The human brain is about 75% water.

The human brain weighs about 1.4 kilograms and is mostly made of water. This water helps maintain the shape of brain cells called neurons. It also forms cerebrospinal fluid which acts as a cushion to protect the brain from injury.Water is necessary for the glymphatic system to remove waste from brain tissue. Even losing 1% to 2% of your body's water can make it harder to focus. This mild dehydration can also cause short-term memory loss and make tasks feel more difficult.When the body lacks water, the brain can physically shrink in size. This happens because the brain loses fluid and pulls slightly away from the skull. This process often causes headaches and forces the brain to work harder to complete simple tasks.The hypothalamus in the brain monitors salt levels in the blood to trigger thirst. Staying hydrated ensures that electrical signals move quickly between brain regions. An adult brain processes about 150 milliliters of protective fluid every day to keep the system clean.