S2Focus

And here is Mr. Vanderzyden engaging in a little target practice:

KC

I found this one from Winaces's compilation cute:

"Bisexual's are the saddest of all. At least Choose one. Lets not forget what happened to SWEDEN for being neutral"

Cheers,

KC

Ted Hoffman

Mr Darwin : It looks like John Vanderzyden has finally conceded all arguments against him.

Intensity : I wouldnt think so. He is down but not out. He admits he is "seemingly down" and promises he will be back. Aah, he took my advice . I guess I dont have to resign.

He calls himself the "seemingly-uninformed but amazingly stubborn invader".

He admits he is in a conundrum and quips:
Then a stroke of genius strikes him and he announces:
And you then think he has it all figured out.
But he makes a craven sterile response:
Then limps away into the darkness in a manner evocative of a wounded toolthless old lion.

[rather disturbing fantasy deleted -- pz]

[ November 08, 2002: Message edited by: pz ]</p>

pz

Intensity:
Please check your PMs.

Vanderzyden

OK, folks, I'm back. Let's go. Forward march.

Oh, scigirl, I apologize in advance for making your head hurt.

First, here are the primary functions of amniotic fluid. Notice the absence of any mention of buoyancy.


Here is some data I've gathered concerning the weights and volumes of the fetus and the amniotic fluid:

800 [ml] = 841 [mg] = 25% the mass of the fetus

-- 250 ml. at 16 weeks gestation [= 89 ml]
-- 800 ml. at 22 weeks (to 37 weeks) [= 414 ml]
-- 800 ml. at 26 weeks [= 945 ml]
-- 800 ml. at 30 weeks [= 1,420 ml]
-- 800 ml. at 37 weeks [= 2,365 ml]
-- 600 ml. at 39 weeks [= 3,300 ml]
-- declines to about 500 ml. at term (40 weeks) [3,430 ml]

At term, the volume of the fetus (at 3,430 [ml]) is over six times greater than the amniotic fluid.

<a href="http://www.obgyngroup.com/Library%5CPREG-a102.HTML" target="_blank">http://www.obgyngroup.com/Library%5CPREG-a102.HTML</a>

<a href="http://www.babycenter.com/timeline.html;prod-bc-sessionid=OCCZCB5USQJ52CUAQIZSJBQ" target="_blank">http://www.babycenter.com/timeline.html;prod-bc-sessionid=OCCZCB5USQJ52CUAQIZSJBQ</a>

As you can see, buoyancy decreases markedly as the fetus grows and fills the amniotic sac. Already, we can see that, in the third trimester, the hydrostatic pressure gradient will diminish significantly as the amniotic fluid volume becomes small relative to the fetus. This is much like immersing yourself in a very large garbage bag; you're cushioned against impact, but you'll receive no "hydrotherapeutic benefits" from it. More on this in a moment.

Something else to consider: skin is tough; even fetal skin. It is not comparable to, say, celophane. Also, only a small volume of blood reaches the surface of the skin, where its pressure would interact with external hydrostatic pressure. Most of the blood volume is well inside. The calculations are complex, since we would need to analyze the interaction of skin with the external fluid (water/amniotic fluid) to obtain the net pressure force that can be transmitted to the capillaries within the skin. However, we may run some numbers, and consider the advanced gestational stages, to see that the fetal CV system will indeed be affected by gravity (only to a slightly lesser degree than a baby outside the womb). Now, let's see what you think of my rough analysis.

First, the simple equation for hydrostatic pressure:


P = r * g * h


where r is the density, which is mass per unit volume:

density of water (and blood) = 62.4 [lb/ft3] = 1.0 [g/cm3] (i.e. grams per cubic centimeter)

g is the gravitational acceleration:

g = 980 [cm/s2] (i.e. feet per second, per second]

h is the height in [cm], or depth below the top of the system, which will be our variable


Doing the math, we have the simplified form:

P = 1.0 [g/cm3] * 980 [cm/s2] = 980 [g/cm2-s2] * h [cm] = 980 [g/cm-s2] = 980 [dyne / cm2]

dyne = 1 [g-cm/s2]

Since blood pressure is typically given in millimeters of mercury [mmHg],

1333 [dyne/cm2] = 1 [mmHg]

we convert and arrive at our function for hydrostatic pressure:

P [mmHg] = 0.735 * h, for h given in [cm]


Amniotic hydrostatic pressure will be the greatest at full term, when its longest dimension is 35 [cm].



