The blood circulation of human body consists of systemic circulation and pulmonary circulation: systemic circulation and pulmonary circulation meet at the heart (blood flow direction): left ventricle-aorta (arterial blood)-branch artery-capillary in organ-vein in organ (venous blood)-superior vena cava and inferior vena cava-right atrium-right ventricle-pulmonary artery (still venous blood)-alveoli.
(See schematic diagram)
Systemic circulation supplies blood and oxygen to various organs of the body and plays a nutritional role (there is also a small amount of blood supplied to the lungs, not shown in the schematic diagram). We call the corresponding blood vessels nutritional blood vessels. The nutrient vessels entering the lungs are branches of bronchial arteries, and arterial blood flows in the middle;
The function of pulmonary circulation is to convert venous blood with low oxygen saturation into arterial blood with high oxygen saturation through alveolar capillaries, so the pulmonary artery leads arterial blood out of the heart and enters alveolar capillaries to complete gas exchange, so there are two sets of blood vessels in the lung, one is nutritional and the other is functional.
PS: Actually, there is another organ in the human body that also has two sets of blood vessels, that is, the liver. However, the function of functional blood vessels in the liver is not to change venous blood with low oxygen content into arterial blood with high oxygen content, but to complete the function of "detoxification" through the microcirculation of the liver. This microcirculation is called arterial blood and leads back to the heart from pulmonary veins. We call this group of blood vessels functional.