Urea

Urea is a waste product produced by the breakdown of proteins by
the liver. Once produced, urea is filtered from the blood by the
kidneys and removed from the body in the urine. Measuring the urea
levels in the blood can therefore show how well both the liver and
the kidneys are functioning. Even if one of your kidneys is severely
damaged but the other is functioning well, the urea level may still be
in the normal range.