At birth, the fetal circulation, where the left and right sides of the heart work in parallel, transitions into the adult pattern of circulation with the two sides of the heart work in series. In the fetal parallel pattern the two sides of the heart pump out different volumes of blood and this does not cause any problem because of the connections between the left and rignt sides, - at the foramen ovale and the ductus arteriosus. Any excessive return to the right side of the heart can be shunted through the foramen ovale. And excess output from one ventricle can be diverted through the ductus arteriosus. After birth with a full functional pulmonary circulation and closure of the shunt connections, the two sides of the heart, working in series, must pump out the same volume of blood if the blood is not to accumulate in the lungs or in the systemic circulation.
This transition needs to be achieved quickly after birth but not instantaneously. The
shunts can remain a little open until both sides of the heart are able to perform
equally. In the fetus the output from the right side of the heart is about 250mls/kg/min
while the output from the left is only about 200mls/kg/min.
The ductus arteriosis allows the excess output from the right side of the heart to
supplement the output from the left side which, after birth, needs to increase in
response to demand from the muscles and gut circulation.
At a physiological and healthy birth the baby's first steps towards transition occurs
with inspiration. This first breath leads to a fall in the resitance of the pulmonary
circulation and, as breathing becomes established, the majority of the output from the right
ventricle goes through the pulmonary circulation. This has a number of effects.
(1) The distended capillaries around the alveoli helps to keep the alveoli open.
(2) The volume of blood returning through the pulmonary vein is greatly increased and the pressure in the left atrium tends to close the valve of the foramen ovale.
(3) The oxygenated blood pumped out by the left ventricle is no longer diluted by blood from the ductus arteriosus, and oxygenated blood arrives at the umbilical arteries. The high oxygen tension together with agents from the newly distended lungs constrict the umbilical ateries.
(4) Less blood is pumped out into the placental circulation but pressure within the uterus maintains a venous return.
(5) As the circulation of the lungs fills, the circulation of the placenta collapses.
(6) Reduced and finally no return of blood from the placenta leads to lower pressure in the right atrium and the valve of the foramen ovale is kept closed by the higher pressure on the left.
(7) High oxygen tension of the blood leads to constriction of the ductus arteriosus.
In the next 24 to 48 hours the left ventricle hypertophies, allowing the ductus arteriosis to close completely by which time the output from the left and right have become equal. The process is slow and controlled. It needs to be. Minor changes in pressure or flow could precipitate a cascade of events. Unfortunately birth is often far from easy for the baby or mother. The baby may already be hypoxic and acidotic. When there is sufficient concern an operative delivery will be carried out. Let's consider what this involves. If it is possible, and considered safe, a vaginal delivery by forceps or ventouse will be carried out, otherwise a caesarean section is necessary. At delivery the umbilical circulation is immediately clamped and the baby handed to the paediatrician for resuscitation. Sometimes the baby may have already taken its first breaths but the pulmonary circulation will not have become established, nor the umbilical circulation closed. Clamping the functioning placental circulation results in a dramatic increase in the systemic resistance and an equivalent increase in the afterload of the heart. ( The placental circulation forms 40% of the combined [left + right] cardiac output). Even a healthy heart may have difficulty in recovering from this sudden load. It is uncertain what effect this will have on the homeostatic and regulatory systems of the circulation of the baby. A further effect is hypovolaemia. The pulmonary circulation has to open up at the expense of the rest of the circulation since the blood in the placenta has been blocked off. The hypovolaemia may lead to hypotension and hypoperfusion of vital organs. This heart simulator demonstrates what must happen to the circulation when the cord is clamped suddenly at birth before the pulmonary circulation is established. Like any simulation it is not perfect. With any medical intervention it is obligatory to establish that the intervention is benficial. All the evidence suggests that the intervention is harmful.
I am grateful to Falstad for providing this computer analogue circuit simulator.
Copy the program below first then load the circuit simulator from