The pulmonary trunk is a large artery originating from the heart’s right ventricle, serving as the starting point for the pulmonary circulation system. It transports blood that needs oxygen to the lungs for gas exchange. It is one of the great vessels of the heart, a term for the major blood vessels that connect directly to its chambers.
Anatomy and Location of the Pulmonary Trunk
The pulmonary trunk is a relatively short and wide vessel, measuring about 5 centimeters in length and 3 centimeters in diameter. It emerges from the top of the right ventricle, passing through the pulmonary valve which prevents blood from flowing back into the ventricle. The vessel is entirely contained within the pericardial sac, the membrane enclosing the heart.
From its origin, the pulmonary trunk travels upward and backward. It starts positioned in front of the aorta, the body’s main artery, and then moves to the aorta’s left side as it ascends. This positioning is a result of the heart’s rotation during embryonic development. The trunk is enveloped in a common sheath of tissue with the ascending aorta.
The most significant anatomical feature of the pulmonary trunk is its division. At about the level of the fifth and sixth thoracic vertebrae, the trunk splits into the right and left pulmonary arteries. The right pulmonary artery travels under the aortic arch to reach the right lung, while the shorter left pulmonary artery moves toward the left lung. This split marks the end of the pulmonary trunk.
Function in Pulmonary Circulation
The primary role of the pulmonary trunk is to transport deoxygenated blood away from the heart and direct it to the lungs. This function is unique among arteries, as most arteries in the body carry oxygen-rich blood. Blood that has circulated the body returns to the right side of the heart and is pumped from the right ventricle into the pulmonary trunk.
Once inside the pulmonary trunk, the blood travels toward the lungs through the branching right and left pulmonary arteries. Upon reaching the vast network of tiny capillaries surrounding the air sacs (alveoli) in the lungs, an exchange occurs. Carbon dioxide, a waste product from bodily processes, is released from the blood into the air to be exhaled. Simultaneously, oxygen from inhaled air moves from the alveoli into the blood.
The pulmonary circulation system operates under much lower pressure than the systemic circulation that supplies the rest of the body. The walls of the pulmonary trunk and its subsequent arteries are thinner and more flexible than those of the aorta. This structure is well-suited to handle the lower-pressure blood flow required for the delicate process of gas exchange within the lungs.
Conditions Affecting the Pulmonary Trunk
One condition is pulmonary hypertension, which is characterized by abnormally high blood pressure within the pulmonary arteries. This increased pressure begins in the pulmonary trunk and extends into the smaller arteries of the lungs, forcing the right side of the heart to work much harder to pump blood. Over time, this can lead to right heart failure.
Another serious condition is a pulmonary embolism. This occurs when a blood clot, often originating from the deep veins of the legs, travels through the bloodstream and becomes lodged in the pulmonary trunk or one of its main branches. Such a blockage can obstruct blood flow to the lungs, which is a life-threatening emergency.
Congenital heart defects can also affect the pulmonary trunk. Pulmonary artery stenosis involves a narrowing of the pulmonary valve or the pulmonary trunk itself, restricting blood flow from the right ventricle to the lungs. Another defect is the transposition of the great arteries, where the pulmonary trunk and the aorta are anatomically switched. In this case, the pulmonary trunk arises from the left ventricle and the aorta from the right, leading to two separate circulatory loops that prevent oxygenated blood from reaching the body.