MrDarwin

<a href="http://www.cayuga-cc.edu/about/facultypages/greer/biol204/heart4/heart4.html" target="_blank">The circulatory system of the human fetus</a> is considerably different from that of the adult, or even of the newborn. Oxygen reaches the fetus from the mother by way of the umbilical cord, rather than through the lungs.

Entering at the chest would allow a direct and efficient bypass of the lungs, yet the umbilical cord of all mammals enters the body through the belly. This necessitates all kinds of odd arrangements in the fetus, including a mixing of oxygenated and deoxygenated blood, and also necessitates a drastic reorganization of the circulatory system at the moment of birth. This odd system fails to work often enough that some serious and potentially life-threatening birth defects occur regularly.

Perhaps not coincidentally, in egg-laying tetrapods (birds, reptiles) nutrients enter the fetal body from the yolk, again through the belly. Indeed, we find the same arrangement in amphibians and fish. But in fish and amphibians, however, the eggs do not have shells and develop in the water; oxygen does not enter at this point of attachment to the yolk because the developing embryos obtain their oxygen directly from the surrounding water.

The evolutionary explanation is a simple one: the position of the umbilical cord has been constrained by the position of its precursors. As we see over and over, evolution doesn't come up with new solutions from scratch, it works with what is already there, even when the resulting solution is not the best one.

pz

I'd also mention that the adult circulatory system carries peculiar vestiges of evolutionary history, as well. The primitive condition was for a single ventral major efferent vessel that branched into a pair and then branched meristically to pass through the branchial arches, where blood was oxygenated; these branches then rejoined to form a dorsal aorta that passed back into the systemic tissues. You see this same condition in modern fish, and most interestingly, in fetal mammals.

In adult mammals, our circulatory system is based on that same structure, but has been extensively modified. That primary ventral vessel pair has been reduced to the carotids, which no longer carry the full cardiac output. Instead, a short circuit has evolved to shunt most of the output directly to the dorsal aorta. That peculiar aortic arch, where the left ventricle pumps blood anteriorly which then immediately makes a hard curve posteriorly, is a consequence of that historical accident.

Copernic

MrDarwin,
Let me get this straight. In fish and amphibians, the umbilicus is only used for nutrient and waste transfer and in more terrestrial animals, oxygen/CO2 transfer merely co-opted this pathway in exchange for harder, non-gas-permeable shells and subsequently, non-gas-permeable wombs and fetal skin.

A good designer should have a completely different umbilicus set up, one that would provide gas exchange in better conjunction with the newborn cardiovascular system. In this way, the switch-over would be less drastic.

is this the gist?

J

[ October 15, 2002: Message edited by: Copernic ]</p>

MrDarwin

Yes, that is the gist of it, although I'm not suggesting a completely different umbilical cord, simply a different placement for it (closer to the lungs and heart) which would require far less internal rearrangement to accommodate it, and thus a less drastic change from one system to another at birth.

[ October 16, 2002: Message edited by: MrDarwin ]</p>

One of the last sane

I'm trying to picture people with bellybuttons on their chests, but it ain't easy.

Albion

Well, it'd probably just be like a third nipple. Is that so hard to visualise? Yeah, maybe...

Vanderzyden

MrD,

First, allow me to thank you for recalling this to my attention, as I have not considered the beauty and complexity of the circulatory system for a long time. I appreciate this opportunity.

With the position that MrD is taking, yet again we see blantant bias in the presentation of this "sub-optimal" design. As with other design critiques, we find:

-- no admission of the (a) inherent complexity and fascinating function of the circulatory system in general or (b) the transition in particular
-- an intense focus on potential defect, which inhibits the consideration of the overall context in which the "defect" might have distinct and significant advantages
-- no elucidation of what is particularly suboptimal, and how "birth defects" would be reduced by relocating the umbilicus
-- no discussion of the problems that would develop from placement of the umbilicus near an organ or at the center of the circulatory system
-- no offer of a "superior" design
-- no mention of the inadequacy of cardiovascular substitutes, such as artifical hearts.

The three vital processing organs in the circulatory system are the heart, lungs, and liver. For the system to function in both the fetus and the infant, the overall specifications of the system must ensure the sustenance of these organs while in development and also prepare them for their mature operation once the baby is born and begins to breath air.

Placement of the umbilicus above the liver means that the bulk of the liver would not get highly oxygenated blood, as it would receive "leftover" blood at considerably lower low pressure (since it would be on the pulmonary side of the fetal cardiovascular system). Also, placement at the chest, as MrD suggests, is perhaps the most problematic alternative. The breastbone, ribs, compressed lungs and diaphragm are packed very densely in the chest cavity, and there is very complex "mainline" plumbing near the heart. Such a placement would not only be very "tricky", but there would probably be an unacceptably high likelihood of a traumatic disruption at the transition to autonomous breathing. It would seem that these same "placement" considerations apply equally to birds and reptiles.

