The brain and heart therefore receive highly oxygenated blood. A small amount of blood from the caudal vena cava passes through the right atrium and ventricle and into the pulmonary artery. Because the pulmonary circulation is a high-resistance pathway in the fetus, the majority of this blood passes through the ductus arteriosus into the aorta, again bypassing the pulmonary circulation. This blood then also enters the systemic circulation. Note that the blood supply to the heart and brain branches off before the ductus arteriosus, ensuring that the heart and brain receive highly oxygenated blood.
Blood returns to the placenta through the umbilical arteries.
At birth, the removal of the placental blood supply makes adaptation to gaseous exchange in the lungs a necessity. The pulmonary circulation must now become low-resistance as it is exposed to oxygen by breathing. The systemic circulation becomes high-resistance, and there is no longer a pressure gradient causing blood to flow into the ductus arteriosus. Exposure to oxygenated blood and prostaglandins released at birth results in closure of the ductus arteriosus, usually between 24 and 72 hours after birth. Pressure in the left atrium increases, causing the foramen ovale to close.