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• Development of Blood Vessels
• Blood vessel formation (angiogenesis)
starts at the beginning of the third week.
• Blood vessels first start to develop in the
extraembryonic mesoderm of the yolk sac,
connecting stalk, and chorion.
• Blood vessels begin to develop in the
embryo about two days later.
• Production of Blood
• Production of blood (hemopoiesis or
hematopoiesis) begins first in the yolk sac wall
about the third week of development.
• Erythrocytes produced in the yolk sac have
nuclei. Blood formation does not begin inside
the embryo until about the fifth week.
• Erythrocytes produced in the embryo do not
have nuclei (eunucleated). Hematopoiesis
inside in the embryo occurs first in the liver, then
later in the spleen, thymus, and bone marrow.
• Vitelline circulation refers to the system
of blood flowing from the embryo to the
yolk sac and back again.
• The yolk-sac is situated on the ventral
aspect of the embryo; it is lined by
endoderm, outside of which is a layer of
mesoderm.
• It is filled with fluid, the vitelline fluid, which
possibly may be utilized for the
nourishment of the embryo during the
earlier stages of its existence.
• Blood is conveyed to the wall of the
sac by the primitive aortæ, and after
circulating through a wide-meshed
capillary plexus, is returned by the
vitelline veins to the tubular heart of
the embryo.
• This constitutes the vitelline
circulation, and by means of it
nutritive material is absorbed from the
yolk-sac and conveyed to the embryo.
• Cardinal veins
• Cardinal veins Scheme of arrangement of
parietal veins.
• During development of the veins, the first
indication of a parietal system consists in the
appearance of two short transverse veins, the
ducts of Cuvier, which open, one on either side,
into the sinus venosus.
• Each of these ducts receives an ascending and
descending vein.
• The ascending veins return the blood from the
parietes(hollow organs) of the trunk and from the
Wolffian bodies, and are called cardinal veins.
• The fetal circulation
• In the fetus the four-chambered heart
circulates blood through the fetal body,
through the placenta and through the
vessels supplying the gut (previously the
vitelline vessels)
• The blood arriving at the right side of the
heart through the superior and inferior
venae cavae has come from the fetus
(body and head), from the digestive
system and liver, and from the umbilical
vein.
• The blood in the umbilical vein is highly
• The blood mixes in the right
atrium and is directed towards the
interatrial septum where it passes
through the foramen ovale into
the left atrium.
• From the left atrium it passes into
the left ventricle and so to the
systemic circulation through the
aorta.
• Any blood which passes into
the right ventricle leaves that
chamber though the
pulmonary trunk to pass to the
aorta through the ductus
arteriosus, but also to a small
extent to the lungs.
• The aorta thus distributes blood to the
head, the body including the gut, and
to the placenta through the paired
umbilical arteries.
• At the placenta the blood is
oxygenated and collected in the
single umbilical vein.
• The umbilical vein travels to the liver
where a connection, the ductus
venosus, shunts most of the blood
into the inferior vena cava.
• This circulation ensures that
the head and upper limbs
receive oxygenated blood, that
the body receives fairly well
oxygenated blood with some
addition of venous blood from
the head and lungs.
• Since at this stage the lungs are
not inflated, the resistance in the
pulmonary circulation is high and
the blood is diverted back into the
systemic circulation through the
ductus arteriosus.
• The ductus venosus diverts blood
from the umbilical vein bypassing
the liver, directly into the IVC.
• Changes at birth
• At birth the lungs will inflate and the
placenta will become disconnected from
the source of oxygenation.
• With the first breath the resistance in the
pulmonary circulation falls as the lungs
inflate.
• Blood in the right ventricle is able to pass
through the pulmonary circulation where
oxygenation occurs.
• Oxygenated blood from the lungs returns
to the left atrium.
• Cutting and tying the umbilical cord closes
both the umbilical arteries and the
umbilical vein.
• Other significant changes in the circulation
occur over the next hours and days.
• The ductus arteriosus starts to constrict
and is usually permanently closed by three
months after birth.
• As pressure in the left atrium rises, the
septum primum is forced against the
septum secundum, closing the foramen
ovale.
• Eventually the two septa fuse to
permanently separate the two atria.
• Absence of flow in the umbilical vein
causes it to become ligamentous - the
ligamentum teres, although a narrow
channel remains.
• The ductus venosus becomes the
ligamentum venosum and the blood
from the portal vein must pass
through the liver before reaching the
IVC.