Why do pulmonary arteries carry oxygen-poor blood and pulmonary veins carry oxygen-rich blood, contrary to the systemic circulation?

Pulmonary arteries carry blood from the right ventricle to the lung capillaries before gas exchange, so that blood is still oxygen-poor. After oxygen moves from the alveoli into the blood and carbon dioxide moves out, the newly oxygen-rich blood drains into venules that form the pulmonary veins, which return it to the left atrium. Thus the reversal is simply because the pulmonary circuit is arranged to deliver deoxygenated blood to the lungs and then carry freshly oxygenated blood away from the lungs, opposite to the direction of oxygen delivery in the systemic circulation.

The pulmonary circulation exists specifically for gas exchange with the alveoli, not for serving the metabolic needs of body tissues. Oxygen-poor blood pumped from the right ventricle enters the pulmonary trunk, which divides into the right and left pulmonary arteries. These arteries branch into lobar arteries, arterioles, and finally the pulmonary capillaries that surround the alveoli. In these capillaries, oxygen moves from alveolar air into the blood and carbon dioxide moves from the blood into the alveolar air, making the blood bright red and oxygen-rich. The capillary beds then drain into venules that form the four pulmonary veins, which deliver this oxygen-rich blood to the left atrium. In contrast, the systemic circulation carries oxygenated blood away from the left ventricle through arteries to body tissues and returns oxygen-poor blood through veins to the right atrium. So the naming of pulmonary arteries and veins reflects the sequence of gas exchange: arteries bring blood to the exchange surfaces, and veins carry it away after oxygenation.