 kg man |
|---|---|---|
| Total body water | 100% | About 42 L |
| Intracellular fluid | About 2/3 | About 28 L |
| Interstitial fluid | About 1/4 | About 10.5 L |
| Plasma | About 1/12 | About 3.5 L |
At the bedside
Of 1 L of normal saline, only about a quarter stays in the blood vessels; the rest moves to the tissues. Dextrose 5% spreads through all compartments, so it does little for blood volume.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).