Subject: Why urea is toxic?

We all heard that it is not a good practice in keeping the urine in the 
body (i.e. the urinary bladder). People says that the urine is toxic. The 
textbook mention about urea toxicity. But actually how toxic it is and how 
does it cause harm to our body?

In addition, as the removal of uric acid requires even less water than 
urea. It can save up more water for terrestrial animals. Why human beings 
do not develop such a mechanism? Is there any negative side-effects from 
excreting uric acid instead of urea?

Re: Why urea is toxic?