Arteries and arterial blood are two concepts, so not all blood flowing in arteries is arterial blood. Arterial blood refers to the blood vessels that exit the heart, while arterial blood refers to blood that is relatively rich in oxygen and relatively low in carbon dioxide. Because of the pulmonary circulation, there is also arterial blood in the pulmonary veins. On the contrary, although the pulmonary artery is a blood vessel out of the heart, the blood in the pulmonary artery comes from the brain and other related tissues and is metabolized by the tissues, so the carbon dioxide content in this part of the blood is relatively more, while the oxygen content is relatively low, so it is venous blood and not arterial blood. The blood in the pulmonary artery is converted into arterial blood by the ventilation of the pulmonary capillaries into the pulmonary veins.