Variation in the venous
It is quite ‘normal' to find
that veins show considerable variation - between sides of the same
body and between people. These differences are usually of little
clinical significance. The venous system operates at a lower
pressure than the arterial system, and like the meandering course of
a slow-moving river, veins sometimes follow irregular courses back
to the heart. This is especially true of superficial veins. However,
even with this variation, the basic pattern is usually recognisable.
When studying the
embryological development of the veins, do not become 'bogged down'
with details. It is however helpful to note the general trend
towards the right side: blood returning to the heart from the
placental and systemic circulations is channelled in increasing
quantities to the right horn of the sinus venosus.
A major factor in this change
is the development and rapid enlargement of the liver just caudal to
the heart, where the great veins pass through the septum
transversum. The only significant exception to this right-hand rule
in seen in the retention of only one of the originally paired
umbilical veins - it is the left umbilical vein which is retained!
Note also the third
circulatory shunt - the ductus venosus - which links the umbilical
vein to the inferior vena cava, by-passing the liver sinusoids and
facilitating blood return to the right atrium. (The other shunts are
the foramen ovale and the ductus arteriosus.)
The pulmonary veins are
derived from an outgrowth from the atrium of the initial heart tube.
This outgrowth branches in relation to the developing lung buds.
Symmetrical veins - note the
pairs of veins entering the sinus venosus. The cardinal veins return
blood from the embryonic body, the vitelline veins from the yolk
sac, and the umbillical veins (pink) return oxygenated blood from
Examples of abnormal veins
Abnormal arrangements of veins
Questions on the
development of the venous system
1 What becomes of the left
horn of the sinus venosus?
2 How is the inferior vena
3 What effect does
development of the liver have on veins passing through the septum
4 Describe the ductus
venosus. Does it carry oxygenated blood or deoxygenated blood?
5 Which vessels pass through
the fetal umbilical cord?
6 Do the vitelline veins
contribute to the formation of any important veins in the body?
7 Into which structure would
an abnormal left superior vena cava drain?
Circulatory changes at
At birth, the baby begins to
breath, the placental circulation closes down, and the three
prenatal ‘shunts' are obliterated.
1 breathing begins -
replacement of fluid in lungs with air
2 cessation of placental
circulation, closure of ductus venosus
3 closure of foramen ovale
4 spasm and closure of ductus
Pattern of prenatal circulation
Pattern of postnatal circulation
Questions on the fetal
circulation and the transition at birth
1 Why is it best to wait for
a few minutes after the baby is delivered before tying and cutting
the umbilical cord?
2 What is the function of the
foramen ovale before birth? In the adult heart, where is the fossa
3 What is thought to initiate
closure of the ductus arteriosus?
4 What causes a lowering of
pulmonary resistance to blood flow after birth?