Extensive bleeding from the upper gastrointestinal (GI) tract will cause an increase in Urea (BUN).
HOW? Blood is full of proteins (i.e Hemeglobin, Immunoglobins) which are absorbed in the GI tract. Since it is an upper GI bleed (above the ligament of treitz) there is time for adequate absorption.
Blood Urea Nitrogen (BUN) reflects the end product of the metabolic breakdown of protein
Hence when there is bleeding => protein breakdown & absorption => increase in Urea
*It is often debated that a GI bleed (if severe enough) can also result in hypovolemia -> causing prerenal azotemia and raise the BUN
NOTE: Cannot be utilized in the setting of diuretics or CKD
Witting MD, Magder L, Heins AE, Mattu A, Granja CA, Baumgarten M (May 2006). “ED predictors of upper gastrointestinal tract bleeding in patients without hematemesis”. Am J Emerg Med 24 (3): 280–5. doi:10.1016/j.ajem.2005.11.005. .