Two thirds of the human body is water, give or take, spread through tissues.

The longer version

Water makes up the bulk of cells, the fluid between them, and the plasma that carries blood cells around. Its share is highest in tissues with plenty of active metabolism, such as muscle and brain, and lowest in bone and in fat, which is why the overall proportion differs between individuals and shifts with age.

Inside cells, water is the medium in which salts, proteins and other molecules meet and react; outside them, it transports nutrients and waste. Because so many processes depend on that balance, the body regulates water and dissolved salts closely through the kidneys and through thirst. Even small changes in that distribution affect how cells function, since enzyme activity depends on concentration.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.