What exactly does urea do that harms our body? Does it interfere with our cellular processes or disturbs the colloidal onchotic pressure of our blood? I have heard that $NH_3$ damages the brain. What does it actually do?
The toxic effects of ammonia is mainly because of its alkaline nature. Mammals can convert ammonia to urea. Urea is a strong denaturant of proteins. Its chaotropic effects can also affect the cell membrane.