Living things need water because it is essential for cells, digestion, temperature control, and chemical reactions, and Earth has it because of its distance from the Sun and its formation history.
Why we need water
- Cells: about 60% of the adult body is water.
- Temperature regulation through sweat.
- Digestion and nutrient transport.
- Cushioning joints and the spine.
- Chemical reactions, blood volume and waste removal.
Daily needs
About 3.7 liters (125 oz) a day for men and 2.7 liters (91 oz) for women from all food and drink (National Academies).
Survival
A person can survive roughly 3 days without water.
Body composition
Water makes up about 60% of an adult’s body weight, about 75% for infants and 50% to 55% for older adults. The brain and heart are about 73% water, the lungs about 83%, muscles and kidneys about 79% and bones about 31%, according to USGS data.
Functions
- Regulates body temperature through sweat.
- Carries nutrients and oxygen in blood.
- Lubricates joints and cushions the brain and spinal cord.
- Helps digestion and flushes waste through urine.
- Forms saliva and mucus.
- Required for chemical reactions in cells.
Daily needs
The National Academies suggest about 3.7 liters (125 ounces) of total water per day for men and 2.7 liters (91 ounces) for women, including water from food (about 20%) and other beverages. Needs rise with heat, exercise, pregnancy and breastfeeding.
Survival limits
A person can live about 3 days without water, though it depends on temperature and activity. Humans can live weeks without food. Losing 1% to 2% of body weight in water reduces mental performance, and a loss of 10% or more is life threatening.
Dehydration signs
Thirst, dark urine, dry mouth, fatigue, headaches, dizziness. Severe: confusion, rapid heartbeat, fainting.
Overhydration
Drinking excessive water in a short time can cause hyponatremia (low blood sodium), which can lead to seizures and death, particularly in endurance athletes.
Planet
About 71% of the Earth’s surface is covered by water, but only 2.5% is fresh water and much of it is frozen or underground.
Clean water
The WHO reports that about 2.2 billion people lack safely managed drinking water.
Water in the body
Adults are about 50 to 60 percent water: roughly 60 percent in men and 55 percent in women, according to the US Geological Survey, with the brain and heart about 73 percent water, lungs about 83 percent and bones about 31 percent. Water carries nutrients, regulates temperature through sweat, lubricates joints and removes waste via urine. The National Academies recommend a total water intake of about 3.7 liters a day for men and 2.7 liters for women, including water from food, which supplies about 20 percent.
Survival limits and global access
A person can survive only about three days without water, depending on conditions. Dehydration signs are dark urine, dry mouth, dizziness and fatigue, and overdrinking can cause hyponatremia, low blood sodium, which is dangerous. Water covers about 71 percent of Earth’s surface, but only about 2.5 percent is fresh, and most of that is frozen. The World Health Organization and UNICEF report that about 2.2 billion people lacked safely managed drinking water in 2022. Clean water prevents diseases such as cholera and diarrhea.
Based on data from the US Geological Survey and the World Health Organization
