What connects the pulmonary artery with the aorta in a fetus, bypassing the lungs?

In fetal circulation, the ductus arteriosus serves a crucial role by connecting the pulmonary artery to the aorta. This structure allows blood to bypass the non-functioning fetal lungs, which are filled with fluid and not yet engaged in gas exchange. Since the fetus receives oxygenated blood through the placenta, the ductus arteriosus facilitates the mixing of blood from the pulmonary artery into the systemic circulation via the aorta, ensuring that oxygenated blood reaches vital organs without passing through the lungs.

The ductus arteriosus closes after birth when the newborn begins to breathe air and the lungs become functional, which shifts the circulation pattern to that seen in adults. This transition is essential for the establishment of normal postnatal respiratory function.

In relation to the other structures mentioned: the ductus venosus is a vessel that allows blood to bypass the liver, the foramen ovale is an opening between the right and left atria that also aids in bypassing lung circulation, and the umbilical vein carries oxygenated blood from the placenta to the fetus. Each of these structures plays a part in fetal circulation, but only the ductus arteriosus specifically connects the pulmonary artery to the aorta.