There are several fundamental differences between foetal and "normal" circulation.
These differences allow the foetus to develop as efficiently as possible.
Whilst the foetus is developing all its oxygenation and nutritional needs are provided for by the placenta as the lungs are not expanded and receive only enough blood flow to allow them to grow. The placenta is also responsible for the removal of waste products such as carbon dioxide from the blood.
Blood passes from the placenta along the umbilical vein, through the ductus venosus (which bypasses the liver) and into the inferior vena cava. From here it then passes into the right atrium where it is directed by the eustachian flap (a flap of tissue that will eventually form part of the tricuspid valve) through the foramen ovale (an opening between the right and left atria) and into the left side of the heart. Once in the left side of the heart the blood is pumped by the left ventricle into the aorta and up to the developing brain.
Deoxygenated blood is returned to the right side of the heart via the superior vena cava and is then pumped into the pulmonary artery where most of it flows across the ductus arteriosus (an extra artery linking the pulmonary artery and the aorta) and into the abdominal aorta. From the abdominal aorta it flows into the umbilical arteries and thus back to the placenta.
The small amount of blood that doesn't flow across the ductus arteriosus goes to the lungs to nourish them and allow them to develop.
Because of these differences in circulation between the foetus and the fully developed and safely delivered neonate even life threatening cardiac abnormalities can remain undetected until after birth.
Changes that occur at and soon after birth
• The umbilical cord is cut and the placenta is removed from the circulation.
• The baby cries and the lungs inflate. This increases blood flow to the lungs and back to the left atrium which in turn causes the foramen ovale to close by sealing a membranous flap over the opening.
• The increase in the amount of oxygen circulating around the body also initiates the closure of the ductus arteriosus (this process may take a few hours or several weeks.)
Babies with cardiac abnormalities may only begin to show signs of their condition as these changes take place.
CHU, Jubilee Building, Leeds General Infirmary, Great George's St., Leeds LS1 3EX
