Pulmonary trunk
The pulmonary trunk (truncus pulmonalis) is a large vessel approximately 5–6 cm in length and up to 3 cm in width. It originates from the conus arteriosus of the right ventricle. At its orifice lies an opening securely closed by semilunar valves (valvulae semilunares trunci pulmonalis), which prevent the backflow of blood into the heart.
Leaving the right ventricle, the vessel courses anteriorly and to the left. At this point, it lies anterior and to the left of the ascending aorta. Reaching the level of the IV thoracic vertebra, the trunk passes beneath the aortic arch and divides into two pathways—the right and left pulmonary arteries. This site is termed the bifurcation of the pulmonary trunk (bifurcatio trunci pulmonalis).
Topographical relations of the pulmonary trunk:
- Anteriorly, it is related to the ascending aorta.
- To the left, it is adjacent to the left atrium and its auricle.
- To the right, the trunk borders the ascending aorta and the superior vena cava.
Pulmonary Arteries and Their Branching
Following bifurcation, blood is distributed into two arteries, each having its own anatomical course and branching pattern within the lung tissues.
Right pulmonary artery (a. pulmonalis dextra) Runs almost horizontally, crossing the posterior wall of the ascending aorta and passing behind the superior vena cava. Upon reaching the hilum of the right lung, it divides into three lobar branches:
- Superior lobar branch (r. lobi superioris) — supplies the apical, anterior, and posterior segments via their respective branches (r. apicalis, r. anterior, r. posterior).
- Middle lobar branch (r. lobi medii) — originates from the inferior branch of the artery and divides into lateral and medial branches (r. lateralis, r. medialis).
- Inferior lobar branch (r. lobi inferioris) — gives rise to the apical branch (r. apicalis) and four basal branches (rr. basales: medialis, anterior, lateralis, posterior).
Left pulmonary artery (a. pulmonalis sinistra) This artery is shorter than the right but exceeds it in caliber. Its course is directed to the left, superiorly, and posteriorly, passing behind the ascending aorta, beneath its arch and the left main bronchus. In the left lung, it divides into only two lobar branches:
- Superior lobar branches (rr. lobi superioris) — supply the apicoposterior and anterior segments, and also give off the superior and inferior lingular branches (rr. lingulares).
- Inferior lobar branches (rr. lobi inferioris) — anatomically similar to the right side, giving off the apical branch and three basal branches (anterior, lateral, and posterior).
Pulmonary Venous System
Oxygenated arterial blood leaves the lungs via the pulmonary venous system (vv. pulmonales). Normally, humans have exactly four such veins: two on the right and two on the left. They drain directly into the left atrium, completing the pulmonary circulation.
Superior and inferior veins are formed on each side:
- Right superior pulmonary vein (v. pulmonalis superior dextra) collects blood from structures of the superior and middle lobes of the right lung (apical, anterior, posterior, medial, and lateral veins).
- Right inferior pulmonary vein (v. pulmonalis inferior dextra) drains the inferior lobe via the superior apical and basal venous branches.
- Left superior pulmonary vein (v. pulmonalis superior sinistra) is responsible for drainage from the superior lobe and lingular segments of the left lung.
- Left inferior pulmonary vein (v. pulmonalis inferior sinistra), similarly to the right inferior vein, returns blood from the basal and apical structures of the inferior lobe.
Topography of the Lung Root
To understand the surgical anatomy of the pulmonary circulation, it is essential to know the spatial arrangement of elements within the lung roots. Each root invariably comprises: a main bronchus (bronchus principalis), one pulmonary artery (a. pulmonalis), and two pulmonary veins (vv. pulmonales).
Syntopy of the elements differs between the right and left sides:
- In the right lung, the pulmonary artery lies strictly anterior to the main bronchus.
- In the left lung, the artery passes superior to the main bronchus.
To avoid confusing the topography of the pulmonary artery relative to the bronchus in the hilum, remember: in the right lung, it lies ANTERIORLY, while in the left, it lies SUPERIOR to the bronchus.
Class and exam questions
Where does the pulmonary circulation begin and where does it end?
It begins in the right ventricle of the heart (conus arteriosus), from where venous blood enters the pulmonary trunk, and ends in the left atrium, where oxygenated blood empties via four pulmonary veins.
At what level does the bifurcation of the pulmonary trunk occur?
The pulmonary trunk divides into the right and left pulmonary arteries at the level of the IV thoracic vertebra, lying directly beneath the aortic arch.
How many pulmonary veins drain into the heart, and what type of blood do they carry?
Four pulmonary veins empty into the left atrium (two on the right and two on the left). Despite the name "veins," they carry oxygen-rich arterial blood from the lungs to the heart.
What else to study on this topic
- Detailed segmental branching of the right pulmonary artery
- Lingular branches of the left pulmonary artery and their perfusion zones
- Formation of tributaries of the right pulmonary veins from segments
- Basal branches of the pulmonary arteries and veins: differences between the right and left lungs
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