cardiovascular mortality (Flores 2013 ). These recent epidemiologicalfindings
suggest that various aspects of blood pressure variability may be as important as the
mean level of blood pressure when it comes to defining cardiovascular health.
Allostasis and the Hidden Adaptability of Blood Pressure
As noted above, the biomedical literature regarding arterial blood pressure is domi-
nated by its role as an indicator of cardiovascular health; however, as blood pressure
variability came into sharper focus in the 1980s, the conceptualization of the funda-
mental nature of blood pressure as a physiological process also began to be
re-examined (James 2013 ). In 1988, Sterling and Eyre ( 1988 ) introduced the concept
of“allostasis”defining it as a process of“achieving stability through change,”using
blood pressure as an exemplar. Blood pressure changes dramatically and continuously
to adapt individuals to their changing daily environmental circumstances (James
1991 ). Because it continuously changes, the individual does not have a dynamic
steady blood pressure state; rather, there are multiple and varied stable blood pressure
states that are each related to the ever-changing internal and external environments
experienced by the individual (Ice and James 2012 ;James 2013 ). It is this property of
blood pressure that defines its hidden adaptability. Blood pressurefluctuates seam-
lessly with physical environmental parameters such as temperature (Modesti et al.
2006 ) or with transient psychosocial stressors such as job demands or child care
(James et al. 1989 ). It changes as people ruminate on emotionally charged memories
or react to the machinations of others (Gerin and James 2010 ;James 2013 ), and it
responds to dietary factors such as salt (James et al. 1994 ; Van Berge-Landry and
James 2004 ) and to a host of other substances that people might ingest on a regular
basis such as alcohol, nicotine, and caffeine (see James and Brown 1997 ). When you
measure the continuous variation in blood pressure over time, you provide a picture of
the experienced trials and tribulations of life, mapping out physiological responses of
which the individual is blissfully unaware.
Interestingly, from the perspective of evolution, a question arises: Is the magnitude
of every change necessary for adaptation and survival? The answer, I suspect, is
probably no, since every reaction and physiological response is not necessarily
appropriate for the circumstance (James2007b). Is the constant change responsible for
the epithet“the silent killer”given to blood pressure? While it is possible that a single
instantaneous blood pressure change could be fatal, it is more likely that it is the
constantfluctuation from state to state, appropriate or otherwise, that contributes to the
vascular system’s ultimate demise, because as with any machine, including a bio-
logical one like the human body, continuous use will wear it out. This, I think, is the
underlying premise of allostatic load as articulated by McEwen (e.g., McEwen 2004 ).
So the answer to this second question is probably yes.
8 Continuous Blood Pressure Variation: Hidden Adaptability 149