A primer of fetal cardiology and circulation.
Bruce Forest
There seems to be a controversy about the fetal heart, and how it
develops, particularly between Nyikos and Cochran. The particular bone of
contention seems to be when the atria and ventricles bifurcate (divide in
two), and the heart becomes fully-four chambered. I think (correct me if
I'm wrong) that Nyikos feels that the complete bifurcation happens either
before 8 weeks fetal age, or after birth. I think Mark Cochran feels the
bifurcation happens much later in fetal development. This subject came up
in an email between Peter any me, and I have to admit, I've forgotten all
my fetal cardiology. So, I went into my dusty storage, and got out some
texts from med school, and threw this together, in the hope of settling
any controversy, and to give someone something to refer to the next time
the subject is brought up.
The short answer? The atria and ventricles have each begun to be divided
by a septum at around 8 weeks fetal age, however, an opening in the septum
called the foramen ovale stays open till up to a year after birth. There
are a lot of differences in the circulation of a fetus, and the
circulation of a one week old baby. (So much for the argument' there is no
difference between a term fetus and a newborn, just location.')
The long answer?
Throughout the fetal stage, the maternal blood supplies the fetus with
oxygen and nutrients and carries away its wastes. These substances diffuse
between the maternal and fetal blood through the placental membrane, and
they are carried to and from the fetal body by means of the umbilical
vessels . Consequently, the fetal vascular system must be adapted to
intrauterine life in special ways, and its pattern of blood flow must
differ from that of an adult.
In fetal circulation, the umbilical vein transports blood rich in oxygen
and nutrients from the placenta to the fetal body. This vein enters the
body through the umbilical ring and travels along the anterior abdominal
wall to the liver. About half the blood passes into the liver, and the
rest enters a vessel called the ductus venosus that bypasses the liver.
The ductus venosus travels a short distance and joins the inferior vena
cava. There the oxygenated blood from the placenta is mixed with
deoxygenated blood from the lower parts of the fetal body. This blood
continues through the vena cava to tht right atrium.
In an adult heart, blood from the right atrium enters the right ventricle
and is pumped through ik pulmonary artery to the lungs. However, in the
fetus the lungs are nonfunctional, and the blood largely bypasses them.
More specifically, as blood relatively rich in oxygen enters the right
atrium of the fetal heart, a large proportion of it is shunted directly
into the left atrium through an opening in the atrial septum. This opening
is called the foramen ovale, and blood passes through it because the blood
pressure in the right atrium is somewhat greater than that in left atrium.
Furthermore, a small valve (septum primum) located on the left side of the
atrial septum overlies the foramen ovale and helps to prevent blood from
moving from left to right. The rest of the blood entering the right
atrium, as well as a large proportion of the deoxygenated blood entering
from the superior vena cava, passes into the right ventricle and out
through the pulmonary artery.
The vessels of the pulmonary circuit have a high resistance to blood flow
since the lungs are collapsed, and the vessels are somewhat compressed.
Enough blood reaches the lung tissues, however, to sustain them. Most of
the blood in the pulmonary artery bypasses the lungs by entering a fetal
vessel called duchtus arteriosus, which connects the pulmonary artery to
the descending portion of the aortic arch. As a result, blood with a
relatively low oxygen concentration returning to the heart in the superior
vena cava bypasses the lungs. At the same time it is prevented from
entering the portion of the aorta that provides branches leading to the
heart and brain.
The more highly oxygenated blood that enters the left atrium through the
foramen ovale is mixed with a small amount of deoxygenated blood returning
from the pulmonary veins. This mixture moves into the left ventricle and
is pumped into the aorta. Some of it reaches the myocardium by means of
the coronary arteries, and some reaches the tissues of the brain through
the carotid arteries.
The blood carried by the descending aorta is partially oxygenated and
partially deoxygenated. Some of it is carried into the branches of the
aorta that lead to various parts in the lower regions of the body. The
rest passes into the umbilical arteries which branch from the internal
iliac arteries and lead to the placenta. There the blood is reoxygenated.
The umbilical cord usually contains two arteries and one vein. In a small
percentage of newborns, there is only one umbilical artery. Since this
condition is often associated with various other cardiovascular disorders,
the number of vessels within the severed cord is routinely counted
following a birth.
This pattern of fetal circulation becomes fully developed near the middle
of the prenatal period. At the time of birth, important adjustments must
occur when the placenta ceases to function and breathing begins.
Fetal Circulatory Adaptions....
Umbilical vein...Carries oxygenated blood from placenta to fetus
Ductus venosus...Conducts about half the blood from the umbilical vein
directly to the inferior vena cava, thus bypassing the liver
Foramen Ovale...Conveys large proportion of blood entering - the right
atrium from the inferior vena cava, through the atrial septum and into the
left atrium, thus bypassing the lungs
Ductus Arteriosus...Conducts some blood from the pulmonary artery to the
aorta, thus bypassing the lungs
Umbilical arteries...Carry blood from the internal iliac arteries to the
placenta for reoxygenation
All disappear, or become vestigal at birth, or soon after. So much for
'the fetus is anatomically just like a baby, only smaller. The differences
are striking.
Prior to birth, the fetus depends primarily on glucose and fatty acids
obtained from the mother's
blood as energy sources. The newborn, on the other hand, is suddenly
without an external source of nutrients, and its mother's milk will not be
produced for 2 to 3 days. Furthermore, the newborn has a relatively high
rate of metabolism, and its liver, which is not Sully mature, may be
unable to supply enough glucose to support the metabolic needs.
