Perinatal adaptation of the circulation

Fetal circulation

After subdivision of the heart into four chambers and parallel alignment of the bloodstream in the heart, blood circulates in the fetus as follows. Oxygenated blood reaches the liver from the placenta via the umbilical vein, flowing through it or bypassing it via the ductus venosus. The blood reaches the right atrium through the inferior vena cava. Most of the blood flows through the foramen ovale from there, into the left atrium and via the mitral valve into the left ventricle. It is expelled from the left ventricle into the aorta. It then returns through the two umbilical arteries to the placenta, full of CO2. Poorly oxygenated blood flows from the superior vena cava into the right atrium. It mixes with a small volume of arterialized blood from the placenta, and reaches the right ventricle via the tricuspid valve. The blood is then expelled into the pulmonary artery. Some of the blood flows through the lungs and reaches the left atrium via the pulmonary veins. The rest flows into the aorta via the ductus arteriosus.