BY GERARD FONTE
The human circulatory system is
not simply a lot of plumbing.
There are a number of subtle factors
that nature has provided to aid in
the ability of the heart to push
blood around the body. One of the
most important is the elasticity of
the walls of the arteries. This elasticity is easily observed by simply
feeling one’s own pulse at the wrist.
The small artery there can be felt
expanding and contracting with
every beat of the heart. If there were
no expansion and contraction, the
pulse would be much harder to feel.
This action sets up something
like a peristaltic wave that helps to
push the blood through the circulatory system. In short, the elasticity
of the arteries is important for good
blood flow. It’s well known that
“hardening of the arteries” from
cholesterol build-up is not good.
There are two general types of
waves found in nature: transverse
and compression. A transverse
wave changes the amplitude of the
medium. Ripples on the surface of
a pond are transverse waves.
Compression waves change the
density of the medium. The most
common compression wave is
sound. Sound changes the density
of the air, not its height.
The elasticity of the arterial walls
means that the pulse wave acts somewhat like a transverse wave. One
characteristic of a transverse wave in
a liquid is that it’s fairly slow. Ripples
on a pond move relatively slowly.
They rely on the external restoring-force of gravity to propagate.
However, the propagation of a
compression wave in a liquid can be
extremely fast. Here, the speed of
propagation depends on the compressibility of the liquid and the walls
containing it. For example, consider
water in a filled copper pipe. There is
very little delay from the time more
water is forced into one end to the
time water comes out of the other
end. In this case, system pressure is
an important factor.
Therefore, it was thought that
the propagation speed of the human
pulse should increase if the elasticity
of the arterial walls decreases
because the pulse wave is changing
from a transverse wave to a compression wave. Initially, no references
could be found. However, recently a
paper by doctors Safer, Henry, and
Meaume (2002) was found (see reference at end) that indicated that the
speed of the pulse does indeed
increase with lower arterial elasticity.
A somewhat more subtle
factor should also be observed.
This is the change in wave shape.
There is every reason to believe
that there will be a different wave
shape when comparing a compression wave to a transverse wave.
This should be especially true
for the same system when the
elasticity of the retaining walls has
changed. It would be expected that
higher frequency components
would appear as the pulse wave
propagation speed increases. No
references concerning that factor
could be found (but that doesn’t
There is a delay
between when your
heart beats and
when the pulse is
felt at your wrist
This delay depends
upon a number of
including the elasticity
of the walls of the
arteries. This project
will allow you to
measure this delay and
will also provide a
platform for acquiring
high-gain analog and
The output of the
project is a digital pulse
with a duration equal to
the difference between
the start of the
April 2007 39