Why does GI haemorrhage increase serum urea?
Any patient with GI bleeding will have an increased serum urea.
Urea is an end product of protein metabolism. Digested blood contains loads of proteins so it's as good as having a meal rich in proteins. Blood proteins include hemoglobin, Immunoglobulins, etc.
Also, in conditions where renal perfusion is decreased, such as hemorrhagic shock, BUN will rise.
Why? In prerenal failure like hemorrhagic shock, you have less renal blood flow, you will filter less and GFR will decrease. When GFR decreases, it gives the proximal tubule more time to reabsorb urea. Thus, there is an increase in serum urea.
Read more on pre renal failure here.