The body sustains life through a vast network of blood vessels that form the circulatory system. This system is divided into two primary circuits: the systemic circulation, which delivers oxygenated blood to the body’s tissues, and the pulmonary circulation, which is dedicated to the lungs. The pulmonary circuit is a shorter, lower-pressure pathway that ensures blood can be refreshed with oxygen after returning from the body. This pathway uses the pulmonary arteries to carry blood from the heart to the lungs.
The Main Pulmonary Artery and Its Immediate Branches
The single main vessel that leaves the heart is known as the pulmonary trunk, also called the main pulmonary artery. This short, wide vessel is approximately 5 centimeters long in adults and acts as the outflow tract from the heart’s right side.
This trunk quickly divides into two major branches: the right pulmonary artery and the left pulmonary artery. Therefore, the system begins with one main vessel that immediately gives rise to two dedicated branches, meaning three main segments are commonly referred to.
The right pulmonary artery travels to the right lung, and the left pulmonary artery heads toward the left lung. These two main branches then continue to divide extensively once inside the lung tissue, creating a complex network of smaller vessels.
Unique Role in Blood Circulation
The pulmonary arteries hold a special designation because they are the only arteries in the adult body that carry deoxygenated blood. By definition, an artery is a vessel that carries blood away from the heart, which is the function the pulmonary arteries perform. Unlike systemic arteries, however, the pulmonary arteries transport blood that has already delivered its oxygen load.
This oxygen-poor blood must be routed to the lungs for gas exchange. The pulmonary arteries deliver this blood to tiny air sacs in the lungs called alveoli. The blood then flows through an extensive network of capillaries, which are the smallest blood vessels.
Across the thin walls of these capillaries, gas exchange takes place. Carbon dioxide diffuses out into the alveoli to be exhaled, and oxygen from the inhaled air diffuses into the blood. Once oxygenated, the blood travels back to the heart via the pulmonary veins, which are the only veins in the body that carry oxygen-rich blood.
Anatomical Journey to the Lungs
The journey of the pulmonary arteries begins directly at the heart’s lower right chamber, the right ventricle. Blood is ejected from this chamber through the pulmonary valve and into the pulmonary trunk. This main vessel travels upward and slightly to the left, positioned near the ascending aorta.
The trunk then reaches the point of bifurcation, where it divides into its two main branches just beneath the aortic arch. The right pulmonary artery takes a longer, more horizontal path, traveling behind the ascending aorta and the superior vena cava to reach the right lung. Conversely, the left pulmonary artery is typically shorter and arches over the left main bronchus to enter the left lung.
Both the right and left pulmonary arteries enter their respective lungs at a central indentation known as the hilum. Upon entering the lung, the vessels begin to branch further alongside the airways (bronchi), ensuring that every segment of lung tissue receives blood for oxygenation.