What Role Do Shunts Play in The Embryonic Circulatory System?

By Chandan Sekhon - Medicine Student @ Peterhouse, Cambridge

 

Various embryonic structures play a huge role in ensuring sufficient blood and nutrient supply to the developing embryo. However, once the baby is born there may no longer be a need for such structures, resulting in their closure or obliteration. In the foetal circulation, there are three main shunts which allow foetal blood to bypass the liver and collapsed foetal lungs. There is no requirement for foetal blood to pass through these structures as the blood has already been detoxified by the liver of the mother of the baby, and all gas exchange occurs via the maternal placenta as the foetus’ lungs are not yet functional. These shunts include the ductus venosus, foramen ovale and ductus arteriosus.


Ductus Venosus:

The ductus venosus is the first encountered shunt and connects the left umbilical vein to the inferior vena cava, allowing oxygenated blood from the placenta to bypass the liver. It is closed off between 4-7 days following birth and this closure is a result of the ductus venosus fibrosing due to increased maternal prostaglandin levels and the clamping of the umbilical cord at birth. Despite this closure, it is possible to open it up again during portal hypertension. Patent ductus venosus is uncommon, and usually results from premature birth or from having a family history of the condition.


Foramen Ovale:

This is the second encountered shunt in the foetal circulation, and shunts oxygenated blood from the inferior vena cava to the left atrium, which allows blood to avoid passing through the nonfunctional lungs. In the developing foetus, the right atrium receives blood from the vena cava, and so has a higher blood pressure than the left atrium, however, this is reversed during the first breath of the baby as pulmonary vessels expand, as do the lungs. This ultimately increases venous return to the left atrium and inhibits any blood flow to the right atrium. Upon closure, the foramen ovale forms the fossa ovalis postnatally.


Ductus Arteriosus:

This is the third and final shunt encountered in this shunting mechanism, connecting the aorta to the left pulmonary artery to prevent blood entering the high vascular resistance of the foetal pulmonary system, as the lungs only require a basal blood supply at this stage. However, once the baby is born, blood flow to the lungs must increase rapidly, increasing blood flow to the left atrium so blood can return via the pulmonary vein. The closure of the ductus arteriosus occurs usually within 1-2 days postnatally. This closure is mediated by a reduction in maternal prostaglandin levels in the blood plasma. As well as this, there is a decrease in the amount of blood passing through the duct as the extremely high foetal pulmonary circulation reduces in resistance after birth. The ligamentum arteriosus fibroses and forms the ligamentum arteriosum.


However, this closure may not always occur completely, as a result of prematurity and maternal rubella, which are potential risk factors for patent ductus arteriosus. This condition results in 10% of all congenital heart defects and causes blood to shunt from left to right. This can lead to symptoms such as pulmonary hypertension and endarteritis (inflammation of the inner lining of arteries).


Further reading:

  1. This website gives a very general overview of the three shunts in the foetal system and is useful to get a general overview of the system: https://mcb.berkeley.edu/courses/mcb135e/fetal.html

  2. This article explains the congenital condition patent ductus arteriosus where the shunt doesn’t close after birth. A very interesting read as it does into great depth about the condition and is useful to get an understanding of it: https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.105.592063

  3. This study explores blood flow through and how it is regulated through the ductus venosus. A very interesting study: https://www.sciencedirect.com/science/article/pii/S0002937800705047

  4. This study explores the relationship between the condition patent foramen ovale and cryptogenic stroke in older patients. The relationship between the two has been suggested so this study is an interesting read: https://www.nejm.org/doi/full/10.1056/NEJMoa071422