How and why do you urinate?

Urine is the body's way of eliminating fluid waste products. Urine contains waste or excess materials and toxic substances. Urine's main content is water. In fact 95 per cent of the urine excreted is water. The other major and common constituents are sodium - 0.4 per cent, ammonia - 0.05 per cent, phosphates - 0.6 per cent, urea - 2 per cent, sulfate - 0.2 per cent and creatine, urobilinogen, casts, in minute quantities. Normally, only water soluble substances are excreted in the urine.
Urine is produced by the kidneys as a process of cleaning the blood and excreting the unwanted soluble waste products from the body. The kidneys filter out the waste products such as toxins, excess water and mineral salts from the bloodstream. They filter, clean and purify about 150-180 liters of blood and recycles it daily. The urine formed in the kidney is drained by ureter into the bladder for storage. The urethra brings out the urine from the bladder, when time and place permit thus you get the "urge" to pee.