It is mostly water and contains a mixture of waste products, minerals, and a wide variety of chemicals that the body needs to operate its daily metabolism. It is not exaggeration that the living body is made up mostly of water. After all is said and done, after living is no longer and all the water leaves us, there is only a handful left. Water gets into the system by the foods we eat and the liquids we drink. It is the case that many people do not drink water itself, so most of our body fluids come from the foods we eat.
Water gets out of the system through the kidneys, bowel, lungs, and skin.
Unnoticeable sweating, called insensible sweat, also accounts for a fair amount. It is not at all unusual for body weight to vary as much as three to five pounds a day, all depending on your fluid balance and fluid retention. The amount of fluids retained can vary with disease conditions that affect the circulation. Some common ones of these include hypothyroidism, high and low blood pressure, heart and kidney disorders, and diseases of the arteries and veins. Excess retention is much more common in women than men because of the effect of female hormones on the body's metabolism and circulation.