The truncus pulmonalis is a short arterial vessel that forms a common trunk connecting the right ventricle and the right and left pulmonary arteries into which the truncus pulmonalis branches. At the entrance to the artery is the pulmonary valve, which closes during the relaxation phase of the ventricles (diastole) to prevent backflow of blood from the pulmonary arteries into the right ventricle.
What is the truncus pulmonalis?
The truncus pulmonalis forms the common trunk of the right and left pulmonary arteries (dextra and sinistra pulmonary arteries), into which the arterial trunk branches after a course of only about 5 centimeters. The pulmonary trunk, together with the two pulmonary arteries, represents the arterial part of the pulmonary circulation. In the arterial part of the pulmonary circulation, “used” blood, which is low in oxygen and rich in carbon dioxide, and which enters the right ventricle from the large systemic circulation via the right atrium, is transported to the two lungs. There, further branching of the two pulmonary arteries takes place down to the capillary level. The capillaries surround the alveoli (alveoli), where the exchange of substances takes place and the blood is enriched with oxygen. The arterial portion of the pulmonary circulation is the only arterial system in the body that transports deoxygenated blood. The pulmonary valve is located in the initial portion of the pulmonary trunk, which prevents backflow of blood from the pulmonary arteries into the right ventricle during the relaxation phase (diastole) of the ventricles.
Anatomy and structure
The truncus pulmonalis begins in the wall of the right ventricle and branches into the left and right pulmonary arteries after only about 5 cm. With its diameter of about 3 cm, it is one of the large vessels. A special feature is that the arterial trunk runs just below the aortic arch, because in the unborn child there is a junction at the point of contact of the two arteries that short-circuits the pulmonary circulation because there is no pulmonary respiration before birth. In arteries, a basic distinction is made between muscular and elastic types, as well as mixed forms and, as a special type, blocking arteries, which can “shut down” and completely interrupt the blood flow if necessary. While the arterial system of the great circulation or systemic circulation offers much more vascular resistance than that of the pulmonary circulation, and resistance must be variable to adjust and change blood pressure, the arteries of the systemic circulation belong mostly to the muscular types. The vascular resistance in the arterial part of the pulmonary circulation is only about one-tenth that in the systemic circulation. This, together with the fact that blood flow at the alveoli must be “regionally” adjustable depending on oxygen supply, means that the arterial trunk and the two pulmonary arteries should respond only minimally to nerve stimuli and messengers to constrict the vessels (vasoconstriction). Therefore, evolution has developed the pulmonary arterial trunk and the two pulmonary arteries as elastic arteries. This means that the middle of three layers (tunica media) of their walls is weak and contains few muscle cells. In contrast, elastic fibers predominate.
Function and tasks
As the trunk of the pulmonary arteries, the truncus pulmonalis provides the central supply of the arterial pulmonary circulation and is the counterpart of the central arterial supply of the systemic circulation through the aorta, from which all arteries of the systemic circulation branch. Due to the respective central supply of the two blood circuits, the heart manages with “only” four heart valves, two of which are designed as pocket valves and close the arterial outlets in the right and left ventricles (pulmonary valve and aortic valve) during the relaxation phase of the ventricles (diastole) and release them during the contraction phase. However, the function of the pulmonary truncus is not only to act as a supply conduit for the pulmonary circulation; it also has the task of ensuring a nearly continuous flow of blood to the alveoli of the lungs and maintaining a minimum pressure, especially during diastole. The arterial trunk and the two pulmonary arteries act as a kind of pressure reservoir, which at the same time must protect the alveoli from pressure peaks and excessively fluctuating pressure patterns during the two phases of the heart.It is therefore important that the arterial trunk and the arteriae pulmonales are formed as elastic arteries that can inflate a little during “pressure filling” by the right ventricle to absorb the pressure peak. Fulfillment of the role of acting as a pressure reservoir during diastole requires proper pulmonary pocket valve function at the entrance of the truncus pulmonalis.
Diseases
Functional impairment of the pulmonary artery trunk attributable to inflammation, infection, or other disease and associated physiologic changes in the vessel walls is rare. The development of malfunction of the pulmonary valve located in the entrance of the truncus pulmonalis due to stenosis or insufficiency caused by disease and inflammation, is also relatively rare. Rather more common are malformations and malappositions of the truncus pulmonalis, including the pocket valve. Usually, such malformations are accompanied by other congenital heart defects such as septal defect and others and lead to mild to severe effects and courses if not treated by corrective and reconstructive interventions. Pathological vascular changes in the arterial part of the pulmonary circulation, resulting in loss of elasticity of the vascular walls, can cause pulmonary hypertension (PH), which, depending on its severity, can lead to right heart strain and insufficiency. A very rare congenital heart defect is the formation of a truncus arteriosus communis. In this case, the prenatal connection between the truncus pulmonalis and the aorta to bypass the pulmonary circulation has not closed, so that oxygen-poor blood from the venous part of the systemic circulation (right ventricle) mixes with oxygen-rich blood from the venous part of the pulmonary circulation with corresponding symptomatic consequences.
