Overview

Right pulmonary artery. This is the longer and wider of the two. It runs horizontally to the right beneath the aortic arch and behind the ascending aorta and SVC, then enters the hilum of the right lung. At the right hilum it lies in front of the main bronchus and below the right upper lobe bronchus, which is therefore called the eparterial bronchus.

Left pulmonary artery. This is shorter. It passes in front of the descending aorta and arches over the left main bronchus, so it is the highest structure in the left hilum. The ligamentum arteriosum connects it to the arch of the aorta.

Within the lungs. The pulmonary arteries follow the bronchi down to the respiratory bronchioles, forming bronchovascular bundles. The pulmonary veins instead run between the segments. The arteries end in a dense capillary network over the alveoli, with a gas-exchange surface of about 70 m². Blood enters this network with a PO₂ of about 40 mmHg and leaves it at about 100 mmHg.

Location: From the bifurcation of the pulmonary trunk (T4/T5) to the hila of the lungs, then throughout the lung parenchyma.

Anatomy

Relations: At the hilum, the pulmonary artery lies anterior on the right and superior on the left (mnemonic "RALS": Right Anterior, Left Superior). The pulmonary veins lie anterior and inferior at both hila.

Function

Deliver mixed venous blood to the alveolar capillaries for gas exchange.
Filter small venous emboli and microthrombi out of the circulation.

Clinical significance

Pulmonary embolism

Emboli lodge in the main, lobar, segmental or subsegmental arteries. They are shown as filling defects on CT pulmonary angiography.

Pulmonary arterial hypertension

Idiopathic, heritable (BMPR2 mutations) or drug-induced remodelling of the small pulmonary arteries progressively raises pulmonary vascular resistance. Treatment targets the endothelin, nitric oxide and prostacyclin pathways.

Test yourself

In the fetus, the ductus arteriosus connects:

The pulmonary trunk (left pulmonary artery) to the aortic arch
The umbilical vein to the inferior vena cava
The right atrium to the left atrium
The ascending aorta to the superior vena cava

Answer: The pulmonary trunk (left pulmonary artery) to the aortic arch

The ductus arteriosus runs from the start of the left pulmonary artery to the underside of the aortic arch, beyond the left subclavian artery. It carries most right ventricular output past the high-resistance fetal lungs. After birth it becomes the ligamentum arteriosum.