Urea
Metabolism of amino acids produces ammonia which is neurotoxic. Ammonia needs to be removed from the body. This occurs in the liver in a metabolic pathway known as the urea cycle to produce urea. Urea is then removed from the body in urine by the action of the kidneys.
The urea cycle
For biochemists and paediatricians the urea cycle is important to know as enzyme defects (in the rectangular boxes eg OTC) will stop this cycle from working properly causing ammonia to accumulate in the body - remember that ammonia is toxic.
Urea
in urine
Urea
in blood
Urea is freely filtered in  the glomerulus in the nephrons of kidneys. It is not actively reabsorbed or secreted by tubular cells. Some urea will diffuse out of the tubules back into the plasma. The rest passes out in the urine.
Nephron
For more about urea