Water makes up a significant portion of the human body, but the exact percentage can vary depending on age, sex, body composition, and overall health. Estimates often range from 10% to over 60%, and in some tissues, even higher. For example, muscle tissue tends to hold more water than fat, and organs like the brain, heart, and lungs have particularly high water content. Hydration plays a vital role in nearly every bodily function—regulating temperature, transporting nutrients, lubricating joints, and supporting cellular activity. Even mild dehydration can affect mood, energy, and concentration. While the body constantly loses water through sweat, breathing, and elimination, it’s also replenished through drinking fluids and eating water-rich foods like fruits and vegetables. Knowing how essential water is to physical function helps explain why hydration is a cornerstone of daily health and performance.