What is hematemesis and what causes it?

Hematemesis is the medical term for vomiting blood. The blood almost always comes from the upper digestive tract. Bleeding from the esophagus could be caused by esophageal varices (dilated superficial blood vessels in the wall of the esophagus), esophagitis (an inflamed esophagus), an esophageal ulcer or a tear in the lower part of the esophagus from violent vomiting. 
Bleeding that comes from a stomach source includes gastritis (stomach inflammation), an ulcer, stomach varices, or rarely cancer. An ulcer in the first part of the small intestine, called a duodenal ulcer, is another potential reason for hematemesis.