In the lungs there are two circulation systems:
Before birth the pressure in the larger branches of the pulmonary circulation system is similar to that of the aorta, because a larger part of the blood is bypassed from the pulmonary trunk through the open arterial duct into the aorta.
Functional and nutritive blood supply of the lungs
The vessels of the pulmonary circulatory system correspond in their construction to those of the somatic circulation system. The proximal arteries are of the elastic type and the peripheral of the muscular. Till birth little blood flows through the vessels of the pulmonary circulation system because the lungs have not yet unfolded and as a result, the peripheral arteries of the muscular type are compressed.
The pulmonary arteries accompany the branching of the bronchial tree. In contrast to the arteries, the pulmonary veins form primarily in the wide mesenchymal septa between and around the large stems of the developing bronchial tree. As a result veins lie more at the periphery of the individual pulmonary segments.
The bronchial circulatory system is also a part of the somatic circulation system and is formed by the bronchial vessels. These vessels take care of those regions of the air-conducting part of the lungs that can no longer be supplied with oxygen directly from the oxygenated blood of the pulmonary artery.
In early development, the bronchial vessels arise from still paired dorsal aortas in the neck region. Later, 1-3 rami bronchiales per lung arise either directly out of the breast aorta or out of the 3rd and 4th intercostal arteries. They extend along and ramify in the peribronchial connective tissue parallel to the bronchi and bronchioli up to the terminal bronchioli (end of the air-conducting part).