Urea is the nitrogen-containing breakdown product of protein metabolism. The nitrogen from the proteins is converted to ammonia in the liver, which is combined with carbon dioxide to form urea. This is continuously excreted via the kidneys but also via sweat. Urea is one of the urinary substances and is a measure of the amount of Toxic, nitrogen-containing urinary substances responsible for uraemia and kidney damage.... in the blood, which also arise from protein metabolism. Its level in the blood varies, depending on the amount of protein consumed.