_From Kimber’s Anatomy._
The peculiarities of the fetal circulation, leaving details aside,
are: the direct communication between the two auricles of the heart,
through an opening called the foramen ovale; the communication between
the pulmonary artery and descending portion of the arch of the aorta,
by means of a tube called the ductus arteriosus; and the communication
between the placenta and the fetus, by means of the umbilical cord.
The arterial blood for the nutrition of the fetus is carried from the
placenta along the umbilical cord by the umbilical vein. Entering the
fetus at the umbilicus, the blood passes upward to the liver and is
conveyed into the inferior vena cava in two different ways. The larger
quantity first enters the liver, and, alone or in conjunction with the
blood from the portal vein, ramifies through the liver before entering
the inferior vena cava, by means of the hepatic veins. The smaller
quantity of blood passes directly from the umbilical vein into the
inferior vena cava, by a tube called the ductus venosus.
In the inferior vena cava the blood from the placenta becomes mixed
with the blood returning from the lower extremities of the fetus. It
enters the right auricle and, guided by a valve—the Eustachian valve,
passes through the foramen ovale into the left auricle. In the left
auricle it unites with a small quantity of blood returned from the
lungs by the pulmonary veins. From the left auricle the blood passes
into the left ventricle and is distributed by the aorta almost entirely
to the upper extremities by the superior vena cava, the blood enters
the right auricle, and, passing over the Eustachian valve, descends
into the right ventricle, and from the right ventricle into the
pulmonary artery. As the lungs in the fetus are solid, they require
very little blood, and the greater part of the blood passes through
the ductus arteriosus into the descending aorta, where, mixing with
the blood delivered to the aorta by the left ventricle, it descends to
supply the lower extremities of the fetus, the chief portions of this
blood, however, being carried back to the placenta by the two umbilical
arteries.
[Illustration: Diagrammatic view of the fetal circulation (Dorland).]
From this description of the fetal circulation it will be seen:
1. That the placenta serves the double purpose of a respiratory and
nutritive organ, receiving the venous blood from the fetus, and
returning it again charged with oxygen and additional nutritive
material.
2. That the greater part of the blood traverses the liver before
entering the inferior vena cava; hence the large size of this organ at
birth.
3. That the blood from the placenta passes almost directly into the
arch of the aorta and is distributed by its branches to the head and
upper extremities; hence the large size and perfect development of
those parts at birth.
4. That the blood in the descending aorta is chiefly derived from that
which has already circulated in the upper extremities and, mixed with
only a small quantity from the left ventricle, is distributed to the
lower extremities; hence the small size and imperfect development of
these parts at birth.