Gastrointestinal bleeding

[Renato]

Bleeding may occur anywhere along the digestive (gastrointestinal [GI]) tract, from the mouth to the anus. Blood may be easily seen by the naked eye (overt), or blood may be present in amounts too small to be visible (occult). Occult bleeding is detected only by testing a stool specimen with special chemicals.
Blood may be visible in vomit (hematemesis), which indicates the bleeding is coming from the upper GI tract, usually from the esophagus, stomach or the first part of the small intestine. When blood is vomited, it may be bright red if bleeding is brisk and ongoing. Alternatively, vomited blood may have the appearance of coffee grounds. It results from bleeding that has slowed or stopped, and the blood looks like coffee grounds because it has been partially digested by acid in the stomach.
Blood may also be passed from the rectum:

As black, tarry stools (melena)
As bright red blood (hematochezia)
In apparently normal stool if bleeding is less than a few teaspoons per day

Melena is more likely when bleeding comes from the esophagus, stomach, or small intestine. The black color of melena is caused by blood that has been exposed for several hours to stomach acid and enzymes and to bacteria that normally reside in the large intestine. Melena may continue for several days after bleeding has stopped.
Hematochezia is more likely when bleeding comes from the large intestine, although it can be caused by very rapid bleeding from the upper portions of the digestive tract as well.
People who have lost only a small amount of blood may feel well otherwise. However, serious and sudden blood loss may be accompanied by a rapid pulse, low blood pressure, and reduced urine flow. A person may also have cold, clammy hands and feet. Severe bleeding may reduce the flow of blood to the brain, causing confusion, disorientation, sleepiness, and even extremely low blood pressure (shock). Slow, chronic blood loss may cause symptoms and signs of low blood count (anemia), such as weakness, easy fatigue, paleness (pallor), chest pain, and dizziness. People with underlying ischemic heart disease may develop chest pain (angina) or a have a heart attack (myocardial infarction) because of decreased blood flow through the heart.
(See also Overview of Digestive Symptoms.)