Because of difference in partial pressure
Simple diffusion
A pulmonary arteriole is a small blood vessel that carries oxygen-poor blood from the right side of the heart to the lungs for oxygenation. These arterioles are part of the pulmonary circulation system that helps exchange carbon dioxide for oxygen in the blood.
The pulmonary artery is the blood vessel that leads away from the heart (Artery=Away), toward the lungs. It is pumping blood to that lungs that just returned from the body (via the vena cava, into the right atrium). So the blood in the pulmonary artery has the lowest concentration of O2 and the highest concentration of CO2. It is just about to pick up lots of oxygen, once it gets to the lungs.
altitude
It comes back through the pulmonary veins. Veins always carry blood TO the heart. This is the only case where veins are high in oxygen.
The pulmonary artery carries oxygen poor blood and the pulmonary vein carries oxygen rich blood.
The exchange of gases between the alveolar air and pulmonary capillary blood is called pulmonary gas exchange. Oxygen diffuses from the alveoli into the capillaries, while carbon dioxide diffuses from the capillaries into the alveoli. This process is essential for the body to obtain oxygen and release carbon dioxide.
During pulmonary gas exchange, oxygen moves from the alveoli in the lungs into the bloodstream. This process occurs due to the difference in partial pressure; oxygen has a higher concentration in the alveoli than in the deoxygenated blood in the pulmonary capillaries. As a result, oxygen diffuses across the alveolar-capillary membrane into the blood, where it binds to hemoglobin in red blood cells for transport to tissues throughout the body.
The pulmonary veins carries oxygen rich blood from the lungs to the heart.
The pulmonary artery carries oxygen-poor blood. The pulmonary vein carries oxygen-rich blood.
Oxygen diffuses from the alveoli into the blood due to higher PO2 (partial pressure of oxygen) levels in the alveoli than in the blood.
Oxygen passes from the air sacs in the lungs to the blood in the capillaries, carbon dioxide passes the other way.