Gastrointestinal (GI) bleeding refers to any form of bleeding that occurs in the gastrointestinal tract, which extends from the mouth to the anus. It can manifest in various ways, such as vomiting blood (hematemesis), passing black, tarry stools (melena), or passing blood in the stool (hematochezia). GI bleeding can range from microscopic amounts detectable only by laboratory tests to massive bleeding that is immediately life-threatening.