The Anatomy of the Pulmonary Vein

The four pulmonary veins play an important role in the pulmonary circulation by receiving oxygenated blood from the lungs and delivering it to the left atrium, where it can then enter the left ventricle to be circulated throughout the body. The pulmonary vein is unique in that it is the only vein that carries oxygenated blood.

Until delivery, fetal blood flow bypasses these vessels, which open at birth upon exposure to oxygen. There are some anatomic variations that may occur as well as several congenital conditions (birth defects) involving these veins that are found in some babies. Medical conditions may occur in adults as well, such as pulmonary venous hypertension.

Anatomy

The pulmonary veins along with the pulmonary arteries make up the pulmonary circulation.

Development

Before birth, the fetus receives oxygen and nutrients from the placenta so that the blood vessels leading to the lungs, including the pulmonary artery and pulmonary vein, are bypassed. It's only at the moment of birth when a baby takes its first breath that blood enters the pulmonary blood vessels to enter the lungs.

Structure

The pulmonary veins are relatively large structures relative to other veins running as large as 1 centimeter in diameter, though they tend to be smaller in women. They are made up of three layers of smooth muscle tissue called tunics. The outer layer is the thick tunica externa, with the middle layer the thin tunica media, followed by the central layer, the tunica intima.

Location

The four pulmonary veins (two from each lung) bring freshly oxygenated blood from the lungs to the left atrium. From the left atrium, the blood travels to the left ventricle from where it is ejected to supply oxygen and nutrients to all of the tissues of the body.

In the lungs, the smallest of blood vessels, called capillaries, are the site where carbon dioxide is removed and oxygen obtained from the alveoli, the smallest structures of the respiratory tree.

From the capillaries, blood enters the bronchial veins, which then drain into the pulmonary veins. The two pulmonary veins exit the lungs through the region known as the hilum, the area where blood vessels as well as the right and left main bronchi enter the lungs in the medial, central area of each lung.

After leaving the hilum, the right pulmonary vein passes posterior to the superior vena cava and right atrium to enter the left atrium. The left pulmonary veins pass in front of the descending aorta to enter the left atrium.

Since the pulmonary veins travel through the lungs and empty into the left side of the heart, they may be affected by any condition that involves these regions.

Anatomic Variations

People often have four pulmonary veins, but variations occur in 38 percent of people. In some cases, there will be five and other times only three pulmonary veins.

The right lung has three lobes and the left lung has two, with the superior right pulmonary vein draining the upper and middle lobe and the inferior right pulmonary vein draining the lower lobe. On the left side, the superior left pulmonary artery drains the left upper lobe and the inferior left pulmonary artery the lower lobe.

In some people, the three right pulmonary veins remain separate instead of merging into two veins, resulting in a total of five pulmonary veins (this is referred to as a single accessory right middle pulmonary vein and is present in roughly 10 percent of people).

A number of other variations can occur as well, such as the presence of two accessory right middle pulmonary veins, one accessory right middle pulmonary vein and one accessory right upper pulmonary vein, and more.

Function

The pulmonary veins serve a very important purpose of delivering freshly oxygenated blood from the lungs to the heart, so it can be sent to the rest of the body.

Clinical Significance

The pulmonary veins can be affected by medical conditions present at birth or acquired later on in life. Due to the pulmonary veins' central role in the heart and pulmonary circulation, congenital conditions are often associated with other heart defects and acquired conditions are often related to other underlying heart conditions.

Congenital Conditions (Birth Defects)

Congenital conditions affecting the pulmonary veins can affect the number of these veins, their diameter, their development, or their connections and drainage (where they deliver the blood that is brought from the lungs). These vessels may also be affected by conditions in the region of the heart where they empty.

Hypoplasia of the Pulmonary Veins

In some children, the pulmonary veins fail to develop completely (hypoplasia). This is usually associated with hypoplastic left heart syndrome.

Pulmonary Vein Stenosis or Atresia

Pulmonary vein stenosis is a condition in which the pulmonary vein is thickened, leading to narrowing. It is an uncommon but serious birth defect and is often combined with other heart abnormalities. Treatment including angioplasty and stent placement can open the vein, but it tends to narrow again (restenosis). Recently, physicians have used chemotherapy as well as biological agents after angioplasty to try to prevent the narrowing from recurring.

