Definitions:

Gastrointestinal bleeding comes in 2 flavors- upper GI bleed (UGIB) and lower GI bleed (LGIB).
Anatomically, the ligament of Treitz is the dividing line between upper and lower GI Bleeding.
Clinically the distinction is often made based on the presence or absence of melena vs. the presence or absence of bright red blood (BRB) and the orifice that the BRB is emanating from - i.e. hematemesis vs. hematochezia.
melena is typically described as black/dark tarry stool. Melena gets its dark color from hemoglobin that has been altered by stomach acid and/or digestive enzymes. The most common causes of melena/UGIB are gastric and duodenal ulcers, and NSAID induced gastritis.
hematemesis is when there's BRB in the vomit. It's pretty much always bad. Unless it's just from a Mallory Weiss tear in which case it's fine, don't worry about it.
hematochezia is when there's BRB in the poop. This is often due to either hemorrhoids or diverticulosis. Note that diverticulitis does not tend to cause LGIB.
Neither the presence of melena nor the presence of hematochezia are definitive for differentiating upper vs. lower GI bleed. ___% of the time UGIB has BRB PR and ___% of the time LGIB has melena. So why are we even talking about this then...?

Epidemiology:

Important Risk Factors

Mortality:

Key Decision Points:

upper or lower source?
requires admission or can be discharged?
requires emergent, urgent, or outpatient specialist attention, or do they require a specialist at all?
requires imaging?
requires transfusion?