Of course, the uterus is substantially off-vertical. Gravity will produce hydrostatic effects over the vertical height from the crown to the base (not the "diagonal" length of the uterus). So, let's multiply the length of the uterus by 0.75 to get:

h = 26.5 [cm]

At most then, the hydrostatic pressure at the bottom of the amniotic sac (AS) is:

P = 0.735 * 26.5 = 19.5 [mmHg]

Note 1: this pressure is not uniform. The hydrostatic pressure is much lower at the top of the AS (.735 [mmHg]) or near the middle (7.35 [mmHg] at 10 [cm])

Note 2: The fetus is also not vertical, either. So, the effects of gravity on its CV system will also be reduced, while still significant. However, our concern here is to understand the potential effects of amniotic fluid hydrostatic pressure on the transmural (external flesh boundary) gradient. If we find that no significant external pressure is exerted, then we know that gravity will indeed affect the fetal CV system.

Now, we know that the fetus has nearly filled the AS, so that its volume is much greater than the amniotic fluid. At about 26 weeks, buoyancy had gone to zero, with the fetus now resting on the bottom of the uterus. For simplicity, let's take the case of the fetus at term, where the volume ratio of amniotic fluid to the fetus (taken from the data above) is approximately:

500 / 3400 = 0.147

This, then, would seem to be the multiplier that we use in determining the effective hydrostatic pressure that would be exerted upon the fetus, assuming that the amniotic sac has not yet burst. The greatest external hydrostatic pressure at the bottom of the amniotic sac will be:

0.147 * 19.5 = 2.86 [mmHg]

Now, remember, this small external pressure must overcome skin and muscular resistance. It would seem that less that 3 millimeters of mercury pressure on the outside of the skin could have precious little effect on capillary pressure. Certainly this pressure is lower than either systolic (~= 68 [mmHg]) or diastolic (~= 40 [mmHg]) fetal blood pressures (at term).

<a href="http://www.netsvic.org.au/nets/handbook/index.cfm?doc_id=450" target="_blank">http://www.netsvic.org.au/nets/handbook/index.cfm?doc_id=450</a>

Now, if the baby is in the cephalic (head-down) presentation, which is typical, external pressures will be essentially negated, since the surface density of the baby's skull and thorax are significantly greater than that of the legs and feet.

Therefore, I think we can safely conclude that the effects of external hydrostatic pressure on the fetus may be considered negligible, as I have been insisting all along. In the late stages of gestation, the internal CV fluid mechanics are similar to the those experienced by an infant outside the womb. Of course, the fetus does not breathe, and has a developmental CV system, complete with umbilical attachments and sophisticated temporary auxilliary "plumbing".

Final note: It is difficult to trust what is written on this web page (the one touted by scigirl and rufus).

<a href="http://www.rehabinternationalpub.com/issues/fall2002/12.asp" target="_blank">http://www.rehabinternationalpub.com/issues/fall2002/12.asp</a>

In the second paragraph, we read:

This is false and grossly misleading, especially when citing the benefits of "increased venous return". Hydrostatic pressure is a function of depth. "Submerged" is relative. Pressure, as measured in millimeters of mercury (the proper scale for blood pressure), varies from the top to bottom.

15 pounds per square inch is 775 [mmHg], which is much, much greater than blood pressure, and is reached at a depth of 35 feet!

775 / 0.735 = 1054 [cm] = ~ 35 [feet]

I won't bother enumerating several other problems I see with the content of that web page.

For your convenience, here is a conversion website: <a href="http://www.sensotec.com/convert.htm" target="_blank">http://www.sensotec.com/convert.htm</a>

You are welcome to check my math and critique my analysis. This was fun.


