There are different reasons for ischemia of the bowel. For the small bowel their blood supply is coming from the SMA and there can be atherosclerosis at the take off of the vessel. What can then happen is critical occlusion or a a plaque breaks off and shoots off to one of the branches to the bowel. It is thought that the straighter shot of this vessel also makes it prone for embolic events. The large intestine bowel supply is actually more tenuous. Being that I work on this area a lot, I see a fair amount of people with large bowel ischemia. Usually it is at the watershed area of the splenic flexure where the branches of the SMA and IMA meet. Ischemia here is more of a low flow state and tends to do better with adequate support. Because of these two etiologies, small bowel ischemia tends to be more of a critical ischemia (like a major MI) and large bowel ischemia tends to be more mild (like angina).
