Nat

Vander,

Let's make this simple. What affects pressure in a closed fluid system? Pump strength, fluid volume, and diameter. Which one of these is changing in Mr. Darwin's proposed solution?

Nat

Rick,

Stop complaining. Those weren't insults - they were characterizations. They are only insults when they are directed at Vander.

Nat

Vander,

Just a quick question - don't want to derail this argument that you are so obvious winning with your incredible insights - do you have any friends? I'd really like to know.

pz

Nat -- please check your pms.

Jesus Tap-Dancin' Christ

<a href="http://medlib.med.utah.edu/WebPath/PEDHTML/PED006.html" target="_blank">http://medlib.med.utah.edu/WebPath/PEDHTML/PED006.html</a>

Way to put the bowels close to the umbilical cord. Bet that kid feels GREAT.

MrDarwin

I'm happy to see that the discussion is going on without me, which it will have to continue to do as I simply have no time to participate right now. I will post a couple more comments, and then will have to retire for a while.

Vanderzyden, I greatly appreciate the admission of error and the apology. But you are still avoiding another of my questions about one of your assertions:

Could you please point out precisely where this mixing will occur?

As I pointed out earlier, the website you are quoting from is not offering these three things as "advantages"; they are merely summarizing the discussion. The first cannot be considered an advantage as it is necessitated by the design in the first place, and the other two are not advantages at all. In fact, if you read a little further in the website you took this from, you will find:

In other words, the "strengthening" you cite as an advantage is actually a disadvantage which needs to be compensated for after birth.

Vanderzyden, I hope you realize that one reason why I am harping on these particular issues is that although you attached great significance to them as demonstrating flaws in my reasoning, instead they strongly suggest that you understand neither the system under discussion, nor the proposed improvements I have suggested.

[ October 28, 2002: Message edited by: MrDarwin ]</p>

Vanderzyden

Rick,

I have to say, despite our rocky start, I find this discussion highly interesting. The cardiovascular system is very, very fascinating, and it is particularly interesting since I have an infant at home. Although internal anatomy is relatively unpleasant to observe, I am coming to see an intense beauty and elegance in the design of the actual system. Thanks for giving me the opportunity to think this through and examine it in more detail.

No doubt you realize now that our conversation has gone longer and off in more tangents than necessary. This is partly my fault. However, it is easy to overlook the significant details in what you and MrD have described (I see that no one else in this thread seems to have noticed, either). Let me highlight what is most important in continuing this discussion:

I see it now. You guys want the pulmonary trunk draining into the umbilical arteries, and the umbilical vein (somehow) joining the four pulomary veins. OK, that's a bit better. It alleviates the concerns I had about the gross misplacement of the heart (pump) in this hypothetical circulatory system. But you still have major problems:

1. Umbilical arteries -- If you branch the umbilical artery anywhere past the apex of the aortic arch, then you are diverting pressure from the carotids and subclavians. In a vertical circulatory system, the lower you place the effluent--in this case, the branch of the umbilical arteries--the more gravity assists the delivery of fluid to the exit, and conversely. So, in the CV system, the closer you place the branching of the umbilical arteries to towards apex (top) of the aorta, the more pressure will need to be diverted from upstream branches in order to push blood out of these arteries. If the effluent is low enough, then gravity will be sufficient, and no pressure diversion will be necessary (as we see in the actual system). If you don't place the umbilical arteries low enough, then you must decrease the diameter of the upper thoracic arteries to lower their pressure requirements. Of course, by doing so, you set off a chain of subsequent design alterations, beginning with the increase in the strength and capacity of the heart. But this is unnecessary if the designer remembers that gravity is a force that is always--and consistently--available.

Note: Observe, in the actual CV system, that the left subclavian artery is positioned precisely at the apex of the arch. In consideration of the foregoing discussion, you can now see why.

