This graph shows the process of homeostenosis as a function of aging. Stenosis refers to the narrowing of a structure. In this case, it is used to signify an encroachment on function, illustrated above by the progressively smaller area under the “physiologic limit” curve, with increasing age. When the body is subjected to stress, it uses physiologic reserves to maintain homeostasis. The greater the stress, the more physiologic reserves are engaged. With age, the physiologic reserves of each organ system diminish. As a result, an insult, easily buffered by the young organ, may push the older organ’s ability to maintain homeostasis beyond the “physiologic limit,” leading to an acute injury or disease state. In the context of the aging heart, the lightning bolt represents exertion. Exercise increases the oxygen demand of the body. Functional reserve allows the heart to respond by increasing cardiac output (CO) accordingly. In the older heart, loss of cardiac functional reserve limits the extent to which the heart can augment CO in response to exertion. This can predispose the heart and other organ systems to disease as a result of an inadequate blood supply. Note the sharp decline in physiologic reserve on the right side of the graph. This illustrates the drop-off in reserves that occur in the very end stages of life. This is one explanation for why it is common for very elderly individuals to be “frail”, susceptible to more disease and injury, and less able to recover from such states.