Which is the watershed area between SMA and IMA that commonly undergoes ischemia?

Anatomic factors predisposing to colonic ischemia include the narrow caliber of the IMA and occasional vascular variations that lack significant collaterals between the SMA and IMA. Predisposing physiologic factors include the low perfusion pressure at the splenic flexure, a watershed area, a decrease in perfusion associated with colonic motility, and sustained mesenteric vasospasm associated with systemic hypotension or other severe physiologic stress produced by sympathetic activity. There are several weak points in the colonic blood supply, known as watershed areas, which result from incomplete anastomoses of the marginal aeries. These watershed areas are more vulnerable to ischemic injury than other pas of the colon. Splenic flexure is the watershed territory between SMA and IMA. Ref: Tyagi S., Cappell M.S. (2012). Chapter 162. Large Bowel Disorders. In G.V. Lawry, S.C. McKean, J. Matloff, J.J. Ross, D.D. Dressler, D.J. Brotman, J.S. Ginsberg (Eds), Principles and Practice of Hospital Medicine. Retrieved March 6, 2013.