Inside the physique, there are a complete of 4 pulmonary veins, and all of them connect with the left atrium of the center. The center pumps oxygen-depleted blood into the lungs by way of the pulmonary arteries. As soon as the blood has been oxygenated, it returns to the center by way of the pulmonary veins. Then, the center circulates this newly oxygenated blood all through the physique. On this manner, pulmonary veins are totally different from different veins within the physique, that are used to hold deoxygenated blood from the remainder of the physique again to the center. The left pulmonary veins join with the left lung, and the lungs themselves are stuffed with hole air sacs referred to as alveoli. That is the place oxygen is faraway from inhaled air. This additionally works as a gasoline alternate. Oxygen enters the blood whereas carbon dioxide leaves the blood stream. This carbon dioxide is then exhaled out of the physique.