Partial or Total Anomalous Pulmonary Venous Return

In this condition, one or more pulmonary veins enter the right atrium instead of the left atrium. When only one vein enters the right atrium, a child is usually asymptomatic, but if two or more enter the right atrium, surgical correction is usually needed. There are several degrees of this condition of varying concern.

Anomalous pulmonary venous return can be a cardiac emergency in newborns, and efforts are being made to diagnose more of these conditions in the prenatal period with ultrasound. It is one of the causes of cyanotic congenital heart disease (conditions that result in a baby being born blue).

Anomalous pulmonary venous return is often associated with an atrial septal defect (ASD), and an ASD is always present with total anomalous pulmonary venous return. In this condition, oxygenated blood from the lungs mixes with deoxygenated blood in the right atrium. The blood then passes through the ASD (the hole in the heart) to the left side of the heart to be ejected to the rest of the body.

Risk factors for the condition include Turner's syndrome (XO), and according to a 2018 study, maternal obesity. Some congenital heart conditions run in families, but this does not appear to be a significant risk factor.

This condition, along with several types of congenital heart disease, can often be diagnosed with an echocardiogram.

Pulmonary Venous Varix (Pulmonary Venous Aneurysm)

This is a condition in which a region of the pulmonary vein is dilated.

Cor Triatriatum

Cor triatriatum is a congenital condition in which the pulmonary veins enter the left atrium through only a single opening. If the opening is large enough, it may be asymptomatic. However, if the opening is small and restrictive, it can be surgically repaired.

Pulmonary Arteriovenous Malformation

This is a condition in which there is communication between the pulmonary artery and pulmonary vein. It may be asymptomatic or cause shortness of breath.

Acquired Conditions

After birth and in adults, the pulmonary veins may be affected by narrowing or obstruction, increased pressure, and blood clots (thrombosis).

Pulmonary Vein Stenosis

Stenosis, or narrowing, can occur in the pulmonary veins, similar to narrowing in arteries such as the coronary arteries. When narrowed, angioplasty may be done or stents placed to maintain the caliber of the vein. Pulmonary vein stenosis sometimes occurs after ablation for atrial fibrillation.

Pulmonary Vein Obstruction

The pulmonary veins may become obstructed in a few conditions such as lung cancer or tuberculosis. Worsening shortness of breath in someone with lung cancer can be a sign of this complication.

Surgical and Procedural Damage

The pulmonary veins may also be damaged during surgical procedures. This includes the different types of surgery for lung cancer. Radiofrequency ablation for arrhythmias may also result in damage.

Pulmonary Venous Hypertension

Pulmonary hypertension is a condition in which the pressure in the pulmonary veins is elevated. It occurs most commonly with left heart failure, as blood backs up into the veins due to inefficient contractions of the heart. Several other types of heart disease can lead to pulmonary venous hypertension as well, including conditions such as mitral stenosis.

Symptoms can include shortness of breath, swelling of the legs, and fatigue. It is diagnosed with a right heart angiogram, which finds an increase in capillary wedge pressure. The primary treatment is to address the underlying cause of the disease.

Pulmonary Vein Thrombosis

Blood clots may form in the pulmonary vein as with other blood vessels but are quite uncommon. When it does occur, it is often related to a malignancy such as lung cancer.

Role in Atrial Fibrillation

The science connecting the pulmonary veins with atrial fibrillation is relatively new. It's thought that the thin layer of myocardial tissue that covers the pulmonary veins can be the focus of atrial fibrillation, with some regions and veins playing a larger role than others. The left superior pulmonary vein is thought to be the focus for roughly 50 percent of cases of atrial fibrillation.

Pulmonary vein isolation is a procedure that is sometimes done to treat atrial fibrillation. In this procedure, scar tissue is created in the left atrium where each of the four pulmonary arteries enters, which can sometimes control the arrhythmia when other treatments such as medications fail. A complication that sometimes occurs with this procedure is pulmonary venous stenosis, which is discussed above.