Consequently, the newborn typically utilizes stored fat as an energy
source.
As a rule, the newborn's kidneys are unable to produce concentrated urine,
so the newborn excretes a relatively dilute fluid. For this reason, the
newborn may become dehydrated and develop a water and electrolyte
imbalance. Also, certain of the newborn's homeostatic control mechanisms
may function imperfectly. The temperature regulating system, for example,
may be unable to maintain a constant body temperature. As a consequence,
the body temperature may be unstable during the first few days of life and
respond to slight stimuli by fluctuating above or below the normal level.
As was mentioned, when the circulation of blood through the placenta
ceases and the lungs begin to function at birth, adjustments are made in
the circulatory system that was formerly adapted to the needs of
intrauterine life.
For example, immediately following birth, the umbilical vessels constrict.
The arteries close first, and if the umbilical cord is not clamped or
severed for a minute or so, blood continues to flow from the placenta to
the newborn through the umbilical vein, adding to the newborn's blood
volume.
The proximal portions of the umbilical arteries persist in the adult as
the superior vesical arteries
that supply blood to the urinary bladder. The more distal portions become
solid cords (lateral umbilical ligaments.) The umbilical vein becomes the
cordlike ligamentum teres that extends from the umbilicus to the liver in
an adult. Similarly, the ductus venosus constricts shortly after birth and
is represented in the adult as a fibrous cord (ligamentum venosum), which
is superficially
embedded in the wall of the liver.
What does all this mean, then? The fetal circulatory system is markedly
different in several major aspects from the newborn's circulation, and
even more different than the adult circulation. The question of when the
four-chambered heart appears is a matter of interpretation, since the
foramen ovale may not close for a year past birth. IMHO, while the
bifurcation, as Nyikos says, appears to begin prior to fetal stage (8
weeks) it does not seem to complete till after birth. So, the answer
'either before 8 weeks, or not till birth' is not entirely correct; yet it
is not fatally flawed either.
My sources:
_Gray's Anatomy_ 15th Edition;
_Human Anatomy and Physiology_, Second Edition, John W. Hole jr. 1978 Wm
C. Brown Co.
_Current Obstetric and Gynecologic Diagnosis and Treatment_ 8th Ed,
DeCherney, Pernoll 1994
--
Bruce Forest...
bfo...@futuris.net
bfo...@interramp.com
bfo...@bliss.demon.co.uk
10416...@compuserve.com
dro...@aol.com...
PGP key on http://www-swiss.ai.mit.edu/~bal/pks-toplev.html
"Throw back the little ones, and pan fry the big ones..
Use tact, poise and reason, and gently squeeze them."
Becker/Fagen 'Katy Lied'
Peter Nyikos
>Hi guys..
>There seems to be a controversy about the fetal heart, and how it
>develops, particularly between Nyikos and Cochran.
Yes, on the talk.abortion thread, "M. Cochran and the telltale
fetal heart", except that Mark is running away from all evidence
I post of his past misdeeds.
The particular bone of
>contention seems to be when the atria and ventricles bifurcate (divide in
>two), and the heart becomes fully-four chambered. I think (correct me if
>I'm wrong) that Nyikos feels that the complete bifurcation happens either
>before 8 weeks fetal age, or after birth.
It's a matter of semantics, depending on whether one counts the
intra-atrial foramen. It's like saying "gestation" means either
"from fertilization" or "LMP": I'm willing to play it either way,
as long as we are consistent about it.
> I think Mark Cochran feels the
>bifurcation happens much later in fetal development.
Unclear from what I have easily accessible. I downloaded some
much earlier stuff in which he even claims that fetuses should
not even be regarded as mammals because of their lack of 4 chambers.
However, it would be utter nonsense to classify them as any other
class of vertebrate since e.g. in reptiles it is the ventricles
that are imperfectly divided from each other, not the atria.
>The short answer? The atria and ventricles have each begun to be divided
>by a septum at around 8 weeks fetal age, however, an opening in the septum
>called the foramen ovale stays open till up to a year after birth. There
>are a lot of differences in the circulation of a fetus, and the
>circulation of a one week old baby. (So much for the argument' there is no
>difference between a term fetus and a newborn, just location.')
Well, the differences are there, but then so are the differences
between someone who is in the midst of open heart surgery and
one on whom it has been completed.
>The long answer?
>Throughout the fetal stage, the maternal blood supplies the fetus with
>oxygen and nutrients and carries away its wastes. These substances diffuse
>between the maternal and fetal blood through the placental membrane, and
>they are carried to and from the fetal body by means of the umbilical
>vessels . Consequently, the fetal vascular system must be adapted to
>intrauterine life in special ways, and its pattern of blood flow must
>differ from that of an adult.
Exactly. A patten EMINENTLY appropriate to placental mammals
such as us humans.
Thanks for the rest. Most of it is familiar to me from
_Williams Obstetrics_ and _Development of Vertebrate Anatomy_,
but it's nice to see different write-ups.
I noted that fetal urine is very dilute. So much for Mark
Cochran's characterization of the fetus as "pissing into
her [the mother's] blood."
Not a very pleasant character, that Cochran.
Peter Nyikos -- standard disclaimer --
Professor, Dept. of Mathematics
University of South Carolina
Columbia, SC 29208