Watershed

Because these zones sit at the end of both supplying arterial trees, they have the least collateral reserve and are the first to suffer in hypotension, shock, or embolism. In the colon the splenic flexure lies at the junction of superior and inferior mesenteric artery territories (Griffith point) and the rectosigmoid junction at the junction of inferior mesenteric and internal iliac supply (Sudeck point). These are the characteristic sites of ischaemic colitis, presenting with crampy left-sided abdominal pain and bloody diarrhoea in an older patient after a hypotensive episode or cardiac surgery, with thumbprinting on imaging from submucosal oedema. The same principle applies elsewhere: cerebral watershed infarcts between the anterior and middle cerebral artery territories cause proximal 'man-in-a-barrel' arm weakness, and between middle and posterior cerebral territories cause higher-order visual deficits; in the heart the subendocardium is the watershed layer and is first to infarct; and centrilobular zone 3 hepatocytes are the furthest from the portal triad and so are first to undergo ischaemic necrosis in shock liver.