The pulmonary veins are a set of four veins located in the thoracic cavity that carry oxygenated blood from the lungs to the left atrium of the heart. There are typically two pulmonary veins from each lung: the right superior and right inferior pulmonary veins, and the left superior and left inferior pulmonary veins. These veins exit the lungs at the hilum, the area where the bronchi, arteries, and veins enter and leave the lungs. The pulmonary veins pass through the mediastinum, posterior to the heart, before draining into the left atrium, completing the pulmonary circulation.
Location
The pulmonary veins are located in the thoracic cavity and run between the lungs and the left atrium of the heart. These veins carry oxygenated blood from the lungs to the heart. There are four pulmonary veins—two from each lung—known as the right superior and right inferior pulmonary veins, and the left superior and left inferior pulmonary veins. The pulmonary veins are positioned posterior to the heart, running through the posterior mediastinum.
Anatomy
Origin
The pulmonary veins originate from the lungs, where they collect oxygenated blood from the capillary networks surrounding the alveoli. The pulmonary veins begin at the hilum of each lung, which is the region where the pulmonary arteries, bronchi, and veins enter or exit the lung tissue.[7] The veins collect blood from the lung parenchyma and converge into larger vessels before exiting the lungs.
- Right Pulmonary Veins: The right superior and inferior pulmonary veins originate from the right lung. The right superior pulmonary vein drains the upper and middle lobes of the right lung, while the right inferior pulmonary vein drains the lower lobe.
- Left Pulmonary Veins: The left superior and inferior pulmonary veins originate from the left lung. The left superior pulmonary vein drains the upper lobe of the left lung, while the left inferior pulmonary vein drains the lower lobe.
Course
The pulmonary veins follow a relatively short and direct course from the hilum of the lungs to the left atrium of the heart.[5] They pass through the posterior mediastinum, which is the part of the thoracic cavity behind the heart and between the lungs. Unlike the pulmonary arteries, which run alongside the bronchi, the pulmonary veins have a more independent course as they do not follow the bronchial tree as closely.
- Right Pulmonary Veins: The right pulmonary veins pass horizontally from the right lung, running posterior to the right atrium and inferior to the right pulmonary artery before draining into the left atrium.
- Left Pulmonary Veins: The left pulmonary veins also pass horizontally, running posterior to the left atrium and the left pulmonary artery before entering the left atrium.
Termination
All four pulmonary veins terminate in the left atrium of the heart, where they deliver oxygenated blood from the lungs. Unlike most other veins in the body, which carry deoxygenated blood, the pulmonary veins are unique in transporting oxygenated blood. The two right pulmonary veins and the two left pulmonary veins enter the posterior wall of the left atrium at four distinct points. The pulmonary veins do not have valves at their junctions with the left atrium, which allows for smooth and continuous blood flow into the heart.
Relations to Neighboring Structures
- Right Pulmonary Veins: The right pulmonary veins are located behind the right atrium and beneath the right pulmonary artery. They pass near the right bronchus at the hilum of the right lung. As they approach the left atrium, they run posterior to the right atrium and lateral to the superior vena cava.
- Left Pulmonary Veins: The left pulmonary veins pass beneath the left pulmonary artery and behind the left atrium. At the hilum of the left lung, they are closely associated with the left bronchus. As they enter the left atrium, they are positioned lateral to the esophagus.
The pulmonary veins are surrounded by mediastinal pleura, a layer of tissue that lines the mediastinum, and they pass through the posterior mediastinum, which contains structures such as the descending aorta, esophagus, and vagus nerves.
Tributaries
The pulmonary veins collect oxygenated blood from smaller veins in the lungs that form a venous network around the alveoli and bronchial tree.[3] These smaller veins drain the lung parenchyma, including the alveoli, bronchial circulation, and other tissues, and converge into the larger right and left pulmonary veins. Each lung has a superior pulmonary vein and an inferior pulmonary vein, which receive blood from the respective lung lobes.
- Superior Pulmonary Veins: These veins collect blood from the upper and middle lobes (right lung) or upper lobe (left lung).
- Inferior Pulmonary Veins: These veins collect blood from the lower lobes of each lung.
Variations
While the anatomy of the pulmonary veins is generally consistent, there are some anatomical variations:
- Accessory Pulmonary Veins: In some individuals, there may be more than four pulmonary veins due to additional veins that arise from the lung lobes. These are known as accessory pulmonary veins and typically drain into the left atrium alongside the main pulmonary veins.
- Single Pulmonary Vein: Rarely, some individuals may have a single pulmonary vein on one or both sides, where the superior and inferior veins join into a common trunk before entering the left atrium.[1]
Function
Transport of Oxygenated Blood to the Heart
The primary function of the pulmonary veins is to transport oxygenated blood from the lungs to the left atrium of the heart. After blood is oxygenated in the lung’s alveoli, the pulmonary veins collect this oxygen-rich blood and carry it to the heart, where it will be pumped into the systemic circulation. This function is vital for maintaining the oxygen supply to tissues and organs throughout the body, as the pulmonary veins are the final link in the pulmonary circulation, connecting the lungs to the heart.
