Fuctional GI disorders can be differentiated from organic GI disorders by

Common causes of Bleeding per rectum hemorrhoids anal fissures (tears in the rectal tissue), diveicula (diveiculitis, including Meckel's diveiculum), infections (bacterial and other pathogens) inflammatory bowel diseases (Crohn's disease and ulcerative colitis), angiodysplasia (fragile blood vessels), tumors,polyps, and bowel trauma. Less common causes include upper GI tract problems like ulcers and Mallory-Weiss tears in the esophagus or a dilated vein or varix. Ref: