By contrast, animals in captivity begin to show survival curves much more rectangular in shape. Such animals are removed from most threats by accident or predator, and for them the second term of the equation, that of the species’ life span, begins to dominate. Figure 2 shows theoretical calculations of this phenomenon after Sacher (1977). Such rectangularization has been documented for many animals, including dogs, horses, birds, voles, rats, and flies….
Figure 3 is drawn from the data Shock developed in 1960, and it is modified only slightly from what has been called “the most frequently shown data in the field of gerontology.” The data show that many important physiological functions decline with age, and the decline is quite close to being a straight line. It is important to emphasize that these data were obtained from healthy human subjects in whom no disease could be identified that was related to the function being measured. Thus, the observed decline does not depend on disease.
Figure 3 is a major oversimplification of complex data…. The lines are not actually as straight as portrayed, and some of the data have been contested. The point is that a considerable body of research supports a gradual, nearly linear decrease in organ function with age.
Figure 2 Theoretical Survival Curves for an Animal Become Progressively More Rectangular as the Environment Progresses From Wild to Domestic
Source: Redrawn from G. A. Sacher, “Life Table Modification and Life Prolongation,” in C. E. Finch and L. Hayflick, eds., Handbook of Aging, Van Nostrand Reinhold, 1977.
Figure 3 The Linear Decline of Organ Function With Age
Source: U.S. Bureau of Health Statistics. Redrawn from N. Shock, “Discussion on Mortality and Measurement of Aging,” in B. L. Strehler, S. D. Ebert, H. B. Glass, and N. W. Shock, eds., The Biology of Aging: A Symposium. Copyright © 1960, American Institute of Biological Sciences.
Normal, healthy organisms maintain an excess organ reserve beyond immediate functional needs. We have four to ten times as much reserve function as we need in the resting state. The heart during exercise can increase its output sixfold or more. The kidneys can still excrete waste products adequately if five-sixths of the functional units, the nephrons, are destroyed. Surgeons can remove one entire lung, and sometimes part of the second, and still have an operative success. Three-fourths of the liver can be removed, under some circumstances, and life is still maintained.
However, the mean level of reserve in many of our organs declines as we grow older. We seldom notice this gradual loss of our organ reserve. Only in the circumstances of exceptional stress do we need all that excess function anyway. Shock and others suggest that the decline may be plotted as a straight line.
Homeostasis and Organ Reserve
The human body may be viewed as a remarkable assembly of components functioning at various levels of organization. Systems of molecules, cells, and organs are all marvelously integrated to preserve life. The eminent nineteenth-century physiologist Claude Bernard emphasized that these integrated components act to maintain a constant internal environment despite variable external conditions. Bernard saw life as a conflict between external threats and the ability of the organism to maintain the internal milieu.
These fundamental observations have stood well the test of time. Indeed, the human organism cannot survive if the body temperature is more than a few degrees from normal, if acid-base balance is disturbed by a single pH unit, or if more than 20% of the body water is lost. Body chemicals are regulated closely, often to within 2% or 3% of an average value. A change in one direction in body constituent is often followed by a complicated set of responses that act to restore equilibrium.
Bernard also noted that living beings change from a period of development to a period of senescence or decline. He stated that “this characteristic of a determined development, of a beginning and an end, of continuous progress in one direction within a fixed term, belongs inherently to living beings.”
The regulation of bodily functions within precise limits was termed homeostasis by Cannon (1932). Living organisms under threat from an extraordinary array of destructive sources maintain their internal milieu despite the perturbations, using what Cannon called the “wisdom of the body.” Dubos (1965) has pointed out that this “wisdom” is not infallible. Homeostasis is only an ideal concept; regulatory mechanisms do not always return bodily functions to their original state, and they can sometimes be misdirected. Dubos sees disease as a “manifestation of such inadequate responses.” Health corresponds to the situation in which the organism responds adaptively and restores its original integrity.
The ability of the body to maintain homeostasis declines inevitably with decreasing organ reserve. Figure 3 shows the decline for lungs, kidneys, heart, and nerves. The decline is not the same for all individuals, nor is the decline the same for all organs. For example, nerve conduction declines more slowly than does maximal breathing capacity. And some organs, such as the liver, intestinal lining cells, and bone marrow red cells, seem to show even less decline with age.
The important point, however, is that with age there is a decline in the ability to respond to perturbations. With the decline in organ reserve, the protective envelope within which a disturbance may be restored becomes smaller. A young person might survive a major injury or a bacterial pneumonia; an older person may succumb to a fractured hip or to influenza. If homeostasis cannot be maintained, life is over. The declining straight lines of Figure 3 clearly mandate a finite life span; death must inevitably result when organ function declines below the level necessary to sustain life….
Implications of the Rectangular Curve
The rectangular curve is a critical concept, and its implications affect each of our lives. The rectangular curve is not a rectangle in the absolute sense, nor will it ever be. The changing shape of the curve results from both biological and environmental factors. Many biological phenomena describe what is often called a normal distribution. This is the familiar bell-shaped or Gaussian curve. If one studies the ages at death in a well-cared-for and relatively disease-free animal population, one finds that their ages at death are distributed on both sides of the average age of death, with the number of individuals becoming less frequent in both directions as one moves farther from the average age at death. A theoretical distribution of ages at death taking the shape of such a curve in humans is shown in Figure 4. This simple bell-shaped curve, with a mean of 85 years and a standard deviation of 4 years, might exemplify the age at death of an ideal disease-free, violence-free human society. The sharp downslope of the bell-shaped survival curve is analogous to the sharp downslope of the rectangular curve. In Figure 5, the first part of the curve becomes ever flatter, reflecting lower rates of infant mortality. Several factors prevent the total elimination of infant mortality and thus prevent the curve from becoming perfectly horizontal. These premature deaths are the result of birth of defective babies, premature disease, and violent death. Improvements in medicine can lower but never eliminate the birth of defective babies and premature disease. It seems likely that the ever dominant proportion of violent deaths during early life will prove recalcitrant to change and will form an ever larger fraction of total premature deaths.
So, the rectangular curve has an initial brief, steep downturn because of deaths shortly after birth, a very slow rate of decline through
