Dr Rick

Thanks to mfaber, Nat, MrD and the others; I expressed frustration, but your support and encouragement does mean that this has been worthwhile, afterall.

Rick

[ November 03, 2002: Message edited by: rbochnermd ]</p>

Jack the Bodiless

No, you have NOT had "advanced training in fluid mechanics". Or, if you have, you were not paying attention in class.

...Because you're still dead wrong about gravity and pressure, and you still show absolutely no inclination to acknowledge or correct your error.

Yes, of course pressure is greater at depth in a liquid, due to the weight of the liquid above. But, when discussing the pressure that an artery must contain, what matters is the pressure differential across the wall of the artery: the difference in pressure between the inside and the outside.

And this is due entirely to the force with which it is being pumped. Gravity is NOT a factor, because blood and amniotic fluid are essentially the same density and are also essentially incompressible.

The fetus is a fluid-filled bag inside another fluid-filled bag. At EVERY depth within the uterus, the pressure inside the blood vessels of the fetus would be in perfect equilibrium with the pressure of the amniotic fluid outside, were it not for the pumping of the heart.

And it's just as easy to pump up as it is to pump down.

This is VERY basic physics, Vanderzyden. If you DO intend to work in a field where this knowledge is important, I suggest you take a refresher course immediately. Or take up scuba diving as a hobby: you will either learn about buoyancy and pressure, or die. Natural selection in action.

Jack the Bodiless

...In fact, there is only one gravity-related pressure gradient that could possibly be of relevance anywhere in this discussion: the gradient across the uterus wall, which would be higher at the base of the uterus when standing upright. And this is relevant only when discussing the point of entry of blood into the uterus from the rest of the mother's body, NOT where it goes once inside.

MrDarwin

Implicit in Jack's comment is that the fetus is so small, and occupies such a small vertical mass, that any effect of gravity on blood pressure from the top of the fetus to the bottom of the fetus (in whichever orientation it finds itself) is negligible.

Jack the Bodiless

If it were not for the pressure generated by the heart, the fetal blood vessels could be as fragile as soap bubbles: the pressure gradient between the interior and the exterior would be precisely zero at all depths within the fetus, because the pressure increases at an identical rate with depth, inside and out.

The only biological effects of pressure per se are phenomena such as nitrogen narcosis, oxygen narcosis, and expanding gas bubbles in blood as pressure decreases: "the bends".

...There are women with unusually large uteri (like tens of meters from top to bottom) in Vanderzyden's neighborhood?

Vanderzyden

Jack,

I hesitate to even respond to you, because of your consistently abrasive and degrading tone. However, you should realize that you are so very much in error with your posts. Perhaps the worst statement you made is:

"And it's just as easy to pump up as it is to pump down."

Go back and read my posts about hydrostatic pressure. Then go pick up a first year physics text. Then go pick up a statics text to see hydrostatic principles reaffirmed. Then go pick up a fluid mechanics text and focus on fluid statics to see the principles yet again affirmed and extended (fluid dynamics also produce significant effects, but we may safely ignore them for our purposes here).

After you have done at least the first step, then you will be somewhat justified in coming back and attempting to argue against the inclusion of significant and substantial hydrostatic effects in the CV system.

Don't come back and simply make claims. Justify your position, which stands against long standing scientific and engineering endeavor.


John

[ November 04, 2002: Message edited by: Vanderzyden ]</p>

Jack the Bodiless

Vanderzyden:

No, YOU are in error. Gravity is irrelevant here. The fluid is neutrally buoyant. It has no tendency to move either up or down within the larger fluid-filled bag of the uterus.

I thought you'd grasped it here:
But the same applies all the way down. At every depth within the balloon, the pressure within and without is identical. The balloon does not have to resist or contain ANY pressure differential.

And that is why everyone here agrees with me. Including those whose knowledge of fluids is evidently superior to yours.

And I AM a recreational scuba diver, by the way.

lpetrich

I don't see how gravity is a serious problem, because a fetus's extremities are no farther away from its heart than the placenta.

And Vandie's goggle-eyed marveling at the construction of the human circulatory system is totally beside the point.

It must be significant that he has not attempted to demonstrate the Panglossian excellence of early-embryonic circulation, with its multiple aortas and aortic arches and other fishlike features; maybe it has been too difficult for him.

And the same can be said of vitelline blood vessels; what does one need a yolk sac for if one will get one's nutrition from one's surroundings?

Jack the Bodiless

...Well, I hope Vander has gotten it now, but just in case:

Let's expand the balloon-in-a-tank model.

The tank is the uterus, the fluid-filled balloon is the fetus.

Put a short length of plastic pipe inside the bag. This is the fetal aorta.

Which way does the fluid inside flow? Nowhere. Even though the water at the top of the pipe is under less pressure than the water at the bottom, nothing happens.

Now let's attach a pump and switch it on. Water flows along the pipe now.

Does it matter whether the pipe is held vertically or horizontally? Does it matter whether the pipe, if vertical, is pumping water up or down? Nope.

If the fluid in the pipe was mecury, then it WOULD be more difficult to pump it "uphill" in the balloon of water, and it would tend to drop out of the bottom. Likewise, if it was gasoline, it would flow up the pipe without pumping, and would have to be forcibly pumped down to make it exit from the bottom.

But water, or amniotic fluid, or fetal blood will go nowhere without pumping and can be pumped in either direction with equal ease.

Asha'man

How about you go into the kitchen and build yourself a siphon? Use a length of plastic tubing and two containers of water, like a pitcher of water.

Notice that the water is able to move up against the pull of gravity on one side of the siphon. Marvel at how my magic works, without invoking any textbooks at all!