Urea is a small organic molecule composed of carbon, nitrogen, oxygen, and hydrogen. Urea is a common constituent of blood and other body fluids. Urea is formed from ammonia in the kidney and liver. Ammonia is produced by the breakdown of protein during tissue metabolism.
Ammonia is very toxic.
Urea is very non- toxic and can be at very high levels without causing any problems. If urea was not produced from ammonia we would very quickly become ill every time we ate a meal containing protein. The conversion of ammonia to urea, primarily in the liver, prevents ammonia toxicity. Urea is then excreted from the body in urine. Urea diffuses readily into body tissue spaces with water.