What protects the urinary system organs from the urine itself?

Answer 1

Nothing; your insides are soaked with urine. A less jarring way of putting it is that urine, by itself, is not all that bad as liquids go. It is, in fact, a filtrate of the blood. The kidneys are filters, and when blood passes through the kidneys the kidney output is urine. Urine is as clean as the blood itself, unless there is some infection in the kidneys, bladder or urinary tract. So wherever there is blood in the body, and that covers very close to all of it, there is also urine ready to be filtered out. Urine does not remain a sterile or nearly sterile liquid for very long outside of the body. There may be germs even at the point of exit that contaminate it. Anyone knows that urine will reek if left out or remains on surfaces for any length of time. Various bacteria live happily in it.