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Heart Pump to Head, From Feet
Name: Ashley
Status: student
Grade: 9-12
Location: IL
Date: March 2008
Question:
Due to gravity, is it harder for the heart to pump blood
upward towards the brain as opposed downwards towards the feet?
Replies:
Yes, but that is not the whole story. Blood has to make a round trip
from the heart and back again, it is not one-way transport unless you
have severed an artery and will die of blood loss in a few moments.
Blood transported to the feet has to come back up; for that, the heart
gets assistance from the muscles in the legs which compress the leg
veins to keep the blood from pooling there. The heart does have to do
all the work pumping blood to the head.
Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
A PPT on blood rheology:
http://www.biomedicalphysics.org/PhysCircCourse/2004/Rheology.ppt
May not be directly applicable to our discussion, but an interesting source
of info nonetheless. Speaks to the myriad other factors involved. The
original source of the data is not cited, but it seems reliable (and is
consistent with other data I have seen).
Gravity pulls on blood just like it pulls on everything else. As a result,
it takes more power to pump against gravity than with it. However, you are
working in a closed loop; blood that goes up must come back down and
vice-versa. The blood coming back down 'pulls' the blood behind it up,
and the blood coming up from your feet is 'pushed' by the blood behind it
coming down. Moreover, pumping through the various vessels and capillaries,
which have mechanisms for adjusting blood pressure, influences your blood
pressure substantially too.
Hope this helps,
Burr Zimmerman
Gravity pulls on blood just like it pulls on everything else. As a result,
it takes more power to pump against gravity than with it. However, you are
working in a closed loop; blood that goes up must come back down and
vice-versa. The blood coming back down 'pulls' the blood behind it up, and
the blood coming up from your feet is 'pushed' by the blood behind it coming
down. Moreover, pumping through the various vessels and capillaries, which
have mechanisms for adjusting blood pressure, influences your blood pressure
substantially too.
Pause.
I thought you were asking to what degree gravity inhibits blood flow. With a
closed system, static pressure head cancels out, and you are only pumping
against minor losses (think of a manometer). Static pressure head is a
factor, but not the only one, and perhaps not a significant one either (and
I do not know enough to quantitatively assess).
Now when I re-read, I think you are asking for a comparative assessment: which
path takes more work, to the head or to the feet? I do not know enough
physiology to definitively assume that the two paths are equivalent other
than for length and location (height). If the paths are equivalent, then
because a longer distance means more minor losses, the path to the feet
takes probably more work. But it is still a closed system -- and a flexible
one at that. With a closed system, the net static pressures cancel out, and
with a longer path you have more minor losses. Again, think of a manometer.
(and if you do not assume the paths are equivalent, then all bets are off)
The flexibility of vessels issue is one I am not sure about either, as well
as the pressure dependence of minor losses. The absolute pressure at the
feet is certainly higher than in the head (when you are standing), and with
higher pressure, the vessels will expand more (or are the vessels
thicker/different to accommodate for this?). Blood is non-Newtonian, but at
high shear, it acts like Newtonian. Does it take more energy to pump at
higher pressure than at low? Are minor losses a function of pressure (e.g.
are losses greater at higher or lower pressure?) Apparent viscosity also
depends on the size and geometry of vessels. Vessels are 'lossy' too --
fluid can flow into or out of them. Is net fluid flow across the vessel
boundaries higher with higher pressure?
It comes down to what we want to assume. I decided to treat this system as a
Newtonian fluid in inflexible vessels, and simply observed that other
factors are in play. This isolates the question of gravity, at least, even
if it ignores potentially more important factors.
With a swimming pool, you have an open system -- you pull water from the
bottom, and eject it at a higher level (water jets near the surface. The
system is open to atmosphere, the energy difference in the water between the
jets and the pump inlet at the bottom is dissipated to kinetic energy and
not captured back into the pumping system. A deeper pool means you have to
pump water up a greater distance. It's not analogous to the human body.
Burr Zimmerman
The "short answer" is yes, although the details can become complex depending
upon the conditions. People with "low" blood pressure sometimes get dizzy if
they stand up too quickly. People who feel like they are going to "faint" are
asked to sit or lay down. People who have poor circulation can experience
swelling in their lower limbs because the fluid cannot be recirculated to their
upper extremities. In most "healthy" people the heart is able to react quickly
to these changing conditions and most people are not even aware there is a
change.
Vince Calder
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Update: June 2012
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