Welcome to JA and thanks for this question. There are a number of conditions which can cause bleeding in the stool and it is good that you are going to the ER, ill quickly go through some of the causes,
1) hemorrhoids which are dilated veins in the rectum which can bleed quite a lot if they grow large in size. These would be diagnosed ona clinical examination and can be removed by simple surgery.
2) Infections - any infections in the intestine can lead to diarrhea as well as bleeding which can be large in amount and the treatment is usually with the help of antibiotics
3) Polyps/growths - any polyps in the intestine can also lead to bleeding of this sort especially if the polyp contains its own blood supply which if damaged can lead to a large amount of bleeding. This would need a colonoscopy to be diagnosed and treated
4) Inflammatory bowel disease - can also cause bleeding of this sort along with abdominal pain as well as mucus in the stools
There are several other causes which include Angiodysplasia in which there are abnormal blood vessels which bleed all of sudden but the point here is that large amount of bleeding is a clear indication for ER management and you should immediately proceed to the ER.
Do you have any more questions?
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