urine and urea

Urine is normally composed of water and wasted products filtered
form the body. The kidney produces urine. The other main function of the
kidney is to regulate fluid balance in the body. It performs this function by
using a selective osmosis system. Basically, the way it works is that
electrolytes (dissolved salts like sodium, potassium, calcium, carbonate,
chloride) are pumped back into or out of urine and blood so that in the end,
just the right amounts of electrolyte and water exit the kidney blood vein.
The rest ends up in urine. Interestingly, normal urine is sterile and has no
bacteria.