A small note regarding aesthestics: personally, I prefer the "belly button" in the lower abdomen! Observe that it is placed in the lower center of the abdomen, directly between the two rows of the rectus abdominus muscles groups.

Mixing of oxygenated and deoxygenated blood is not a problem. If the oxygen concentration at the placenta is sufficiently high, then the mixing of deoxygenated blood at the junction of the ductous venosus and the inferior vena cava is a non factor. More important, the design critic must realize that placement of the inflow juncture anywhere in system will result in a confluence of oxygenated and deoxygenated blood. Regardless, there are several other locations where such confluences occur in the fetus, such as the left atrium and the exit from the liver. Many such junctions also remain in the adult human.

There is no "drastic reorganization of the circulatory system at the moment of birth. It is not reorganized. Three "valves/ducts" close, the umbilical cord is tied off, and the baby is on its own. The original schematic of the system remains largely intact. Amazingly, at birth, the lungs begin to function (immediately!). This relaxes the pressure in the lungs, which in turn invites more blood to flow from the pulmonary arteries. Instead of crossing from the right to the left atrium (through the foramen ovale), blood now flows freely from the right atrium down to the right ventricle and into the pulmonary arteries. This reduces pressure on the foramen ovale from the right side. Since the lungs are now operational, more blood flows from the pulmonary veins into the left atrium, increasing the pressure on the foramen ovale to the point of complete closure! As the web site says, the heart therefore becomes two pumps, one that is high pressure for the arterial system and one that is low pressure for the pulmonary system! Eventually, the foramen ovale fuses to become the fossa ovalis--a permanent part of the septal wall (divider between the left and right atria).

Also, the ductus arteriosis, which connects the right ventricle with the aortic arch, normally closes within 2 days. This duct then becomes--amazingly--connective tissue which lends additional structural rigidity and cohesiveness between the aorta and the pulmonary trunk. It also prevents the one from "pinching off" the other when the upper torso is in motion. Surely this is the mark of a caring, anticipatory designer! The same thing occurs with the unbilical remnants: the umbilical vein and ductus venosus both become ligaments for the liver. In summary, the nearly simultaneous closure of the foramen ovale, the ductus arteriosis, and the ductus venosus seals off both ends of the now autonomous circulatory system.

Before I finish, let me respond to this observation:

This narrow assessment ignores the fact that the heart is located in the extreme anterior (front) of the chest cavity. The aorta has three branches immediately downstream of the exit from the heart. These branches are the brachiocephalic, the left carotid, and the subclavian. The right carotid branches downstream on the brachiocephalic. These arteries supply the brain and the arms, the access points of which are located above and to the rear of the heart. So, there must be some rotation to the posterior to serve the head (through the neck) and the arms (which are connected at the upper posterior of the torso). These parts of the body require less blood supply than the vital organs and the legs, and so do not need to "carry the full cardiac output". In fact, much blood must be supplied to the lower abdomen and the legs. Another oversight concerns the extremities of entire cardiovascular system, also known as the circulatory system. Throughout the entire body, the blood is reoxygenated and returned 180 degrees back towards the heart. Turns in the system are not defects whatsoever!

MrD's conclusion is a gross non-sequitur:

Yet again, we see a failed attempt to raise suspicion about the necessity for a designer. Evolution does not work. Period. Certainly, MrD has not been persuasive of its successful operation in this case. Nor has he shown the cardiovascular system to be poorly designed.

I find it amazing that a human can dare to play the role of design consultant to the Creator. MrDarwin must be very highly qualified, although he is incapable of choosing the place of his birth or preventing his eventual death. He does not have power in this very small thing, and yet he is so bold to criticize the design of the wonderfully spectacular human body. To top it off, MrD does not offer an alternative, nor does he really describe his critique. He justs floats a few "toy balloons" that are easily deflated.

Tell us, MrDarwin, do you think you will stand before God with such pitiful defiance?


Vanderzyden

Doubting Didymus

Insane religious preaching belongs in other forums. If you can keep the discussion scientific, that would be nice.

It should not surprise you that MrD 'dares' to challenge the designer. In case you missed it, this is an atheist board. You might want to look up the term?

[ October 16, 2002: Message edited by: Doubting Didymus ]</p>

S2Focus

Heh heh... every time I read one of Vanderzyden's posts, I'm reminded of an old Mark-Twainism that goes along the lines of: I'd prefer Heaven for the weather, but Hell for the company.

If you subscribe to fundamentalist Christian doctrine, then it seems that you are confronted with a bit of a tradeoff: You can spend eternity in a place with good weather or good company (but not both).

[ October 16, 2002: Message edited by: S2Focus ]</p>

scigirl

Yeah except for all those babies where it fails to close, and they either 1) DIE or 2) pay a surgeon a crap-load of money to fix it!

Congenital heart defects are common, vanderzyden. Did god just hate those kids?

scigirl