This is what I came up with. Pulmonary Artery carries deoxygenated blood into the lungs. The pulmonary artery turns into arterioles and eventually into alveoli. The blood is pushed through to that alveoli. From that specific alveoli the now oxygenated blood cell makes it's way into the left ventricle via pulmonary artery. That obviously makes no sense. How does the blood go from the Pulmonary Artery to being oxygenated and in the left atrium?