In a fetus in the uterus, blood flow from the pulmonary artery would have too much pressure for the collapsed, unused lungs in the developing fetus. Blood flow at full pressure would damage all the arteries in the developing lung. As such, in a fetus, there is a hole between the pulmonary artery and the aorta called the ductus arteriosus. Instead of all the blood flowing to the unused lungs, most of it is diverted to the aorta where it is distributed to the rapidly growing body.