Bleeding from the gastrointestinal tract can cause blood to be passed through the rectum or vomited.
Rectal bleeding is blood or the digested metabolites of blood. There are two types, melena, and hematochezia. Melena is bleeding originating high in the digestive tract, usually the stomach or duodenum. The pigments from the partly digested blood make the stools appear black and tarry. Hematochezia is the appearance of blood mixed or associated with the passage of stool. If the source of the bleeding is the distal large intestine or rectum, the blood will be fresh red. The higher up the digestive tract the source of the bleeding, the darker the blood appears.
Small volumes of blood loss from a source high in the digestive tract may not be obvious but can be detected with the chemical assay of a stool sample. This is known as occult bleeding. Up to 200 mL of blood per day can be lost into the upper gastrointestinal tract without being clearly visible. When chronic, occult bleeding can be a major contributing factor to anemia and debility.
Vomiting of blood, or hematemesis, originates from the stomach or esophagus. The blood may be fresh red, mixed with partially digested food, or dark and lumpy, resembling coffee grounds. Fresh red blood suggests an arterial or variceal source. Dark lumpy blood suggests that the bleeding is sufficiently slow for the blood to linger in the stomach long enough to be digested.
In Chinese medicine, rectal bleeding and vomiting of blood are associated with similar underlying pathologies in differing degrees of intensity.