Anatomy of the pulmonary veins

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

Before delivery, blood flow to the fetus bypasses these blood vessels, which are opened when exposed to oxygen at birth. There may be some anatomical variations, as well as several congenital disorders (birth defects) involving these veins found in some babies. Adults can also develop medical conditions, such as pulmonary venous hypertension.

anatomy

The pulmonary veins together with the pulmonary artery constitute the pulmonary circulation.

developing

Before birth, the fetus receives oxygen and nutrients from the placenta, thereby bypassing the blood vessels leading to the lungs, including the pulmonary artery and pulmonary vein. Only when a baby takes its first breath at birth does blood enter the pulmonary vessels into the lungs.

It’s the exposure to oxygen that closes the shunts that bypass the pulmonary veins and other blood vessels — which relaxes those vessels so blood can enter.

structure

Pulmonary veins are relatively large structures compared to other veins up to 1 cm in diameter, but they tend to be smaller in women. They are composed of three layers of smooth muscle tissue called the coat. The outer layer is the thick adventitia, the middle layer is the thin tunica, and then the middle layer, the inner tunica.

Place

Four pulmonary veins (two in each lung) carry fresh oxygenated blood from the lungs to the left atrium. Blood flows from the left atrium to the left ventricle, where it is ejected, providing oxygen and nutrients to all the tissues of the body.

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

Blood flows from the capillaries into the bronchial veins and then into the pulmonary veins. The two pulmonary veins leave the lungs through an area called the hilum, which is the area where the blood vessels, as well as the left and right main bronchi, enter the lungs at the inner, central area of ​​each lung.

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After leaving the hilum, the right pulmonary vein enters the left atrium via the superior vena cava and posterior to the right atrium. The left pulmonary vein enters the left atrium anteriorly through the descending aorta.

Because the pulmonary veins pass through the lungs and drain into the left side of the heart, they can be affected by any disease that involves these areas.

Anatomical variation

People usually have four pulmonary veins, but 38% have a variant. In some cases, there are five pulmonary veins, and sometimes only three.

The right lung has 3 lobes, the left lung has 2 lobes, the right upper pulmonary vein drains the upper and middle lobes, and the right inferior pulmonary vein drains the lower lobe. On the left, the left superior pulmonary artery drains the left upper lobe and the left inferior pulmonary artery drains the lower lobe.

In some people, the three right pulmonary veins remain separate, rather than merging into two, resulting in a total of five pulmonary veins (this is called a single accessory right middle pulmonary vein and is present in about 10% of people).

The two left pulmonary veins often fuse, resulting in a total of three pulmonary veins.

Many other changes may also occur, such as the presence of two accessory right middle pulmonary veins, one accessory right middle pulmonary vein, and one accessory right upper pulmonary vein.

Function

The pulmonary veins play a very important role in transporting fresh oxygenated blood from the lungs to the heart so it can be transported to the rest of the body.

clinical significance

Pulmonary veins can be affected by medical conditions present at birth or acquired later in life. Because of the central role of the pulmonary veins in the heart and pulmonary circulation, congenital diseases are often associated with other heart defects, while acquired diseases are often associated with other underlying heart conditions.

Congenital Disorders (Birth Defects)

Congenital diseases that affect the pulmonary veins can affect the number of these veins, their diameter, their development, or their connection and drainage (they carry blood from the lungs). These blood vessels can also be affected by the conditions of the area of ​​the heart that they empty.

Pulmonary vein hypoplasia

In some children, the pulmonary veins do not develop fully (hypopplasia). This is often associated with hypoplastic left heart syndrome.

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Pulmonary vein stenosis or atresia

Pulmonary vein stenosis is a condition in which the pulmonary veins thicken, causing them to narrow. This is a rare but serious birth defect that is often combined with other heart abnormalities. Treatments including angioplasty and stenting can open the vein, but it tends to narrow again (restenosis). More recently, doctors have used chemotherapy and biologics after angioplasty to try to prevent the stenosis from recurring.

Partial or total abnormal pulmonary venous return

In this case, one or more pulmonary veins enter the right atrium instead of the left atrium. When only one vein enters the right atrium, the child is usually asymptomatic, but if two or more veins enter the right atrium, surgical correction is usually required. This situation has varying degrees of concern.

Abnormal pulmonary venous return can be a cardiac emergency in neonates, and efforts are underway to diagnose more of these conditions by ultrasonography in the prenatal period. It is one of the causes of cyanotic congenital heart disease (the condition that causes babies to be born blue).

Abnormal pulmonary venous return is often associated with atrial septal defect (ASD), which is always associated with abnormal pulmonary venous return. In this case, oxygenated blood from the lungs mixes with deoxygenated blood in the right atrium. The blood then travels through the ASD (hole in the heart) to the left side of the heart, where it is ejected to the rest of the body.

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

This condition, as well as several types of congenital heart disease, can often be diagnosed by echocardiography.

Pulmonary varicose veins (pulmonary venous aneurysm)

This is a case of dilation of the pulmonary vein area.

heart disease

Tri-atrial is a congenital disorder in which the pulmonary veins enter the left atrium through only one opening. If the opening is large enough, it may be asymptomatic. However, if the opening is small and restricted, it can be repaired surgically.

Pulmonary arteriovenous malformation

This is when there is communication between the pulmonary arteries and the pulmonary veins. It may have no symptoms or cause shortness of breath.

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Get conditions

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

pulmonary vein stenosis

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

pulmonary vein obstruction

In rare cases, such as lung cancer or tuberculosis, the pulmonary veins may become blocked. Worsening shortness of breath in lung cancer patients may be a sign of this complication.

Surgery and Procedural Injuries

Pulmonary veins can also be damaged during surgical procedures. This includes different types of lung cancer surgery. Radiofrequency ablation of arrhythmias may also cause injury.

pulmonary venous hypertension

Pulmonary hypertension is a condition in which pressure in the veins of the lungs increases. It most commonly occurs in left heart failure, because the heart contracts inefficiently and blood returns to the veins. Several other types of heart disease can also cause pulmonary venous hypertension, including conditions such as mitral valve stenosis.

Symptoms may include shortness of breath, swollen legs, and fatigue. It was diagnosed on right cardiovascular angiography, which found increased capillary wedge pressure. Primary treatment is to address the underlying cause of the disease.

pulmonary vein thrombosis

Blood clots can form in the pulmonary veins, like other blood vessels, but are rare. When it does occur, it is often associated with malignant tumors such as lung cancer.

role in atrial fibrillation

The science linking pulmonary veins to atrial fibrillation is relatively new. It is thought that the thin layer of myocardial tissue covering the pulmonary veins may be the focal point of atrial fibrillation, with some areas and veins playing a greater role than others. The left upper pulmonary vein is considered to be the focal point in approximately 50% of cases of atrial fibrillation.

Pulmonary vein isolation is a procedure sometimes used to treat atrial fibrillation. During this process, scar tissue is created in the left atrium where each of the four pulmonary arteries enter, which can sometimes control arrhythmias when other treatments, such as drugs, fail. A sometimes complication of this procedure is pulmonary vein stenosis, as described above.

function of the pulmonary artery