Complex sentence: Urobilin is produced when Urobilinogen is oxidized by intestinal bacteria; it can also be produced when urobilinogen is exposed to the environment upon urination, which again oxidizes it to urobilin.
Split into simpler sentences: Urobilin is produced when Urobilinogen is oxidized by intestinal bacteria. Also when urobilinogen is exposed to the environment upon urination, it is oxidized to urobilin.