Oxygenation Process
In the lungs, blood from the pulmonary arteries passes through the capillary networks surrounding the alveoli, where gas exchange occurs. Oxygen from the inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses into the alveoli to be exhaled. The pulmonary veins are responsible for collecting this oxygenated blood from the lungs, ensuring it is rapidly transported to the heart for distribution.
Drainage of Blood from the Lung Parenchyma
The pulmonary veins also play a role in draining blood from the lung parenchyma, which includes the functional tissue of the lungs involved in gas exchange, such as the alveoli, bronchi, and blood vessels. The veins collect oxygenated blood from the alveolar capillaries and smaller venous tributaries in the lung tissue, preventing blood from pooling in the lungs and ensuring continuous circulation.
Regulation of Cardiac Output
By delivering oxygenated blood to the left atrium, the pulmonary veins support the regulation of cardiac output.[7] The volume of blood returning to the left atrium through the pulmonary veins influences the preload, or the amount of blood filling the heart before contraction. This, in turn, affects the stroke volume—the amount of blood the heart pumps with each beat. Efficient venous return from the lungs helps maintain adequate cardiac output, ensuring the heart can supply oxygen-rich blood to the rest of the body.
Maintenance of Efficient Pulmonary Circulation
The pulmonary veins are essential for the proper functioning of the pulmonary circulation, which is the system responsible for moving blood between the heart and lungs. Pulmonary veins work in concert with the pulmonary arteries, which carry deoxygenated blood from the right ventricle of the heart to the lungs. After oxygenation, the pulmonary veins return the oxygen-rich blood to the heart. This closed loop allows for continuous circulation between the lungs and heart, enabling efficient gas exchange and blood oxygenation.
Balance of Blood Flow between the Lungs and Heart
The pulmonary veins also help to balance the flow of blood between the lungs and heart. The lungs receive the entire cardiac output from the right ventricle via the pulmonary arteries, and the pulmonary veins must match this output by returning an equal volume of oxygenated blood to the left atrium. This balance is critical for preventing fluid overload in the lungs and maintaining proper blood flow through the heart and lungs.
Support for Systemic Oxygen Delivery
The pulmonary veins enable the heart to supply oxygenated blood to the systemic circulation by ensuring the heart has a steady supply of oxygen-rich blood from the lungs.[5] Once the pulmonary veins deliver the oxygenated blood to the left atrium, the blood is pumped into the left ventricle and then into the aorta, where it is distributed to tissues and organs throughout the body. The pulmonary veins are therefore a key part of the process that supplies the body with the oxygen needed for cellular respiration and energy production.
Prevention of Venous Congestion in the Lungs
The pulmonary veins also help prevent venous congestion in the lungs. Without the efficient drainage of blood through the pulmonary veins, blood could accumulate in the lung parenchyma, leading to increased pressure and potential complications such as pulmonary edema, which is the abnormal accumulation of fluid in the lung tissues. The pulmonary veins ensure that blood is continuously removed from the lungs, preventing congestion and allowing for proper respiratory function.
Facilitating the Left Heart Function
The pulmonary veins play a role in facilitating the function of the left atrium and left ventricle, which are responsible for pumping oxygenated blood throughout the body. The continuous flow of blood from the pulmonary veins into the left atrium ensures that the heart’s left side is always filled with oxygenated blood, which can be pumped into the aorta and distributed to the systemic circulation.[3] This uninterrupted flow of oxygenated blood is essential for maintaining the heart’s efficiency and ensuring that the body receives a steady supply of oxygen.
Connection to the Pulmonary and Systemic Circulatory Loops
The pulmonary veins serve as the final connection between the pulmonary circulation, which involves gas exchange in the lungs, and the systemic circulation, which distributes oxygenated blood to the body. This vital function makes the pulmonary veins the bridge between two major circulatory loops, ensuring that oxygenated blood can enter the heart and be delivered to all tissues.
Role in Regulation of Left Atrial Pressure
The pulmonary veins also influence left atrial pressure, which affects the overall function of the left atrium and left ventricle. When the pulmonary veins deliver a steady flow of blood into the left atrium, it maintains the appropriate pressure needed to fill the left ventricle before contraction. Proper venous return from the pulmonary veins helps regulate the pressure within the left atrium and ventricle, ensuring the heart functions effectively as it pumps oxygenated blood to the systemic circulation.
Clinical Significance
The pulmonary veins are clinically significant due to their essential role in transporting oxygenated blood from the lungs to the heart. Conditions affecting the pulmonary veins can lead to severe cardiovascular and pulmonary complications. One notable condition is pulmonary vein stenosis, where the veins become narrowed, leading to restricted blood flow from the lungs to the heart, which can cause pulmonary hypertension and right heart failure.
In the context of atrial fibrillation (AF), the pulmonary veins are often the site of abnormal electrical activity that triggers the arrhythmia. Pulmonary vein isolation is a common procedure used in the treatment of AF, where specific areas of the veins are ablated to prevent irregular heart rhythms.
Other potential issues include pulmonary vein thrombosis, a rare condition in which a clot forms in the pulmonary veins, leading to impaired blood flow and oxygenation.[2] Pulmonary veins are also involved in certain surgical procedures, such as lung resection or heart surgeries, where their anatomical position and function must be carefully managed to avoid complications.
