Fig. 3. Reactive oxygen species (ROS) are continually generated as byproducts of oxidative metabolism. A self-perpetuating, physiologic “cycle of aging” has been proposed  96,180 in which oxidative stress within mitochondria slowly degrades the components needed for energy production and self-repair of damage done by ROS. The resulting decrease in bioenergetic capacity could eventually compromise organ system functional reserve and predispose to increased probability of adverse perioperative outcome.