Thank you, and good night,

John

[ November 08, 2002: Message edited by: Vanderzyden ]</p>

Vibr8gKiwi

I like the picture of the baby. I notice its umbilical is above its heart. Is it dead yet?

pz

Irrelevant. It also doesn't mention that it keeps the skin moist, but it does. It does say "cushion from trauma"...how do you think that works?
Again, this is irrelevant. This is most definitely not like just being in a garbage bag; it's a fluid filled sac within a cushioning environment of muscle and connective tissue and several epithelial layers and various other fluids.
Are you kidding me? Ever been in a preemie ward? The skin of the fetus is so fragile that just a touch can tear it.
I think there is a flaw somewhere in your calculations.

Standard atmospheric pressure is 760mm Hg. I don't think the fetus is developing under conditions equivalent to the atmospheric density at 25000 feet. Perhaps you are neglecting to consider a few factors?
This is another of those statements that suggest that there are obvious flaws in your reasoning. This simply cannot be true. The fetus is head down. Would you take a newborn baby and prop it up to rest on its head? What would happen? Yet now you are arguing that it spends the entire third trimester with no bouyancy, no support, resting on a floor in an extraordinarily awkward position.
OK, let's think this through.

Let's say I'm in the pool, doing a backfloat. I'm completely immersed, except for my face...let's just use your numbers, and say that my depth is 26.5cm, so the hydrostatic pressure is about 20mm Hg.

Now I open my mouth and take a deep breath. Atmospheric pressure is 760mm Hg; you want to claim that the pressure on my body is only 20mm Hg. That's a rather large pressure differential of 740mm Hg. Air rushes into me with great force, rupturing my lungs and killing me.

I don't know. Something just seems wrong with this scenario. But then, what do I know...I'm no engineer.

Dr Rick

Please pay no attention to the limp; it's to be expected when one shoots one's own foot-off:

<strong> Blood is much, much thicker and denser than water: it is typically almost one-half solid, and the remaining liquid portion is a complex solutive colloidal suspension.

<strong> Our purpose is to discuss the suboptimal design of the fetal circulatory system or provide a reasonable counter to the op's initial assertion; John hasn't done either.

<strong> Bouyancy is a physical property of any fluid system surrounding a confined mass within a gravitational field; to deny this is to deny physics.

<strong> Just because a reference doesn't mention the obvious doesn't mean that the obvious isn't true. Notice the absence of any mention of gravity; so what?

<strong> That's just in the drawing; imagine how "off-vertical" the womb is every night when a pregnant woman goes to sleep.

If the assertions John has made on this thread had any merit, no baby could ever survive a breech presentation, no one could ever stand on his/her head, and even lying down to sleep would be a bitch.

This is a brilliantly succinct observation; dammit, I just wish I had thought of it, first.

Rick

[ November 08, 2002: Message edited by: rbochnermd ]</p>

Happy Wonderer

Er, no. You are talking out of your... hat again, John.

Take a gander at this:

<a href="http://www.mtsinai.org/pulmonary/books/scuba/sectiond.htm" target="_blank">ambient pressure at 33 feet below sea level is 1520 mmHG</a>

This is from a scuba instruction manual. If they get it wrong they die, so they are generally likely to be correct.

And, of course, you are completely misunderstanding what blood pressure is measured in relation to. Think about the consequences of what you claim. The "average" systolic blood pressure is 120mmHG and sea level atmospheric pressure is about 760mmHG depending upon the weather...

Oh, right, that is why air rushes into my veins when I cut myself shaving. I hate when that happens.

Or think about it this way -- what is the blood pressure of a dead person? Higher than that of a living one? Well, perhaps they are stressed out because they are dead...

How to solve this puzzle? Well, the answer to
<a href="http://gumbo2.bae.lsu.edu/mmailand/2350/tests/Test2-1999-key.htm" target="_blank">Question 5</a> might help.

HW

I won't bother enumerating the other problems that I see with your post.

Blinn

[ November 08, 2002: Message edited by: Zetek ]</p>