Overview

Length × width≈5 cm × 3 cm

Bifurcation levelT4/T5 (sternal angle)

Pressure≈25/10 mmHg (mean ≈15)

The pulmonary trunk is about 5 cm long and 3 cm wide. It starts at the pulmonary valve, at the top of the conus arteriosus, behind the left 3rd costal cartilage. It passes upward and backward, first in front of and then to the left of the ascending aorta. The two vessels twist around each other, a legacy of the spiral aorticopulmonary septum.

The trunk lies inside the pericardium and shares a sleeve of serous pericardium with the ascending aorta. The transverse pericardial sinus lies directly behind both vessels. The trunk divides into its two branches at about T4/T5 (the level of the sternal angle), in the concavity of the arch, in front of the left main bronchus. The ligamentum arteriosum, the remnant of the fetal ductus arteriosus, joins the beginning of the left pulmonary artery to the underside of the arch.

The pulmonary circulation is a low-pressure, high-compliance circuit. Pulmonary artery pressure is about 25/10 mmHg (mean about 15 mmHg), and pulmonary vascular resistance is about one-tenth of systemic resistance.

Location: Middle and superior mediastinum, from the right ventricle up to its bifurcation beneath the aortic arch.

Anatomy

Branches: The right and left pulmonary arteries.

Function

Carries the whole right ventricular output, about 5 L/min of deoxygenated blood, to the lungs.
Its elastic wall buffers pulsatile right ventricular ejection.

Physiology: Unlike systemic arteries, pulmonary arteries constrict when alveolar oxygen is low (hypoxic pulmonary vasoconstriction). This matches perfusion to ventilation. When the whole lung is hypoxic, as at altitude or in chronic lung disease, the same response causes pulmonary hypertension.

Clinical significance

Pulmonary embolism

A large thrombus can lodge across the bifurcation (a saddle embolus). The sudden rise in afterload causes acute right ventricular failure and obstructive shock. The ECG commonly shows sinus tachycardia, and sometimes an S1Q3T3 pattern or right bundle branch block.

Pulmonary hypertension

Pulmonary hypertension is a mean pulmonary artery pressure above 20 mmHg (2022 ESC/ERS definition). It dilates the trunk, produces a loud P2 and leads to right ventricular hypertrophy and failure (cor pulmonale).

Conotruncal anomalies

In transposition of the great arteries, the pulmonary trunk arises from the left ventricle and the aorta from the right. In persistent truncus arteriosus, a single great artery leaves both ventricles. Both are cyanotic lesions caused by abnormal aorticopulmonary septation.

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.