Around two-thirds of the water contained in our bodies is held within the cells, while the other third travels around the body in blood and fluids. The water in our bodies comes mainly from the fluid we drink. The body loses water all the time by evaporation from the skin and lungs and in the form of urine and faeces. When fluid is lost from the body, the concentration of salt in the blood increases, causing our thirst mechanism to be activated. Fluid loss increases when the body temperature is raised, for example in a centrally-heated house or office or during vigorous exercise.
Water is expelled from the body through perspiration and from the lungs, to cool the system.
Respiratory losses can be particularly high during vigorous exercise. Unfortunately, our thirst mechanism only switches on to signal an urgent need for fluid replacement in the body. If we only drank when we felt thirsty, it is unlikely that we would consume enough fluid to stay healthy It is important, therefore, to keep fluid intake high throughout the day, especially when in a heated environment, and before or after exercise. The amount of fluid we need depends on our circumstances. Select the following questions to find out more.