Foetal Circulation

Foetal Circulation

Comparison to Adults

The adult heart consists of 4 chambers, each with inflow and outflow. Its objective is to take deoxygenated blood from the body, transport it to the lungs for oxygenation, and then take this oxygenated blood to the tissues.

For more information on this, you can look at the physiology and anatomy of the heart.

A fetus is surrounded by amniotic fluid and will use the placenta as its source of oxygen and nutrients. While the lungs are developing, they are not functional and provide no oxygenation.  There are also high energy demands of developing tissue, in particular the brain.

As a result, foetal circulation must direct blood away from non-functional organs and ensure that growing tissues receive their oxygen requirements.

Foetal Heart Structure

Due to these differences, the foetal heart has a number of different structures to direct blood flow:

• The umbilical vein delivers oxygenated blood from the placenta to the fetus, providing oxygen and nutrients.
• The umbilical arteries are used to transport deoxygenated blood away from the foetal tissue and back towards the placenta for re-oxygenation.
• The ductus venosus allows blood from the placenta to bypass the relatively inactive liver.
• The ductus arteriosus is the fusion of the primitive pulmonary artery to the aorta, therefore allowing blood to pass straight from the right ventricle into the aorta and bypass the inactive lungs.
• The foramen ovale creates a shunt between the right atrium and the left atrium so oxygenated blood from the placenta can move to the left atrium. This allows for the oxygenated blood to pass through the left ventricle and into the ascending aorta, oxygenating the brain.

Made by Amy Haeffner 2021

Fig 1
Illustrative diagram of the foetal heart structure and foetal shunts

Oxygenation in Utero

The highest partial pressure of oxygen in the feto-placental circulation is approximately 4kPa. This is compared to 13kPa in an adult. However, the fetus can maintain adequate oxygen delivery to tissues through the use of the shunts above, assisted by a relative polycythaemia and the properties of foetal haemoglobin.

How is Foetal Haemoglobin Different?

Foetal haemoglobin has a different quaternary structure to adult haemoglobin. Adult haemoglobin (HbA) is formed of 2 alpha subunits and 2 beta subunits. However, foetal haemoglobin contains different subunits, namely 2 alpha subunits and 2 gamma subunits.

This change in the three-dimensional structure of the protein means that foetal haemoglobin can bind more readily to oxygen from maternal circulation. This allows for adequate oxygenation of tissues.

Leticia, via Wikipedia, 2010. Accessed with Creative Commons

Fig 2
Oxygen dissociation curve showing the relative properties of myoglobin, HbF, and HbA

Foetal Heart to Neonatal Heart

The foetal heart changes from birth to allow the newborn to effectively oxygenate itself with newly open lungs. This occurs in a series of steps:

1. The first breath causes a rise in the partial pressure of oxygen
2. An increase in the partial pressure of oxygen causes pulmonary vasodilation
3. Pulmonary vasodilation leads to a drop in right heart pressure.
4. Simultaneously, placental circulation ceases, causing left heart pressure to rise. 
5. These factors combine to cause the foramen ovale to shut.
6. The pulmonary and systemic circulations become separate, and the whole output of the right ventricle passes through the pulmonary circulation.
7. The final step in the sequence is the closure of the ductus arteriosus, occurring 2 to 3 days after birth.
8. The structural remnants of the foetal circulatory structures are referred to as the fossa ovalis (foramen ovale), the ligamentum arteriosum (ductus arteriosus), and the ligamentum venosum (ductus venosus).