AGING AS A PROCESS OF COMPLEXITY LOSS - Complex Systems Science in Biomedicine - page 619

Table 1 . Decreased complexity of anatomic structures with aging

Structure Measure Age effect

Neurons Dendritic arbor Loss of branches and

neural connections

Bone Trabecular meshwork Trabecular loss and

disconnection

Kidney Glomerular capillary tuft Degeneration and loss

of capillaries

Subepidermis Elastic fibers and Loss and collapse of

dermal papillae subepidermal structure

Table 2 . Decreased complexity of physiologic systems with aging

System Measure Age effect

Heart rate dynamics 1/ f slope, DFA, & approx. Decreased fractal scaling,

entropy of interbeat intervals more regular dynamics

Blood pressure dynamics 1/ f slope, DFA, & approx. Decreased fractal scaling,

entropy of BP fluctuations more regular dynamics

Respiratory dynamics DFA of interbreath intervals Loss of long-range

correlations in

elderly males

Postural control 1/ f slope of center-of- Decreased fractal scaling

pressure trajectories

Gait dynamics 1/ f slope, DFA Decreased fractal scaling

secreted in pulsatile fashion in order to promote glucose metabolism. When an
individual stands up, blood pools in the lower extremities, and blood pressure
suddenly falls. In response, the sympathetic nervous system secretes norepi-
nephrine to restore the blood pressure to its resting state. Thus, the complex dy-
namics observed during resting conditions differ from the more focused singular
response that occurs during stress. The dynamics of this adaptive response have
been referred to as "reactive tuning" (2). In the field of nonequilibrium statistical
mechanics, the relationship between the correlation properties of the fluctuations
of a system and its relaxation to equilibrium is described by the fluctuation-
dissipation theorem (16). If the complex interactions of physiologic systems
during rest enable an organism to mount a focused adaptive response during a
perturbation, the loss of complexity in resting dynamics may indicate an im-
paired ability to adapt to stress and a predisposition to functional decline.