Why do we need water?
Water is a fundamental part of our lives. It is easy to forget how completely we depend on it. Human survival is dependent on water. The average adult body is 75% water. Our brain is made up of 85% of water. 2/3 of your body weight is water. A human embryo is more than 80% water. A newborn baby is 74% water. Everyday your body must replace 2 1/2 quarts (about 2l) of water. Since such a large percentage of our body is water, it plays a very important role in the body’s functions. Water carries nutrients and oxygen to cells, thus keeps our cells alive and healthy. Water aids in digestion and absorption of food, water regulates body temperature and blood circulation and removes toxins and other wastes from cells and from our body. ‘Body water’ also cushions joints and protects tissues and organs, including the spinal cord, from shock and damage.

Chronic dehydration may cause certain problems for the body, including hypertension, asthma, allergies, and migraine headaches. An average person needs to drink about 8 glasses of water per day. By the time you feel thirst the body is already mildly dehydrated. Water is essential to life. Of our body water about two thirds is contained within the cells, and one third is extra cellular in body fluids. Much of the water in the body is used repeatedly, for example digestive juices, which are excreted by the glands into the digestive tract, and then reabsorbed to carry simple substances into blood and lymph.

Water is however lost from the body in urine, faeces, from the lungs (when breathing out), and from the skin (in perspiration). The kidney is the main organ for regulating the amount of water in the body, and output reflects intake. The kidney can deal with excess fluid but there is a minimum volume of urine which an individual must excrete to eliminate waste products from tissues. This is approximately 1 litre per day, so the minimum intake of fluids is 0.75l. A greater intake is highly desirable to allow for variations in the amount of water lost through the skin and lungs; so daily intake should be 1.5l or more.

Fluid intake is largely determined by social custom and habit, and in most cases, if thirst is satisfied; the body’s needs for water will be met. Water intake comes from various sources, including all types of drinks, and some solid foods. This is called "metabolic water", being water which is resulting from the metabolising of foods. Vomiting and diarrhoea will result in water loss.

If salt is lost through excessive sweating, the thirst mechanism may not work adequately, and more water than is necessary to satisfy thirst should be drank. The salt loss should also be replaced as the electrolyte balance will be disturbed. Sodium and potassium are important in maintaining electrolyte and chemical balance between the tissue cells and blood:

  • sodium remains outside the cell in the extracellular fluid
  • potassium is located inside the cell wall
  • these ions are in constant flux

When a muscle undergoes contractions or an impulse travels along a nerve, there is a rapid exchange of these two elements across the cell membrane. The exchange creates an electrical impulse.

The water balance in the body is also important for acid-base equilibrium. Human life is only possible if the blood is kept within a narrow range of alkalinity. In health, the blood is maintained at a pH of 7.4. This level is maintained by two mechanisms, the excretion of carbonic acid (CO2 + H2O) through the lungs, and the excretion through the kidneys of urine, which is either acid or alkaline.
Water has three basic functions to perform in our bodies:
  • to transport nutrients and oxygen to our cells
  • to carry away toxic wastes from the cells
  • to regulate our body temperature through hydration
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