Causes of blood in the stool range from harmless, annoying conditions of the gastrointestinal tract such as hemorrhoids and anal tears from straining against hard stools with constipation to serious conditions such as cancer. Blood in the stool should be evaluated by a health-care professional. Rectal bleeding (known medically as hematochezia) refers to passage of bright red blood from the anus, often mixed with stool and/or blood clots. Most rectal bleeding comes from the colon, rectum, or anus.