Final answer:
The ductus arteriosus serves as a critical connection between the proximal left pulmonary artery and the descending aorta in a fetus, diverting blood away from the non-functional fetal lungs and directing it towards the aorta.
Step-by-step explanation:
The connection between the proximal portion of the left pulmonary artery and the descending aorta in a fetus is the ductus arteriosus. This is a crucial part of fetal circulation as it allows most of the blood from the right ventricle to be diverted away from the non-functional fetal lungs, which have high resistance to blood flow. The ductus arteriosus shunts blood from the pulmonary trunk directly into the aorta, ensuring that the developing fetal tissues receive the oxygen-rich blood they need.
When a newborn takes its first breath, the lungs expand, and the smooth muscles in the wall of the ductus arteriosus begin to constrict due to the increase in oxygen, eventually leading to closure of the vessel. This transformation is part of the transition from fetal to neonatal circulation. The ductus arteriosus typically seals off completely and transforms into the ligamentum arteriosum shortly after birth.