Total body water is close to 40 L. It is divided into intracellular and extracellular fluids. Intracellular fluids are those within the cells. They make up 2/3 of total body water (about 25 L). Intracellular ions are mostly K and phosphate ions. Extracellular fluids (ECF) are those outside the cells. They are about 1/3 of total body water and have primarily Na and Cl ions. The ECF is divided into interstitial fluids that bathe the tissues (12 L) and intravascular fluids that are within the blood vessels (3 L). Interstitial fluids are essentially plasma without proteins. Plasma and interstitial fluids are in free exchange (but not their proteins or cells).
Body water must remain in balance for health. The input must equal the output (about 2.5 L in and out daily). Input is from the diet (foods and liquids). Output is through sweat, insensible water loss through the skin and lungs, and in the urine and feces. Most of this is not regulated to preserve body water. Urine produces the most water loss at rest. Its volume is regulated to maintain water balance.
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