One of the more common causes of blood in the stool is the development of anal fissures. Anal fissures are very small tears (usually less than 1 1/2 inch) that occur in the skin of the anus (which is where the bowel movements leave the body). These tears often occur as a result of unusually hard bowel movements tearing the skin of the anus as they pass through. Hard bowel movements can be caused by constipation because as the fecal matter spends more time in the large intestine, it is additionally compacted, which makes it harder. Thus, constipation is able to lead to anal fissures which can then cause blood to appear in the stool.
Another cause of blood appearing in the stool can be hemorrhoids. Hemorrhoids are made up of swollen veins that appear in the rectum and anus. These veins, when they get swollen, can become inflamed and swollen which can lead to rectal bleeding. Hemorrhoids can either be located inside the body (where they are generally less painful) or externally, where they are more likely to itch and cause pain. One of the most common causes of hemorrhoids is straining to make a bowel movement, which is commonly a feature of constipation. The extreme muscular effort necessary to pass a bowel movement when one is constipated can lead to hemorrhoids, which can cause blood to appear in the stool.
Although constipation has not been shown to have a direct link to colon cancer, cancerous growths within the large intestine can lead to both constipation and blood appearing in the stool. The constipation would be due to obstruction of the colon that a tumor can cause. Colon cancer also typically causes blood in the stool. A possible diagnosis of colon cancer would be considered for any patient who suffers from both constipation and blood in his stool if no anal fissures or hemorrhoids could be identified.