Ischaemic colitis, gross specimen. Ischaemic colitis is inflammation of the colon due to reduced blood supply. Here, blood supply is being interrupted to part of the colon (seen as the dark red/purple region) resulting in inflammation and necrosis (death of tissue). Ischaemia is an uncommon cause of colitis and is caused by narrowing of the arteries supplying the intestines by atherosclerosis (the build up of plaque on the inner lining of an artery). Bloody diarrhoea and abdominal pain may be a symptom of chronic conditions.