Urea is the major metabolite product of protein catabolism. The biosynthesis of urea from ammonia is exclusively carried out by hepatic enzymes. More than 90% of urea is excreted through the kidneys, with the remainder excreted through the gastrointestinal tract or skin. Blood urea concentrations can be increased by numerous factors linked to prerenal causes (increased protein catabolism, as in haemorrhage into gastrointestinal tract, shock, some chronic liver diseases) or renal/postrenal causes (acute or chronic renal diseases, postrenal obstruction to urine flow). Uremia is also increased by high-protein diet, state of dehydratation, muscle wasting (as in starvation). The determination of urea rate is used together with the determination of creatinin rate to discriminate between prerenal (normal creatinin) and renal/postrenal (increased creatinin) disorders.