One important internal process that can change the color of stool is bleeding into the intestines. The stool may turn black because of chemical changes to the hemoglobin in blood. This is caused, in part, by the action of intestinal enzymes, particularly if bleeding is more rapid and there is a large amount of hemoglobin within the intestines. A color change to black also is more likely if the bleeding occurs in the upper intestine, since there is more time for the chemical changes to the hemoglobin to take place as the blood travels through the intestines.
Stool that is black due to bleeding is also "sticky" (tarry) and smells bad. These latter characteristics help distinguish black stools due to internal bleeding from black stools due to the ingestion of iron or bismuth-containing medicines, for example, bismuth subsalicylate (Pepto-Bismol).
On the other hand, bleeding that occurs lower in the intestines, particularly in the colon, is likely to cause red or maroon-colored stools, since there is little time for chemical changes to the hemoglobin to take place.
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