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.
Jeff
jcrows.com