We can live quite a long time without food, until we have used all our reserves, but we can survive only a couple of days without water. This is because water is the largest component in our body, from 75% body weight for a baby to 50% for elderly people and we cannot make any reserves. As we lose around 2.6 liters daily (sedentary person of normal weight) we must replenish it daily too.
What is the role of water in our body?
Two thirds of our water is found in tissue cells and one third is extracellular, that is in organs, blood, lymph and other fluids.
In the cells, water makes chemical reactions possible by dissolving and transporting minerals and other nutrients.
In the blood, lymph and urine, water helps transport nutrients, cells, hormones, and products of metabolism.
In organs, water allows secretions, such as hormones, enzymes and salts used for digestion as well as the products of digestion. Water is also an important element of eyes and tears.
In muscle and joints, water plays a role in structure and mechanical function.
Finally, in skin, water participates in body temperature regulation through evaporation by perspiration/transpiration.
How is the water balance regulated?
As water cannot be stored in our body, it must remain at a constant level; water intake must equal water losses.
We lose water through the skin (350 ml) on top of normal perspiration (150 ml), through respiration (450 ml), through the intestines (150 ml) and in urines (1500 ml). Some of those losses can vary; for example perspiration increases if we are in a warm environment, if we practice sport, if we have a fever or under a strong emotion.
Losses in urine and water intake are the main ways by which water balance is regulated in our body.
Our kidneys filter and reabsorb water permanently before excreting the excess. If the degree of hydration (level of water) in our body is insufficient, the kidneys increase reabsorption and the volume of urine is decreased to spare the water. On the opposite, when the level of water in the body is appropriate or in excess, kidneys do not change reabsorption, and get rid of the excess through increased excretion, i.e. a larger volume of urine.
We get around 800 ml of water from our food and an additional 300 ml is produced by metabolism (transformation of food into energy). Since we need around 2.6 liters to replenish our daily losses, 1.5 liter should come from the water