When a baby is born, there are rapid changes in the heart that occur. This is due to the very different demands of the heart when a baby is breathing air normally upon birth or when receiving oxygenated blood from their mother in utero.

In the mother’s uterus, the foetus is not required to breathe, because it receives oxygenated blood from the mother’s circulation via the umbilical cord. The foetus is also surrounded by amniotic fluid, meaning his or her lungs remain deflated. This creates a great amount of resistance for blood flowing from the heart to the lungs, which ultimately results in blood having to bypass the lungs via two shunts. In order to understand these circulatory shunts we must first look at the a diagram of the adult heart and understand the roles of each of the chambers (shown below in figure 1).

figure 1. Adult heart

In a normal adult heart deoxygenated blood arrives from the body at the right atrium and passes through a tricuspid valve into the right ventricle. From here it is ejected via the pulmonary artery to the lungs. The lungs subsequently add oxygen and remove carbon dioxide from the blood, which returns as oxygenated blood to the heart via the pulmonary vein. After leaving the pulmonary vein oxygenated blood enters the heart through the left atrium, passing through the mitral (or bicuspid) valve to the left ventricle. The left ventricle then pumps oxygen rich blood up through the aorta to the body. The body then uses the oxygen within this oxygen rich blood to carry out aerobic respiration.

Shunts of the foetal circulation

There are two major shunts of the foetal circulation in order to bypass the deflated lungs. The first is the foramen ovale, whichi is shown in figure 2 below.

figure 2. Foramen ovale

This allows blood in the foetus to bypass the lungs and move from the right atrium into the left atrium, to be circulated around the body.

The second shunt is the ductus arteriosus, which is shown in figure 3 below.

figure 3. Ductus arteriosus

This is a shunt from the pulmonary artery to the aorta, and much like the foramen ovale, is used to bypass the deflated lungs of the foetus. Both these shunts described are extremely useful in the foetus in order to allow the rest of its body to receive blood from the heart and not get trapped by the deflated lungs.

But why do these shunts need to close at birth?
It may not be obvious as to what problem these shunts can present to an adult person if they remain open. The answer is all to do with pressure difference!

In the foetus the collapsed lungs give a very high pressure on the right side of the heart. Those physicists among the readers will know the fluids move from high pressure to low pressure if given the opportunity. This means in the foetus, blood moves from the right atrium to left atrium through the foramen ovale, and from pulmonary artery to aorta through the ductus arteriosus. We have no problems at all as blood is bypassing the lungs!

However, when you are born, your lungs inflate (obviously), and this reduces the pressure on the right side of the heart to below the left side. This will mean blood will move from the left side to the right side from high to low pressure. Oxygenated blood will move from the left atrium to right atrium through the foramen ovale and from the aorta to pulmonary artery via the ductus arteriosus.

Hang on, this still isn’t a problem is it?
Correct. Oxygenated blood is just mixing with deoxygenated blood, so we currently do not have a problem in the short term. However, as there is more blood flowing through the right side of the heart, it has to work harder and harder. Eventually the right heart muscle will grow and the pressure will rise, pumping blood into the left side of the heart again, mixing deoxygenated blood into oxygenated blood. Now we have a problem. The body will not be getting enough oxygenated blood and may not be able to function properly, and you will get cyanosis. This is a blue tinge to the skin, lips and nail beds due to lack of oxygenation of tissues.

So, how do these shunts close?
This is relatively simple! Due to the pressure differences at birth, when the lungs inflate a flap is pushed over the foramen ovale to close it as the pressure in the left atrium increases. The ductus arteriosus is closed by decreasing prostaglandin levels in the mother and the release of bradykinin from the lungs (these are both hormones).

References:
http://www.stanfordchildrens.org/en/topic/default?id=blood-circulation-in-the-fetus-and-newborn-90-P02362
https://pediatricheartspecialists.com/heart-education/15-congenital-heart-defects/172-patent-ductus-arteriosus
http://www.embryology.ch/anglais/pcardio/umstellung02.html