Life on earth began in water. Water is the largest molecular constituent of the body. It makes up for about 60% of the total body weight in males and 55% in females.
It provides a medium for metabolic functions in the body. Enzymatic reactions occur in aqueous media. It maintains the form of cells, tissues and organs, helps in digestion, evacuation of body wastes, maintenance of body temperature, lubrication in joints, prevention of infections and many more such functions.
Water Distribution in the body
Of the total body water, 2/3 is in the cells and 1/3 is extracellular. Of the extracellular water, 2/3 is outside the vascular system and 1/3 is in blood.
Regulation of water balance
The osmolality( a measure of solute concentration in water) is held in a very narrow limit in the human body. This typically is 280 to 295 mosm/kg. If the osmolality increases in the body fluids, a hormone called ADH is produced at the base of the brain. This hormone increases thirst and decreases water excretion in the urine thus restoring osmolality.
If excess water is taken, in healthy humans, it is promptly excreted in urine. Even an intake of 8-10 liters in healthy persons may be tolerated.
Water quantities for balance in the body
Body losses of water
Water is required for cooling down of the body by formation and evaporation of sweat. About 1 ml of sweat is required for losing .58 ml kcal of body heat. Thus about 400-500 ml of water is used in this way depending on environmental temperature, wind velocity, dryness of air etc. 100 -200 ml may be lost in the formation of stools. Small amounts are also lost in vapour form during breathing.
Kidneys excrete about 800 mosm of daily solutes in salt and electrolytes, urea etc. Since maximum urine concentration in healthy adults is about 1200 mosm, 2/3 litre of water is lost in this way daily.
Gains of water from outside
Apart from water ingested directly in fluids and beverages, water is also contained in food. Some fruits may have almost 100% water while most will have 50 to 60% water content. Thus 500 ml to 600 ml of water is ingested in foods. Water in similar quantity is produced by metabolism of proteins and carbohydrates in the body.
Thus to maintain a balance a minimum of about 600-800 ml of water is required to be ingested in the liquid form. An intake of 1.5 to 3 l is sufficient for most cases for healthy living. In old people a litre more water results in better function of the kidney and bladder.