Urine is not a dirty and toxic substance rejected by the body. Urine is a by-product of blood filtration, not waste filtration.
Medically it is referred to as "plasma ultrafiltrate". It is a purified derivative of the blood itself, made by the kidneys--whose principal function is not excretion but regulation of all the elements and their concentrations in the blood. Urine can be compared to leftovers from a meal, and this metaphor may help us understand why our bodies excrete elements that are valuable to our health and well-being.
Nutrient-filled blood passes through the liver where toxins are removed to be excreted as solid waste.
Eventually, this purified "clean" blood undergoes a filtering process in the kidneys, where excess water, salts, vitamins, minerals, enzymes, antibodies, urea, uric acid and other elements not usable at that time by the body are collected in the form of a purified, sterile, watery solution that is urine. The function of the kidneys is to keep the various elements in the blood balanced.
The important elements in the blood are not filtered out because they are toxic and harmful to the body, but simply because the body does not need a particular concentration of an element at that specific point in time.
It is this very regulating process of the kidneys that allows us to eat and drink more than our bodies need at any one time.