Mammalian aging clearly involves a gradual, cumulative process of damage and deterioration.
Protective mechanisms against aging are costly to the organism. The “disposable soma” theory of aging states that antiaging mechanisms only need to be good enough to give the next generation the best opportunity to reproduce. In fact, most of the gains in average human lifespan have been as the result of reducing the factors that cause premature death (accidents, violence, disease).
The inability to completely thwart aging implies that the average human lifespan is limited and that if everyone only died of “old age,” the age at death would end up being a bell-shaped curve centered at a certain value, probably around age 85 years.
A variety of deleterious processes continually attack DNA, proteins, and lipids (free radicals and nonenzymatic glycosylation of sugars and amines).
Collagen becomes stiffer from aromatic ring cleavage and by cross-linking to other collagen molecules. In the cardiovascular system, arteries, veins, and the myocardium all stiffen with age.
DNA damage occurs, and mitochondrial DNA sustains more damage than nuclear DNA.
Caloric restriction, well documented to increase lifespan in small mammals, probably does so by decreasing the rate of oxidative damage.
Functional Decline and the Concept of Frailty
Functional reserve represents the degree to which organ function can increase above the level necessary for basal activity. For healthy individuals, reserve peaks at approximately age 30 years, gradually declines over the next several decades, and then experiences more rapid decline beginning around the eighth decade of life. Anesthesiologists often assess the patient's reserve.
The ability to achieve the desired minimum of four metabolic equivalents (METs) presumably provides enough cardiovascular reserve to tolerate the stress of most surgical procedures.
Even without formal assessment, an intuitive sense of reserve is often obtained through simple observation (loss of subcutaneous tissue, unsteady or slowed gait, stooped body habitus, minimal muscle mass [sarcopenia]).
Diminished mentation is a risk factor for postoperative delirium.
The rate at which a given individual ages is highly variable and is determined to a great extent by genetics and luck at avoiding illnesses, trauma, or environmental exposure that may contribute to functional loss. Nevertheless, successful aging can be promoted through avoidance of obesity, good nutrition, and regular exercise.
The older we get, the less likely our chronologic age reflects our physiologic status and functional reserve.