Ans. It is always wise to contact your vet if your dog’s diarrhea causes him to expel blood – this is especially true if the blood appears dark (like coffee grounds) or the stools appear tarry, as these signs may indicate bleeding high in the gastrointestinal tract. Bright red blood, by contrast, usually originates near the end of the intestinal tract or from the anus. This occurs pretty commonly in dogs with diarrhea and is rarely serious unless it occurs over a lengthy period. It is still wise to contact your vet anytime you see blood, but bright red blood is rarely cause for panic.