Nitric oxide is important in the operation of the endothelium - the lining of blood vessels - but diminishes with age: "The normal endothelium exerts a major vascular protecting role by secreting substances, above all nitric oxide (NO). In disease conditions (such as the presence of cardiovascular risk factors) the activation of endothelial cells can lead to the production and release of contracting factors, which counteract the beneficial effects of NO, and reactive oxygen species (ROS), which in turn cause NO breakdown. ... ageing has been demonstrated to be associated to a progressive impairment in endothelial function both in conduit arteries and resistance vessels, mainly because of an increased production of ROS. Therefore, it is conceivable that endothelial dysfunction plays a major role in favoring age-related increased cardiovascular risk in the elderly." This is an example of the way in which age-damaged cells cause problems in the normal operation of surrounding tissue: cells taken over by damaged mitochondria are exporting reactive oxygen species that breakdown NO, and senescent cells are pushing out their own cocktail of unhelpful chemical instructions as well.