Chapter 28
Pregnancy and Human Development
1079
28
Development of the Fetal Circulation
Embryonic develop-
ment of the cardiovascular system lays the groundwork for the
fetal circulatory pattern, which is converted to the adult pattern
at birth. Te first blood cells arise in the yolk sac. Before week
3 of development, tiny spaces appear in the splanchnic meso-
derm. Tese are quickly lined by endothelial cells, covered with
mesenchyme, and linked together into rapidly spreading vascu-
lar networks, destined to form the heart, blood vessels, and lym-
phatics. By the end of week 3, the embryo has a system of paired
blood vessels, and the two vessels forming the heart have fused
and bent into an S shape. By 3½ weeks, the miniature heart is
pumping blood for an embryo less than a quarter inch long.
Unique cardiovascular modifications seen only during pre-
natal development include the
umbilical arteries
and
vein
and
three
vascular shunts
(Figure 28.14)
. All of these structures
are occluded at birth. As you read about these vessels, keep in
mind that the blood is flowing from and to the fetal heart. Te
large umbilical vein carries freshly oxygenated blood returning
from the placenta into the embryonic body, where it is conveyed
to the liver. Tere, some of the returning blood percolates
through the liver sinusoids and out the hepatic veins. Most of
the blood coursing through the umbilical vein, however, en-
ters the
ductus venosus
(duk
9
tus ve-no
9
sus), a venous shunt
Epiblast
ECTODERM
MESODERM
ENDODERM
Notochord
Somite
Intermediate
mesoderm
Lateral plate
mesoderm
Somatic
mesoderm
Splanchnic
mesoderm
• Epider
mis, hair, nails,
glands of skin
• Brain and spinal cor
d
• Neural cr
est and
derivatives (e.g.,
cranial, spinal, and
sympathetic ganglia
and associated
nerves; chromaffin
cells of the adrenal
medulla;
pigment
cells of the skin)
Nucleus
pulposus of
intervertebral
discs
• Scler
otome:
vertebrae and
ribs
• Der
matome:
dermis of dorsal
body region
• My
otome:
tr
unk and limb
musculatur
e
• Kidn
eys
• Gonads
Parietal serosa
• Der
mis of ventral
body region
• C
onnectiv
e tissues
of limbs (bones,
joints, and
ligaments)
Wall of digestive
and respiratory
tracts (except
epithelial lining)
Visceral serosa
• Hear
t
• Blood v
essels
Epithelial lining
and glands of
digestive and
respiratory tracts
Figure 28.13
Flowchart showing major derivatives of the embryonic germ layers.
that bypasses the liver sinusoids. Both the hepatic veins and the
ductus venosus empty into the inferior vena cava where the pla-
cental blood mixes with deoxygenated blood returning from the
lower parts of the fetus’s body. Te vena cava in turn conveys this
“mixed load” of blood directly to the right atrium of the heart.
AFer birth, the liver plays an important role in nutrient
processing, but during embryonic life the mother’s liver per-
forms these functions. Consequently, blood flow through the
fetal liver during development is important only to ensure that
the liver cells remain healthy.
Blood entering and leaving the heart encounters two more
shunt systems, each serving to bypass the nonfunctional lungs.
Some of the blood entering the right atrium flows directly into
the leF atrium via the
foramen ovale
(“oval hole”), an opening
in the interatrial septum loosely closed by a flap of tissue. Blood
that enters the right ventricle is pumped out into the pulmonary
trunk. However, the second shunt, the
ductus arteriosus
, trans-
fers most of that blood directly into the aorta, again bypassing
the pulmonary circuit. (Te lungs
do
receive adequate blood to
maintain their growth.) Blood enters the two pulmonary bypass
shunts because the heart chamber or vessel on the other side of
each shunt is a lower-pressure area, owing to the low volume of
venous return from the lungs. Blood flowing distally through
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