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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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