Pulmonary Trunk

Overview

The pulmonary trunk is a large arterial vessel that emerges from the right ventricle of the heart and serves as the main conduit for deoxygenated blood being transported to the lungs for oxygenation. It is the first component of the pulmonary circulation and quickly bifurcates into the right and left pulmonary arteries. Unlike most arteries that carry oxygen-rich blood, the pulmonary trunk and its branches carry oxygen-poor blood — a unique feature of the pulmonary vascular system.

Location

The pulmonary trunk originates from the right ventricle of the heart and ascends slightly posteriorly and to the left. It lies:

• Anterior to the ascending aorta at its origin

• Left of the aorta as it ascends

• Below the aortic arch before it bifurcates

At the level of the T5–T6 vertebrae (around the sternal angle), it bifurcates into:

• Right pulmonary artery – passes horizontally across the midline to the right lung

• Left pulmonary artery – passes horizontally to the left lung

Structure

The pulmonary trunk is a thick-walled, elastic artery designed to accommodate large volumes of blood ejected from the right ventricle during systole. Key structural features include:

• Origin: Conus arteriosus (infundibulum) of the right ventricle

• Length: Approximately 5 cm

• Diameter: Roughly 3 cm in adults

• Wall layers: Tunica intima (endothelium), tunica media (smooth muscle and elastic fibers), and tunica adventitia (connective tissue)

• Pulmonary valve: A semilunar valve located at the origin of the trunk, preventing backflow into the right ventricle during diastole

Function

The primary function of the pulmonary trunk is to:

• Transport deoxygenated blood from the right ventricle to the lungs via the pulmonary arteries

• Serve as the starting point of the pulmonary circulation

This ensures that blood is directed toward the alveolar capillaries in the lungs, where gas exchange occurs — carbon dioxide is released, and oxygen is absorbed.

Physiological Role(s)

The pulmonary trunk plays multiple vital roles in cardiovascular physiology:

• Manages low-pressure, high-volume flow: Unlike systemic arteries, the pulmonary trunk operates under lower pressure (normally 15–25 mmHg systolic) to protect the delicate pulmonary capillaries.

• Supports gas exchange: It ensures timely delivery of blood to the lungs for continuous oxygenation and carbon dioxide removal.

• Coordinates with cardiac cycle: The pulmonary valve and elastic nature of the trunk allow smooth systolic ejection and diastolic closure, minimizing energy loss.

Clinical Significance

The pulmonary trunk is involved in various congenital, structural, and hemodynamic disorders:

• Pulmonary Hypertension: Elevated pressure in the pulmonary trunk and its branches can result from chronic lung disease, left heart disease, or idiopathic causes. Leads to right ventricular strain and eventual failure.

• Pulmonary Embolism: A life-threatening condition where a thrombus lodges in the pulmonary trunk or its branches, causing acute right heart failure, hypoxia, or death.

• Truncus Arteriosus: A rare congenital defect where the pulmonary trunk and aorta fail to separate during development, resulting in a single outflow tract — requires early surgical correction.

• Transposition of the Great Arteries (TGA): A condition in which the pulmonary trunk arises from the left ventricle instead of the right, disrupting normal circulation and requiring neonatal intervention.

• Dilation or Aneurysm: Pulmonary trunk dilatation may occur in pulmonary hypertension or connective tissue disorders and is visible on imaging studies (CT, MRI, echocardiography).

• Valve Disorders: Pulmonary valve stenosis or regurgitation affects flow through the trunk and may lead to post-stenotic dilation or backward flow into the right ventricle.

Assessment of the pulmonary trunk is routinely performed using echocardiography, CT angiography, and cardiac MRI. Early recognition of abnormalities is critical for preventing right-sided heart failure and improving outcomes in cardiopulmonary disease.