39. Fetal Circulation

Question 1

Can you describe the fetal

circulation?

Answer 1

Fig. 39.1 Schematic of fetal circulation

• Gas and waste exchange occurs at the placental bed.

Oxygenated blood then flows from the placenta towards the fetus,

via the umbilical vein.

• It enters the fetus 
and 
flows up the inferior vena cava 
to the 
right atrium.

• From here,
~60% of the blood flows through the
foramen ovale,
from right to left atrium.

This blood travels
from the left atrium
to the left ventricle

and from here is

pumped out into the
aorta to circulate

to the brain, heart and body.

• The other 40% passes

from the right atrium
to the right ventricle.

This ventricle contracts ejecting blood
into the pulmonary artery.

However, because of the high resistance
afforded by the lungs,

only 10% of this blood flows

through the pulmonary bed;

the other 90% follows

the path of least
resistance and
flows through

the ductus arteriosus

to the descending aorta.

Here, it joins the
blood that originally
flowed through
the foramen ovale to left side of the heart.

• The blood continues its
journey to and
through the body,

ultimately leaving the fetus to

return to the placental bed,

via the umbilical arteries,

which arise from the common iliac arteries.

(N.B. There are two umbilical arteries and one vein.)

Question 2

How is preferential oxygenation to

the major organs achieved?

Answer 2

• The pO2 in the umbilical vein is only
~4.7 kPa, representing saturations of
80–90%,

and so the fetus is hypoxaemic
when compared to the mother.

• Blood arriving in the right atrium
from the inferior vena cava has
saturations of ~60%,

as it is composed of blood
from the umbilical vein,
mixed with deoxygenated
blood returning from the body.

• The blood returning to the
RA from the superior vena cava is even
more deoxygenated,

with sats of ~25%, because of the
relatively high
oxygen extraction of the brain.

Clearly, it is sensible for this blood
to be returned to the placental bed

for re-oxygenation via
as direct a route as possible,

and certainly not to be
re-introduced to the cerebral circulation.

This is achieved by a structure called the
Eustachian valve,

a tissue flap found at the junction
of the inferior vena cava and right atrium.

It causes preferential ‘streaming’
of the more highly oxygenated
inferior vena cava
blood straight across the foramen ovale,

to the left atrium to be expelled
in to the aorta and
therefore supply the brain.

The less oxygenated blood
of the superior vena cava ‘falls’ 
into the right ventricle 
to be distributed to
the lungs and through 
the ductus arteriosus.

 This blood rejoins the aorta
at a point distal to the origin of the carotids, 
therefore ensuring that the
brain and heart are supplied with the 
most oxygen-rich blood.

Question 3

Can you describe the changes that
take place in the fetal circulation
at birth?

Answer 3

Several things happen here, we think it’s best to break it down in to stages when trying to explain it:

1
The gasp

Question 4

The gasp

Answer 4

As the baby is born it takes its first breath,
often called a gasp.

The generation of
negative intrathoracic pressure
alters Starling’s forces

across the pulmonary vessels

and helps to reduce the
amount of interstitial fluid in the lungs.

A dramatic drop in
pulmonary vascular resistance follows,

allowing an increase in blood flow through the lungs.

As a result, more blood returns to
the left atrium from the lungs.

This increases left-sided pressures
in the heart,

causing mechanical closure
of the foramen ovale.

So, yet more blood flows though 
the pulmonary bed 
as it now has no other path to go by, 
and this maintains 
the foramen ovale’s closure.

Question 5

Clamping the cord

Answer 5

As the cord is clamped,
the ultra low-pressure placental circulation

is removed from the baby.

This causes a rise in
systemic vascular resistance,

and the removal of the blood
from the placental circulation
causes a reduction in venous
return to the heart.

This decreases right atrial filling
pressures, therefore increasing
further the pressure gradient
between the left and right atria,

helping further to
keep the foramen ovale closed.

Question 6

The ductus arteriosus

Answer 6

There is a net reduction in the
amount of blood being shunted
across the ductus arteriosus,

as the pulmonary circulation is
no longer a high-pressure system.

The gradient between pulmonary
vessels and the aorta is reversed,

making the path through the
lungs the one of least resistance.

In addition to this, the baby’s blood
now has a higher pO2 than it did as a fetus.

The movement of this more
highly oxygenated blood across the
ductus arteriosus stimulates its closure.

The ductus arteriosus is closed
physiologically at around 15 hours after birth,
and anatomically by 15 days.

Oxygen, 
bradykinins and
prostaglandin antagonists
 (e.g. indomethacin) 
accelerate ductus arteriosus closure, 

while PGE1 (e.g. alprostdil) 
and conditions causing a 
raised pulmonary vascular resistance 
such as cold and acidosis help to keep it open. 

It is important therefore to keep neonates
warm, oxygenated and hydrated
(warm, wet and pink)

to prevent their reverting to a fetal circulation,

which can be disastrous as they obviously
no longer have the ability to
oxygenate themselves via a placenta.