Because water can absorb and transfer heat well, the human body uses it to stabilize temperature. Water has a relatively high heat capacity, meaning it can absorb a lot of heat before its temperature rises. This trait allows the water in every cell of the human body to act as a buffer against sudden temperature changes. Blood, which is made largely of water, moves heat away from the extremities and toward the vital organs when it needs to conserve heat, it flows towards the skin surface to release excess heat when necessary, and it transports muscle heat away as needed. Water also helps expel excess heat from the body as water vapor from the lungs and sweat on the skin.
TL;DR (Too Long; Didn't Read)
Water has a relatively high specific heat, or heat capacity, meaning it can absorb a lot of heat before its temperature rises. This trait helps it to stabilize temperature in its surroundings. The water in every cell of the human body act as a buffer against sudden temperature changes. Blood is largely composed of water, and it moves toward vital organs when the body needs to conserve heat, and toward the extremities and skin when the body is in danger of overheating. Water evaporates from the skin and lungs to cool the body.
Water Absorbs and Moves Heat
Burning calories through physical work or exercise generates heat from muscles. Water comprises up to 75 percent of muscle mass. One calorie will heat one gram of water by one degree Celsius - that's ten times more heat absorption than copper. The water in the muscle cells exchanges heat with the water in blood, which carries the heat away. In the brain, the hypothalamus senses the heat increase in the blood, and activates the sweat glands.
Sweat Cools By Evaporation
Your skin, made wet by its sweat glands, serves as a heat exchanger. Evaporative cooling occurs because the fastest-moving (hotter) water molecules escape as vapor, leaving behind the slower-moving (cooler) molecules. The heat driving the vapor is called the heat of vaporization. This is why a hot beverage or bowl of soup goes cold; the escaping vapor robs the heat. Air flow across the skin increases this effect. That's why, when you're wet with sweat, fans or a breeze help to cool you off more quickly.
Water Cools the Body
The normal human body temperature is 98.6 degrees Fahrenheit, yet people survive when the ambient air temperature is higher. Air temperatures in deserts can reach higher than 120 degrees Fahrenheit, and sunlight adds to the heat absorbed by the body. In these conditions, loose, billowing clothing allowing air flow to help evaporate sweat is the norm. In heat-stressing conditions or in heavy exertion, the body might need as much as 10 liters of water per day to maintain healthy hydration.
The Importance of Hydration
The water lost cooling the body through sweat (as well as during other body processes) must be replaced. That's why you often hear "drink plenty of fluids" as advice for anyone working or playing hard. But sweat also excretes electrolytes such as sodium, potassium, chloride and calcium. That's why sport beverages include these among their ingredients.