If a person's stool appears black and tarry, where is the likely source of bleeding?

When stool appears black and tarry, a condition known as melena, it indicates the presence of digested blood, which is usually associated with bleeding originating from the upper digestive tract. This can include areas such as the esophagus, stomach, or duodenum.

The reason for this coloration is that when blood is digested in the gastrointestinal tract, it undergoes a chemical change that alters its color to a dark, tarry shade. This typically happens because the blood has a longer transit time through the digestive system, allowing more time for digestion and processing.

While bleeding from the lower digestive tract or rectum can lead to blood in the stool, that blood is usually bright red as it has not been digested. Similarly, while the stomach is also considered part of the upper digestive tract, the term "upper digestive tract" encompasses all areas where upper gastrointestinal bleeding can occur, making it a broader and more accurate classification for the source of the bleeding reflected in black, tarry stools.