and was substantially more elevated than at home, as opposed to the women with
children who had higher pressures at home than at work, similar to thefindings of the
earlier study. The work pressures of men with children were also significantly elevated
over that of childless men, who showed a similar pattern of work–home difference as
that of childless women. Schlussel et al. ( 1992 ) speculated that the difference in work–
home blood pressure pattern among the two groups of men was also related to a
perceived change in sociological role among the men who had children. Specifically,
they suggested that the work pressures among the fathers increased due to the fact that
they now had to provide for a family, not just themselves. That is, their role perception
changed from worker to provider, so they invested more effort into the job to insure
security, as it was now needed to support a family.
As with women, the impact of a circadian blood pressure pattern change in men
on long-term cardiovascular health is unknown. However, since the circadian
pattern changes from being childless to having children may be different between
men and women, one could speculate that the difference could contribute to the
gender differences in CVD risk. Interestingly, it is also not known as to whether
circadian blood pressure pattern shifts with children are adaptive. One might see
them as such, under the assumption that the shifts might indicate an enhanced
parental investment of men and women in the survivability of their offspring
(during the times their pressures are highest), thus improving their respective
reproductivefitness.
Venous Blood Pressure and Gravity: Another Hidden
Adaptation
Once the ancestors of Homo sapiens evolved bipedalism some four and a half
million years ago, a necessary concomitant circulatory adaptation needed to occur.
In the human circulatory system, there is arterial blood pressure (the numbers we
are all familiar with and which have been the focus of discussion up until this point)
and venous blood pressure, which is much lower given that it arises once the
oxygenated blood hasflowed through the capillary beds where it loses much of the
pressure generated by the push from the heart contractions. Because of the lower
venous blood pressure, there arises in upright bipeds a problem with venous blood
return to the heart (Rowell 1986 ).
When humans stand upright, the heart is some 1.2–1.5 m above the feet, some
75% of the blood volume is in the veins and 70–75% of the total blood volume is
below the level of the heart (Rowell 1986 ). Practically, what this means is that the
unoxygenated blood that is mostly below the heart and in the veins needs enough
pressure to be pushed back to the right atrium against the force of gravity so that it
can be shunted to the lungs, oxygenated, and then circulated back to the body,
including the brain which sits atop the entire vascular system. Mechanically, the
heart is simply not strong enough to provide a sufficient pressure via its contraction
162 G.D. James