Circulatory System: Embryonic Veins
The
vitelline circulation is the flow of blood between the embryo and the yolk sac
through a collection of vitelline arteries and veins that pass within the yolk
stalk (Figure 30.1).
The
vitelline arteries are branches of the dorsal aortae, and most of them
degenerate in time. Those that remain fuse and form the 3 unpaired ventral
arterial branches of the aorta that supply the gut: the celiac trunk, superior
mesenteric artery and inferior mesenteric artery.
The
vitelline veins will give rise to the hepatic portal vein and the hepatic veins
of the liver.
The
umbilical circulation is the flow of blood between the chorion of the placenta
and the embryo. The umbilical arteries carry poorly oxygenated blood to the
placenta and the veins carry highly oxygenated blood initially to the heart of
the embryo (Figure 30.1), and later into the liver when it forms (see Figure
31.1). The right umbilical vein is lost arou ng only the left to carry blood
from the placenta.
The
formation of the ductus venosus during the foetal period causes about
half of the blood from the umbilical vein to flow directly into the inferior
vena cava, bypassing the liver (Figure 31.1). This, with other mechanisms,
preferentially shunts highly oxygenated blood to the foetal brain.
Of the
umbilical arteries only the proximal portions persist as parts of the internal
iliac arteries and superior vesical arteries in the adult. The distal portions
do not remain as arteries but become the medial umbilical ligaments. The
umbilical vein becomes the ligamentum teres, passing from the umbilicus to the
porta hepatis in the adult (see Chapter 31).
The common
cardinal veins initially form an H‐shaped structure, with the horizontal bar
being the sinus venosus that links the major veins and the atrium of the early
heart tube (Figure 30.2). The left and right anterior (or superior) branches
drain blood from the head and shoulder regions and the posterior (or inferior)
branches drain from the abdomen, pelvis and lower limbs.
At 6 weeks a
subcardinal vein arises on either side of the embryo caudal to the heart
and anastomoses with the posterior cardinal veins (Figure 30.3). The
subcardinal veins also form an anastomosis with each other anterior to the
dorsal aortae, and tributaries are sent into the developing limbs. The right
subcardinal vein joins vessels of the liver. Similarly, at 7 weeks supracardinal
veins form and link to the posterior cardinal veins (Figure 30.3).
The
posterior cardinal veins degenerate, although the most caudal parts continue as
a sacral venous plexus and later as the common iliac veins.
An important
junction between the right supracardinal and right subcardinal vein forms and
both will become sections of the inferior vena cava (IVC). Parts of the right
posterior cardinal veins, common, subcardinal and supracardinal veins also
contribute. A shift towards the right side occurs, with degeneration of venous
structures on the left side and the formation and enlargement of the inferior
vena cava on the right (Figure 30.4).
Similarly,
the degeneration of much of the left anterior cardinal vein gives a shift to
the right side as the right anterior cardinal vein forms part of the superior
vena cava (SVC) and the right brachiocephalic vein (Figure 30.4). An
anastomosis between the 2 anterior cardinal veins persists as the left
brachiocephalic vein.
The right
supracardinal vein becomes much of the azygos vein, and the left
supracardinal vein forms part of the hemiazygos vein and the accessory
hemiazygos veins (Figure 30.4). Branches from the subcardinal vein network form
renal, suprarenal and the gonadal veins.
Clinical relevance
The
formation of the venous system is somewhat variable and complicated, and can
give rise to variations in adult SVC and IVC anatomy. The hepatic section of
the IVC may fail to form, for example, and blood instead flows back to the
heart through the azygos and hemiazygos veins from the inferior parts of the
body (azygos continuation). Persistence of supracardinal veins can leave
double inferior vena cavae, and persistence of the left anterior
cardinal vein can give double SVC. In this case the right anterior vena
cava may even dege left SVC. These variations are not common.
