After the liver processes metabolites to produce urea and other byproducts, these travel with the blood to the heart, then they are oxygenated, and some of it travels through the renal artery to the kidneys.

Urea in water can decompose into ammonia which is toxic. 

The chance of urea decomposing into ammonia is high while travelling in the blood (i.e taking long route) and in the bladder, but it still does not happen. Why not?