The blood of the fœtus is oxygenated, receives nutrient matter, and
gives off waste matter by close contiguity with the maternal blood in
the placenta. The chief differences in the blood-vascular system as
compared with that which obtains after birth are correlated with this
interchange.
The =umbilical arteries=, right and left, are large vessels which arise
from the internal iliac arteries and pass downward and forward in the
umbilical folds of peritoneum on either side of the bladder to the
umbilicus. Here they are incorporated with the umbilical vein and the
urachus in the umbilical cord, ramify in the allantois, and end as the
capillaries of the fœtal placenta. They conduct the impure blood to the
placenta. After birth these vessels retract with the bladder to the
pelvic cavity; their lumen becomes greatly reduced and the wall
thickened so that they are cord-like and are termed the round ligaments
of the bladder.
The =umbilical vein= receives the oxygenated blood from the placenta.
Its radicles converge to form in the horse a single large trunk which
separates from the other constituents of the umbilical cord on entering
the abdomen and passes forward along the abdominal floor in the free
border of the falciform ligament of the liver. It enters the latter at
the umbilical fissure and joins the portal vein, so that the blood
conveyed by it passes through the capillaries of the liver before
entering the posterior vena cava.
In the ox and dog some of the blood in the umbilical vein is conveyed
directly to the vena cava by the =ductus venosus= (Arantii). This vessel
is given off within the liver from a venous sinus formed by the
confluence of the portal and umbilical veins and passes directly to the
posterior vena cava.
The =foramen ovale= is an opening in the septum between the atria, by
which the latter communicate with each other. It is guarded by a valve
(Valvula foraminis ovalis) which prevents the blood from passing from
the left atrium to the right. After birth the foramen soon closes, but
this part of the septum remains membranous, and there is a deep =fossa
ovalis= in the right atrium which indicates the position of the former
opening. In some cases the foramen persists to a variable extent in the
adult without apparent disturbance of the circulation.
The =pulmonary circulation= is very limited in the fœtus, and most of
the blood which enters the pulmonary artery passes through the =ductus
arteriosus= to the aorta. This vessel is larger than the divisions of
the pulmonary which go to the lungs and joins the left side of the
aortic arch. After birth the pulmonary circulation undergoes promptly an
enormous increase and the ductus is rapidly transformed into a fibrous
cord—the =ligamentum arteriosum=.
The only arterial blood in the fœtus is that carried by the umbilical
vein. This blood is mixed in the liver with the venous blood of the
portal vein, and after passing through the capillaries of the liver is
carried by the hepatic veins to the posterior vena cava. The latter
receives also the venous blood from the posterior part of the trunk and
the pelvic limbs. It is generally believed that the blood carried into
the right atrium by the posterior vena cava passes largely, if not
entirely, through the foramen ovale into the left atrium, while the
blood flowing into it through the anterior vena cava passes into the
right ventricle. On this basis the blood received by the left atrium
consists chiefly of mixed blood from the posterior vena cava, since the
small amount of blood conveyed by the pulmonary veins is venous. This
mixed blood passes into the left ventricle and is forced into the
systemic arteries. The venous blood from the anterior part of the body
and the thoracic limbs is conveyed by the anterior vena cava to the
right atrium, passes into the right ventricle, and is forced into the
pulmonary artery. A small amount is carried to the lungs, but the bulk
of it passes by the ductus arteriosus into the aorta behind the point of
origin of the brachiocephalic trunk (anterior aorta), and is carried to
the posterior part of the body, a large part passing by the umbilical
arteries to the placenta.