<P> The fetal circulation of humans has been extensively studied by the health sciences . Much is known also of fetal circulation in other animals, especially livestock and model organisms such as mice, through the health sciences, veterinary science, and life sciences generally . </P> <P> Blood from the placenta is carried to the fetus by the umbilical vein . In humans, less than a third of this enters the fetal ductus venosus and is carried to the inferior vena cava, while the rest enters the liver proper from the inferior border of the liver . The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein . The blood then moves to the right atrium of the heart . In the fetus, there is an opening between the right and left atrium (the foramen ovale), and most of the blood flows through this hole directly into the left atrium from the right atrium, thus bypassing pulmonary circulation . The continuation of this blood flow is into the left ventricle, and from there it is pumped through the aorta into the body . Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the maternal circulation . </P> <P> Some of the blood entering the right atrium does not pass directly to the left atrium through the foramen ovale, but enters the right ventricle and is pumped into the pulmonary artery . In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus, which directs most of this blood away from the lungs (which are not being used for respiration at this point as the fetus is suspended in amniotic fluid). </P> <P> The circulatory system of the mother is not directly connected to that of the fetus, so the placenta functions as the respiratory center for the fetus as well as a site of filtration for plasma nutrients and wastes . Water, glucose, amino acids, vitamins, and inorganic salts freely diffuse across the placenta along with oxygen . The uterine arteries carry blood to the placenta, and the blood permeates the sponge - like material there . Oxygen then diffuses from the placenta to the chorionic villus, an alveolus - like structure, where it is then carried to the umbilical vein . </P>

What connects the pulmonary trunk to the aorta during fetal circulation