Dehydration is the result of a negative fluid balance within the body. It occurs when more water leaves the cells and the body than is replaced by drinking water and consuming water-rich foods. Every day you lose roughly two liters of water in the form of vapor when you exhale, and more noticeably when you sweat and urinate.
Mild dehydration can occur simply by not consuming enough water on a daily basis. However, severe dehydration is often the result of serious injuries to the skin, diseases, and illnesses, extreme heat exposure, excessive physical training or a lack of safe drinking water.