Human Anatomy - Kotsan I. Y. 2009

Blood vessels of the pulmonary (lesser) circulation

The lesser (pulmonary) Circulation ensures gas exchange between the Blood in the pulmonary capillaries and the air in the pulmonary alveoli. The Vessels of the Pulmonary Circulation include: the pulmonary trunk, the right and left pulmonary Arteries along with their branches, the pulmonary microvasculature, and the two right and two left Pulmonary Veins with all their tributaries.

The pulmonary trunk (truncus pulmonalis) carries deoxygenated blood from the right ventricle of The Heart to the Lungs. It is 5—6 cm long and 3—3.5 cm in diameter. Originating from the right ventricle, it passes anterior to the aortic arch and divides at the level of the IV thoracic vertebra into the right and left pulmonary arteries. This site of division is called the bifurcation of the pulmonary trunk (bifurcation trunci pulmonalis). Situated between the bifurcation of the pulmonary trunk and the aortic arch is the short fibro-muscular ligamentum arteriosum, which represents the obliterated ductus arteriosus (Botallo's duct).

The right pulmonary artery (a. pulmonalis dextra), with a diameter of 2.0—2.5 cm, is slightly longer than the left; its total length before branching into lobar and segmental branches is about 4 cm. It runs transversely from left to right, lying posterior to the ascending aorta and the SUPERIOR VENA CAVA. In the region of the hilum and beneath the right main bronchus, the right pulmonary artery divides into three lobar branches, corresponding to the number of lobes in the right lung. Each lobar branch, in turn, divides into segmental branches.

In the upper lobe of the right lung, the following are distinguished: the apical branch (r. apicalis), the descending and ascending posterior branches (rr. posteriores descendens et ascendens), and the descending and ascending anterior branches (rr. anteriores descendens et ascendens), which supply the apical, posterior, and anterior segments of the right lung.

The middle lobe branch of the right lung divides into three segmental branches: the lateral and medial branches (rr. lateralis et medialis), which supply the lateral and medial segments of the middle lobe, and the apical (superior) branch of the lower lobe (r. apicalis (superior) lobi inferioris)), which runs to the superior (apical) segment of the lower lobe.

The lower lobar branch (basal part) of the right pulmonary artery divides into four branches: the medial (cardiac), anterior, lateral, and posterior basal branches (rr. basales medialis (cardiacus), anterior, lateralis et posterior)), which deliver blood to the corresponding basal segments of the lower lobe: medial (cardiac), anterior, lateral, and posterior.

The left pulmonary artery (a. pulmonalis sinistra) also runs transversely, but from right to left, anterior to the descending aorta and the left bronchus. It divides into two lobar branches corresponding to the two lobes of the left lung.

In the upper lobe of the left lung, the following segmental branches are distinguished: the apical branch (r. apicalis), the ascending and descending anterior branches (rr. anteriores ascendens et descendens), the posterior branch (r. posterior), the lingular branch (r. lingularis), and the apical (superior) branch of the lower lobe (r. apicalis (superior) lobi inferioris)). All listed branches, except the last one, supply the segments of the upper lobe of the left lung (apical, posterior, anterior, as well as superior and inferior lingular segments). The final branch (the apical branch of the lower lobe), just as in the right lung, enters the lower lobe of the left lung to supply its superior (apical) segment.

The second lobar branch (basal part) of the left pulmonary artery divides into four segmental branches: the medial, lateral, anterior, and posterior basal branches (rr. basales medialis, lateralis, anterior et posterior), which ramify within the medial, lateral, anterior, and posterior basal segments of the lower lobe.

The segmental branches subdivide into smaller arteries, arterioles, precapillaries, and capillaries that envelop the alveoli. This is where gas exchange takes place between the blood of the pulmonary capillaries and the alveolar air. Oxygen diffuses from the alveolus across the ultrathin alveolar-capillary membrane into the blood, while carbon dioxide passes from the blood into the alveolar cavity, thereby converting deoxygenated blood into oxygenated blood.