About 60 percent of an adult human’s body mass is water, a proportion that varies with age, sex and body composition but underlies nearly every physiological process from nutrient transport to temperature regulation.
Where water is stored
Body water is distributed between intracellular fluid inside cells and extracellular fluid that includes blood plasma, lymph and interstitial fluid; together these compartments enable circulation of nutrients and removal of waste products throughout the body.
Key functions of body water
Water regulates body temperature through sweating and evaporation, aids digestion by dissolving and transporting nutrients, supports blood circulation by maintaining plasma volume and helps cushion joints and organs, making hydration essential for everyday function and long‑term health.
How much is needed
Daily water balance depends on intake and losses from urine, sweat, breathing and bowel movements; typical daily losses range with activity and climate so regular fluid intake and a diet containing water‑rich foods help keep the body’s ~60 percent composition stable.
Clinical and practical notes
Dehydration impairs cognitive and physical performance, strains the cardiovascular system and affects digestion; monitoring urine colour, drinking when thirsty and adapting intake to exercise or hot weather are simple practical steps to protect hydration and the many bodily processes that depend on water.