There are many reasons that blood can appear in urine and stools: disease, trauma, infection, tumors and anatomic abnormalities. Under normal circumstances, there should be no blood present in either system, so even the presence of a small amount can be indicative of abnormal processes occurring.
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Hematuria, or blood in the urine, can be caused by the irritation of a urinary tract infection. With an infection comes inflammation and swelling of the tissues, such as the urethra (tube leading from the bladder to the outside world), bladder lining, ureters (tubes connecting the bladder to the kidneys) and renal structures. This severe inflammation and swelling can cause the tiny blood vessels to become engorged with blood and the vessel walls to become thin and fragile, breaking down and allowing for the bleeding. The irritation perpetuates the other symptoms, and soon it becomes a vicious cycle.
Trauma, or injury, can cause hematuria as well. Motor vehicle accidents and athletic injuries that cause direct blows to the abdomen and low back can traumatize the bladder or kidneys. In some instances, trauma can lead to a temporary presence of blood from such causes as bruising of the kidney or bladder lining. Persistent bleeding that won't stop can be a sign of something more severe, such as a lacerated kidney, severe renal contusion or punctured bladder.
Hemorrhoids are small veins located low in the rectum just inside the anal sphincter. For any number of reasons, these veins can become incompetent, engorged with blood and very fragile to pressure and friction. In the presence of hemorrhoids, bowel movements that require straining put undue pressure on these veins, causing them to tear and bleed. The bleeding can be intermittent and is usually stopped by pressure exerted against them from the wall of the rectum--that is, until the next bowel movement.
Gastric ulcers can cause serious damage to the lining of the stomach. The normally present hydrochloric acid in the stomach can severely irritate these ulcers, causing additional erosion of the lining. When small blood vessels in the stomach lining are affected, they begin to bleed. These blood cells are carried throughout the intestinal tract until they leave the body in the feces (stool). By this time, all the oxygen content in the cells is gone, creating a dark, "tar-looking" stool.
Chronic disease such as irritable bowel syndrome and diverticulitis can cause a longstanding irritation to the lining of the large intestine. In turn, this chronic inflammation can cause erosion of the delicate lining of the intestine, causing minimal but persistent oozing. Bleeding from the large intestine, especially at its farthest point, can show up as brighter red blood in the stool as opposed to lesions of the beginning of the bowel farther away from the rectum, where it will present as darker brown in coloration.
Bladder cancer, prostatic cancer and renal (kidney) tumors can be major factors in the presence of bloody urine and stool. Invasive tumors alter the integrity of tissues, making them more subject to damage and bleeding. Colon cancer is often first diagnosed after concern over constipation/diarrhea and bloody stools.