After the liver processes metabolites to produce urea and other by-products, these travel in the blood to the heart, then they are oxygenated, and some travel through the renal artery to the kidneys.

Urea in water can decompose into ammonia which is toxic, as mentioned in a [textbook][1] page no 170 (6), under production of ammonia.

Reactant and product are at equilibrium. Also a gaseous product, CO<sub>2</sub>, is present which could potentially escape during oxygenation of blood. Hence, according to Le Chatelier's principle, an equilibrium shift towards  the product is possible

NH<sub>2</sub>CONH<sub>2</sub> + 2H<sub>2</sub>O → (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> ⇌ 2NH<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub>

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?


  [1]: http://ncert.nic.in/ncerts/l/lech107.pdf