Before birth the foetus is not capable of respiratory function and relies on maternal circulation to carry nutrients, waste exchange and gas exchange. The blood of the foetus and mother never mix, but interface in the placenta. There are a series of shunts in the foetal circulation to by-pass the liver and lungs. At birth there placenta is replaced by the lungs for respiratory exchange. When the newly born animal takes its firth breath, the lungs & pulmonary vessels expands- lowing the resistance to blood flow. This lowers pulmonary pressure in the right side of the heart, and increased pressure on the left side of the heart- this triggers the closures of the foetal shunts.
Foremen Ovale– is an opening between the two atria (RA-LA) which enables blood to channel directly into the systemic circulation, bypassing the lungs (pulmonary circulation). The covering provided by the remnant of the septum primus forms the valve of the foramen ovale. The septum secundum directs the majorty of the blood entering the right atrium through the foramen ovale into the left atrium. Here it mixes with a small volume of blood returning from the non- functional lungs.
In the foetus the foramen ovale is kept open by a higher pressure of blood int he right atrium compared to the left atrium. When the pressure decreases at birth, due to the termination of blood flow from the placenta + the left atrium pressure increases because of increased pulmonary flow- the septum primus is pressed against the septum secundum, closing the foramen ovale. The scar is termed the fossa ovalis. Failure to close results in atrial septal defect.
Ductus Venosus – Umbilical vein bring oxygenated blood to developing heart from the placenta. Portal veins mix with oxygenated blood at ductus venosus- which shunts the blood past before too much blood is mixed- bypassing the hepatic circulation to join caudal vena cava. The ductus venosus links the umbilical vein to the caudal vena cava and the flow of blood is controlled by a sphincter, enabling the proportion travelling to the heart via the liver to be altered
At birth there is an absence of umbilical blood volume and pressure of the caudal vena cava streat that causes the sphincter in the ductus venosus to constrict- thereby diverting the blood to the liver. The remnants of ductus venosus forms the ligamentum venosum. If it remains patent, a congenital partacaval anastamosis is created.
Ductus arteriosus- is the foetal blood vessel that joins the aorta and the pulmonary artery. It is formed by the dorsal part of left 6th aortic arch. The blood from the right ventricle is pumped to the pulmonary trunk where, due to the high resistance in the collapsed foetal lungs, a larger volume passes through the ductus arteriosus to the caudal aorta. Most of the blood in the aorta is then returned to the placenta for oxygenation through the umbilical arteries. The ductus arteriosus empties blood into the aorta after the artery to the head has branched off thus ensuring that the brain receives well-oxygenated blood.
The ductus arteriosus is a muscular artery and immediately after birth, contraction of the musculature closes the shunt- which is stimulated by the raised oxygen tension of the perfusing blood. Expansion of the lungs reduced the resistance of their vascular bed and the drop in pulmonary arterial pressure results in the flow through the ductus normally being form the aorta. Anatomical closure takes about two months and occurs by infolding of the endothelium and proliferation of the subintimal connective tissue layer. The residual ligament is termed the ligamentum arteriosum. Abnormal persistence of an open lumen after birth results in a persistent or patent ductus arteriosus.