Final answer:
The pulmonary arteries are the unique arteries that carry deoxygenated blood after birth, transporting it from the right ventricle to the lungs for gas exchange. The pulmonary trunk and its branches are essentially responsible for this function.
Step-by-step explanation:
The pulmonary arteries are the only arteries in the post-natal body that carry relatively deoxygenated blood. These arteries originate from the right ventricle of the heart, which pumps deoxygenated blood into the pulmonary trunk.
The pulmonary trunk then bifurcates into the left and right pulmonary arteries, which further branch out before reaching the pulmonary capillaries. At the capillaries, gas exchange occurs: carbon dioxide exits the blood, and oxygen is absorbed. After this exchange, the now oxygen-rich blood is carried back to the heart via the pulmonary veins, the only veins in the post-natal body that carry highly oxygenated blood.