From: bmc@mayo.edu (Bruce Cameron)
Subject: Re: I'm a bit pissed...
Organization: Mayo Foundation, Rochester MN. Campus
Date: Mon, 27 Jul 92 13:30:07 GMT

In article <1992Jul24.185004.16938@uceng.UC.EDU>, vlsi06@nest.ece.uc.edu (Gregory S Clausen) writes:
|> 
|> For the nth time.  The heart begins beating at 18 days, and by 21 days it is
|> pumping, through a closed circulatory system, blood whose type is different
|> from that of the mother.
|> 
|> J.M. Tanner, G.R. Taylor, et al.,
|> _Growth_,
|> New York:  Life Sciences Library, 1965, p. 64.
|> 
|> If you have difficulties with this, please write Messrs. Tanner and Taylor.

Please provide their address and I'll be glad to take this
up with them, however...

Cardiac development begins in humans during the 8 somite
stage (third week [day 21-28] of gestation). It begins to
beat when the cardiac rudements start to fuse (forming the
ventricles) at some point between the 10-12 somite stage.

At this point, the beating is uncoordinated due to the
fact that the pacemaker nodes have yet to form and the
heart is not yet connected to the rest of the circulatory
system as the aortic root, vena cava and the atriums have
not yet formed.

Now, even if we were at some early stage of gestation, 
and the heart was fully formed, undergoing rythmic and
controlled beating, and there was an adequate supply of
fetal blood, there are still differences between fetal
and adult circulation. To recap:

Fetal Circulation

        Differs from that of the parent in that:

                a) fetal lungs are functionally inactive

                b) fetus depends completely on the placenta for oxygen and
                nutient supply.

        Oxygenated fetal blood from the placenta passes through the umbilical
        vein to the liver. A major fraction passes through the liver, while a
        small fraction bypasses the liver and enters the inferior vena cava via
        the ductus venosus. The larger stream in shunted in the left atrium,
        while the smaller joins with blood returning from the superior vena
        cava and enters the right atrium.

        In contrast to the adult, fetal ventricals pump in parallel and only
        one third of the right ventrical output goes to the lungs; the rest
        enters the aorta directly via the ductus arteriosus. One third of
        blood pumped goes to head, upper thorax and arms, the rest goes to 
        the remaning organs and the placenta (more then half of cardiac output
        goes directly back to the placenta).

Circulatory changes that occur at birth:

        a) infant blood pressure rises
        b) ductus venosus closes in responce to a cessation of blood flow in
           umbilical vein.
        c) asphyxia due to closure of umbilical veins & cooling of body triggers
	   respiration centers
        d) ductus arteriosus begins to close (process takes 1 - 2 days)
        e) right ventrical thickness begins to diminish.

        Faber J.J., and Thornburg K. L., Placental Physiology, Raven Press,
        New York, 1983

        Mott, J. C. and Walker D. W., Neural and endocrine regulation of
        circulation in the fetus and newborn. Handbook of Physiology,
        section 2: The Cardiovascular system -- peripheral circulation
        and organ blood flow, vol III, American Physiological Society,
        Bethesda, MD. 1983

Now, outside of being a wonderfully inflamitory rallying
point and a trite truism is there any difference between
abortion stops a beating heart and eating meat stops a
beating heart?

-- 
--Bruce
-----------------------------------------------------
Bruce M. Cameron		bmc@mayo.edu
Mayo Foundation			voice: (507) 284-3288
200 1st St SW			fax:   (507) 284-9623
Rochester, MN 55905		ARS -- WD9CKW
Newsgroups: talk.abortion