Water is a critical ingredient for humans, animals and plants. Though humans can live without food for several days or weeks, they can live only a few days without water. Water, therefore, must be replenished often in order to fulfill its numerous bodily functions.
To Maintain Body Temperature
Water has a high heat capacity, which means it requires more energy than many other substances to increase its temperature. An example of the high heat capacity of water is that heating watery foods often take longer to heat than foods with low water content. Because the body is constituted mainly of water, it takes a large amount of energy to raise or lower one's body temperature, therefore allowing the body to maintain a relatively stable body temperature.
To Cool the Body
Body temperature often rises, triggering the body to sweat due to exercise, illness, environment or other factors. Water keeps bodies cool by releasing sweat when the body gets too warm. When this happens, the sweat evaporates off the skin and cools the body, allowing the body to maintain homeostasis. On a humid day, water does not evaporate as well, and sweat is significantly less effective in cooling.
To Balance Electrolytes in the Body
The human body contains electrolytes, which are ions that can conduct electrical currents. Examples of these include sodium, chloride and potassium. Every cell requires a certain balance of electrolytes, and extracellular fluids also require a very particular balance of electrolytes. The proportion of these electrolytes must stay within a precise range. The body is able to stay within this range by excreting water when necessary and triggering thirst mechanisms in the body when necessary.
To Provide Body Fluids
All body fluids are comprised of mostly water. About two thirds of the body's fluid is in intracellular fluid, while one-third is in extracellular fluid, like the fluid between cells and the fluid in the blood. Water provides shock absorption, cleansing and protection for the body. A fetus, for example, is protected by amniotic fluid, which is mostly water. Water also allows the body to produce tears and saliva, and lubricates joints, allowing them to move easily.
To Maintain Body pH
The body's pH balance is important to carry out metabolic reactions in optimum conditions. Water is a major component of the maintenance of pH in the body. One of the major reactions in the body occurs when carbon dioxide reacts with water, forming carbonic acid. Carbonic acid then detaches to form hydrogen and bicarbonate, thereby increasing the body's acidity and lowering the body's pH.