Pulmonary venules connect the pulmonary capillaries to the pulmonary veins. Venules range from 7 to 50 micrometers in diameter. These venules surround the alveolus of the lungs and serve as carriers of oxygen-rich blood.
The lungs serve as a very important part of the circulatory system; this is because they are where the blood absorbs oxygen. The oxygenated blood is transported from the lungs into the heart through the pulmonary veins. Since blood travels in the different parts of the body through complex capillaries, it becomes deoxygenated along the way, as it releases oxygen to the different tissues of the body.
Pulmonary capillaries, venules and veins are different from the other capillaries, venules and veins in the body, as they surround millions of alveoli. The alveoli are the tiny air sacs in the lungs, where the blood takes up oxygen from the air inhaled by the lungs. As the blood leaves the alveoli, pulmonary capillaries join to form pulmonary venules, and then venules join into pulmonary veins that gather in the crevices that divide various segments of the lungs. From the lungs, the oxygenated blood is pumped back to the heart through the pulmonary veins. The heart, lungs, pulmonary arteries, pulmonary venules and pulmonary veins are what comprise the so-called pulmonary circuit.