Tumors contain regions of hypoxia and necrosis because their vasculature cannot supply oxygen and other vital nutrients to all the cells. Whereas normal vasculature (a) is hierarchically organized, with vessels that are sufficiently close to ensure adequate nutrient and oxygen supply to all cells, tumour vessels (b) are chaotic, dilated, tortuous and are often far apart and have sluggish blood flow. As a consequence, areas of hypoxia and necrosis often develop distant from blood vessels. In addition to these regions of chronic (or diffusion-limited) hypoxia, areas of acute (or perfusion-limited) hypoxia can develop in tumours as a result of the temporary closure or reduced flow in certain vessels.
