Pulmonary Arteries and Veins
Embryologically, the pulmonary arteries originate from the truncus arteriosus (as does the aorta), and in the developed heart, the pulmonary trunk (pulmonary artery or main pulmonary artery) begins at the base of the right ventricle. The pulmonary trunk is a short and stout (wide) structure that is about 5 cm in length and 3 cm in diameter, which branches into 2 pulmonary arteries; the left and right pulmonary arteries, which act to deliver deoxygenated blood to its respective lung.
On the other hand, pulmonary veins are large blood vessels that receive oxygenated blood from the lungs to delivery to the rest of the body. There are 4 total pulmonary veins—with 2 pulmonary veins coming from each lung, left and right—that empty into the left atrium of the heart. Two pulmonary veins emerge from the hilus of each lung, and each pulmonary vein receives blood from 3-4 bronchial veins apiece before draining into the left atrium. The pulmonary veins are fixed to the pericardium travel alongside the pulmonary arteries.
The right superior pulmonary vein passes in front of and a tad below the pulmonary artery at the root of the lung, and the inferior pulmonary vein is situated at the lowest part of the lung hilum. In reference to the heart, the right pulmonary veins pass behind the right atrium and superior vena cava return, and the left pulmonary veins pass in front of the descending thoracic aorta. Finally, the bronchus is located behind the pulmonary artery.
While veins usually carry deoxygenated blood from tissues back to the heart, in this case, pulmonary veins are among the few veins that carry oxygenated blood instead. Oxygenated blood from the lungs is circulated back to the heart through the pulmonary veins that drain into the left atrium. Once blood is pumped from the left atrium through the mitral valve into the left ventricle, this oxygenated blood will then be pumped from the left ventricle through the aortic valve to the rest of the body’s organs and tissues through the aorta.
Deoxygenated blood that has circulated through the system will be collected from the superior vena cava and inferior vena cava, which drain into the right atrium of the heart. Once deoxygenated blood is pumped from the right atrium through the tricuspid valve into the right ventricle, contraction of the right ventricle will push blood through the pulmonic valve into the pulmonary artery that will carry deoxygenated blood to the lungs. Within the lungs, the blood passes through capillaries adjacent to alveoli and becomes oxygenated through respiration (breathing). Branches of the pulmonary artery travel closely alongside the bronchial tree on their way to the alveoli. However, the bronchial tree itself is supplied by the bronchial artery, which arises from the aorta and carries systemic blood. Each alveolus is surrounded by a nest of blood capillaries that are supplied by small branches of the pulmonary artery.
In summary, the pulmonary circuit begins with the pulmonary trunk, which is a large vessel that ascends diagonally from the right ventricle and branches into the right and left pulmonary arteries. As the circuit approaches the lung, the right pulmonary artery branches into two arteries, and both branches enter the lung at a medial indentation called the hilum of the lung. The upper branch is the superior lobar artery, which feeds into the superior lobe of the lung. The lower branch divides again within the lung to form the middle lobar and inferior lobar arteries that supply the lower 2 lobes of the lung, since there are 3 lobes of the right lung. The left pulmonary artery is more variable in number and gives off several superior lobar arteries that feed into the superior lobe before entering the hilum of the lung to branch off into inferior lobar arteries that feed the left lower lung lobe.
Histology of Arteries and Veins
Large veins have diameters greater than 10 mm. They have some smooth muscle in all three tunics. They have a relatively thin tunica media with only a moderate amount of smooth muscle; the tunica externa is the thickest layer and contains longitudinal bundles of smooth muscle. Large veins include the venae cavae, pulmonary veins, internal jugular veins, and renal veins. Since the pressure of the pulmonary veins cannot be easily measured, the pulmonary capillary wedge pressure is used instead, and the normal range is 2-15 mmHg. The pulmonary arteries have thin distensible walls with less elastic tissue than the systemic arteries. Thus, they have a blood pressure range of 15-30 mmHg systolic, and 4-12 mmHg diastolic.
Pulmonary Hypertension: Conceptually, the output by the 2 ventricles of the heart must be equal to insure homeostasis. However, if the right ventricle pumps more blood into the lungs than the left ventricle can handle on return, blood will accumulate in the lungs and cause pulmonary hypertension and edema. Too much edema can put a patient at risk of drowning in one’s own body fluid. Clinically, respiratory distress or shortness of breath can be a sign of left ventricular failure. Excess fluid accumulation from insufficiency of ventricular pumping (whether it be insufficiency of the right or left ventricle) can lead to congestive heart failure.
Heart Failure: As mentioned under “Pulmonary Hypertension,” insufficiency of ventricular pumping can lead to congestive heart failure since failure of one ventricle will lead to an increased workload on the other ventricle, often leading to eventual failure of both ventricles. For this reason, right-sided heart failure is the most common cause of left-sided heart failure. In this case, pulmonary hypertension is the primary condition, and heart failure is a secondary or tertiary effect of the chronic hypertension.
Pulmonary Embolism (PE): A very common cause of morbidity and mortality results from the obstruction of a pulmonary artery by a blood clot (embolus). Formation of an embolus in the pulmonary artery can happen when a blood clot, fat globule, or air bubble travels in the blood to the lungs. For example, this can occur after a long plane flight that disposes passengers to a deep vein thrombosis in the legs that results in a PE. Another example would be an embolus following a compound bone fracture. The embolus passes through the right side of the heart to a lung through the pulmonary artery, and it may obstruct the artery or one of its branches. If this blockage is complete instead of only partial, then the patient will suffer from acute respiratory distress due to a major decrease in blood oxygenation. In this case, the right side of the heart may become acutely dilated since the volume of blood systemically trying to return to the heart cannot be pushed through the pulmonary circuit, thus causing acute cor pulmonale. A partial obstruction can result in a pulmonary infarct, or area of necrotic lung tissue.
Hypoxia: There is a unique characteristic of the pulmonary arteries is their response to hypoxia. Whereas systemic arteries will dilate in response to local hypoxia to improve tissue perfusion, pulmonary arteries will oppositely constrict instead. Presence of pulmonary hypoxia indicates that a part of the lung is not being ventilated properly. This can be due to airway congestion (due to mucous, etc.) or a degenerative lung disease/condition. Vasoconstriction will thusly occur in poorly ventilated regions of the lung in order to redirect blood flow to better-ventilated regions of the lung.