2. Umbilical vein diverging into one, two, or four pulmonary veins -- How will the small pulmonary veins handle the large volume of blood from the umbilical vein? On which side will you place the junction(s)? Perhaps you will say both. Then, which set will it be--upper or lower? You can see how easy it is for me to envision a huge mess. Certainly the design is a hack job, at best. Even with four small branches from the umbilical vein, you must increase the diameter of the pulmonary veins to accomodate higher volume blood flow coming from umbilical vein. However, this diameter will be too large for the needs of the lungs once the newborn begins breathing, when the lungs will expand and the new requirements will call for low pressure, high velocity blood flow in the pulmonary veins. This cannot be accomplished if these veins have been enlarged previously for fetal circulation. So, while the design critic has provided direct, close proximity access for the placental flow, he has also introduced serious, non-trivial complications. In particular, if we consider the overall CV system, it is clear that four branches--one to each pulmonary vein--are unnecessary (and malfunction-prone) complexity.

You suggested yet another alternative:

3. Umbilical vein joins SVC -- This configuration could handle the volume of umbilical vein, but it is essentially no different than the CV system as it is now. With the superior vena cava (SVC) and inferior vena cava (IVC) both entering the right atrium, we have the same volume and pressure configuration as in the actual CV system (if we ignore small gravitational effects coming down the SVC). You are still combining the blood from the superior and inferior vena cavas into the right atrium. And, while this blood is at low pressure, it is a high volume. In fact, it is the highest volume of blood at any point in the entire CV system (like the return or sump in a mechanical system). Without the foramen avole (FA) and the ductus arteriosus (DA), this entire volume will be pumped into the pulmonary arterial trunk, and yet the lungs are completely compressed!!! It is clear, then that, in the actual fetal CV system, the FA and DA function as critical bypass valves. They are necessary in a development configuration where (1) the lungs are not yet operational and (2) the pulmonary arteries cannot sustain a high volume of blood (the pulmonary arteries would burst!). Therefore, you still need both the FA and DA if the umbilical vein empties into the right side of the heart.

4. Organ ligaments -- Also, you must install a replacement for the ligamentum arteriosum in order to dampen the heavy vibrations of the artic arch and the pulmonary trunk. Similarly, you must also install replacement ligament for the liver. (How would purposeless macroevolution processes make such provisions?).

5. Placement of the umbilicus at a thoracic boundary -- If you agree to place the effluents to the umbilical arteries below the diaphragm, then you must either (1) loop them back towards the thorax--which requires even more strengthening of the heart, or (2) make arrangements for a second umbilicus!

To address scigirl's previous concern: All of these considerations still apply if the baby is inverted (or horizontal) in the womb. The gravitational force simple works in the opposite direction--where it assists in the effluent in one case, it assists in the influent in the other.

Final notes: In your "design improvement", it is likely that optimization concerns arise for the distribution of oxygenated blood (although the coronary concern has been alleviated). I have not taken the time to consider them here, since the foregoing argument is strong enough to show that the actual CV system cannot be improved as you say.


John

[ October 28, 2002: Message edited by: Vanderzyden ]</p>

Vanderzyden

* BUMP *

No objections or corrections? Rick? MrD?

MrDarwin

You mean aside from the ones I posted at the top of this very page, which you are still ignoring after several days?

Vanderzyden

MrD,

I expect more from you, given the high confidence that you have displayed here.

You did not give strong support for your design critique in your OP, nor have you done so in recent posts. You have done nothing to refute my core arguments--neither the first line of reasoning (if only by clarifying the direction and routing of the blood) nor this second one. The objections you have raised are minor, even trivial. Some of them (e.g. the strength of the right side of the heart) are erroneous. But I will not take time to address them until you assess my reasoning. Courtesy requires that you provide an exposition and critique, or indicate concession. I should think this is quite reasonable, since I have taken time to present a cogent set of arguments. Am I to construe your silence as complete agreement?

Should you respond again in the same fashion, I will take it that you have nothing more of substance to write in defense of your suggestion of sub-optimal design of the CV system. Perhaps Rick will lend you a hand.

The design of the actual fetal and adult CV systems are amazingly elegant and are the best design for the given purpose. The human body is a work of fanstastical wonders.

When I consider your heavens,
the work of your fingers,
the moon and the stars,
which you have set in place,
what is man that you are mindful of him,
the son of man that you care for him?

--Psalm 8:3-4


John

[ October 30, 2002: Message edited by: Vanderzyden ]</p>