to do this. Rowell ( 1986 ) notes that given the parameters of the heart and vessels, a
hydrostatic analysis of the human circulation would lead to the conclusion that
human beings simply could not stand up because the force of gravity will overcome
venous blood pressure. But obviously, people do stand up. How is this possible?
Humans have evolved a“second heart”on the venous side of the circulation
when in upright posture. Specifically, contractions of the soleus muscles of the calf,
which are richly embedded with valveless veins called venous sinuses, serve as an
effective pump, driving blood back to the heart (Rowell 1986 ; Madhavan et al.
2006 , 2008 ; Madhavan 2009 ). This calf muscle pump is stimulated by neural
signals transmitted from cutaneous mechanoreceptors on the plantar surface of the
foot which are used in postural maintenance (Kennedy and Inglis 2002 ; Fallon et al.
2005 ; Madhavan et al. 2008 ). The mechanoreceptors, known as exteroceptors, are
involved in the sensory transduction of external stimuli such as touch, pressure,
vibration, temperature, and pain (Madhavan 2009 ). There are several types of
plantar exteroceptors, but the ones that appear to affect posture are the tactile
Meissner’s corpuscles (Madhavan 2009 ). These corpuscles dominate the plantar
surface, providing nearly 60% of the sensory input, and are distributed widely
across the surface with particular density across the toes (Kennedy and Inglis 2002 ;
Madhavan 2009 ). The density of these corpuscles declines with age, which may
contribute to the increasing incidence of falls in the elderly, but also to the
increasing incidence of orthostatic hypotension in the group as well (Madhavan
2009 ). Interestingly, the neurons that innervate these receptors respond to
low-frequency vibrations in the range of 30–80 Hz, with peak sensitivity at 45 Hz
(Madhavan 2009 ). Studies involving blood pressure variation and vibration of the
feet at 45 Hz confirm that it is the Meissner’s corpuscles that stimulate the soleus
muscle pump, as blood pressure is increased with continuous foot vibration at this
frequency (Madhavan et al. 2006 ; Madhavan 2009 ). So, with the development of
bipedalism, the tactile Meissner’s corpuscles appear to have taken on the additional
role of maintaining upright posture, as they signal the soleus muscles to contract,
which in turn provides additional push to venous blood that is sufficient to over-
come gravity thus allowing humans to stand up! One might speculate that without
this postural“hidden”adaptation, human beings might still be arboreal, or perhaps
knuckle walkers like our nearest primate relatives.
Summary and Conclusions
Arterial blood pressure is a property of blood circulation. It is pulsatile, as it is
generated when blood is expelled from the heart into the aorta. It varies continu-
ously in an allostatic fashion and adapts people to their ever-changing circum-
stances. The adaptive cycle of blood pressure during the day may change as people
age and take on differing sociological roles. However, it may do so differently in
men and women, due to differences in how sociological roles are perceived. There
is also ample evidence that the system of hormonal regulation of blood pressure in
8 Continuous Blood Pressure Variation: Hidden Adaptability 163