Introduction
Definition of pulmonary veins
Pulmonary veins, also known as lung veins, are blood vessels that transfer freshly oxygenated blood from the lungs to the left atria of the heart. This is a key differentiator from other veins, as they typically carry deoxygenated blood back to the heart from the rest of the body. Pulmonary veins are also where atrial fibrillation (A-Fib) begins, making them the target of A-Fib treatment.1
Importance in circulatory system
The pulmonary veins are a critical part of the circulatory system. However, to understand the importance of pulmonary veins, we must dive into the circulatory system itself. The circulatory system serves to carry oxygen, nutrients, and hormones to cells while removing waste products, such as carbon dioxide. To keep things running smoothly and maintain functionality it functions in one direction.
The circulatory system is essential to maintaining the balance and coordination of many physiological processes. Without the circulatory system, our bodies cannot survive.2
Anatomy of pulmonary veins
Location within the body
Pulmonary veins are located between the lungs and the heart. Within each lung, left and right, several smaller blood vessels combine to form a pair of pulmonary veins. Each pair leaves its respective lung through the hilum or root.1
The hilum of the lung is a depressed surface at the very centre of the surface of the lung. Many structures use it as an entry and exit to the lungs.3 Pulmonary veins are one of the structures as, from the hilum, they travel to the heart and connect with the left atrium at the top left chamber of the heart.1
Number and arrangement
Most of us have four pulmonary veins with two connected to each lung. Typically, each pulmonary vein is connected directly to the left atrium. The arrangement of the pulmonary veins is described in the bullet points below:
- The left inferior pulmonary vein drains the left lung’s lower lobe
- The left superior pulmonary vein drains the left lung’s upper lobe and the lingula
- The superior pulmonary vein drains the right lung’s upper and middle lobes
- The inferior pulmonary vein drains the right lung’s lower lobe1
Structure and composition
Delving deeper into the anatomy of pulmonary veins, let's examine their structure and composition. These veins have three wall layers: tunica adventitia, tunica media, and tunica intima.
Wall layers
Tunica adventitia
Tunica adventitia, also known as tunica externa, is the outer layer of the blood vessel. It comprises connective tissue with Vasa and Nervi Vasorum and is critical to vascular health. This is the strongest wall layer, composed of collagenous and elastic fibres. Additionally, it acts as a barrier that protects the vessel from overexpansion.4
Tunica media
Tunica media is the middle layer of blood vessel walls and is primarily comprised of thin, cylindrical, smooth muscle cells and elastic tissue. The arrangement of the smooth muscle cells is in circular layers around the vessel, and the thickness of the coat is according to the size of the vessel. Since arteries are actively vasoconstricting or vasodilating, the tunica media is much thicker in arteries than in veins. This difference allows the body to regulate blood flow into specific areas and blood pressure.5
Tunica intima
The tunica intima layer is closest to the lumen. The lumen is the inside of a tubular structure (like an artery or intestine). The intima is the thinnest layer of the vascular wall, consisting of a sheet of endothelial cells that rest on a basement membrane and a thin subendothelial extracellular matrix (ECM) composed of collagen and elastin. It is important to note that the tunica intima is sometimes mistaken as the tunica “interna”. Tunica “intima” is the correct term because it is found in the inner layer of blood vessels. Its primary function is to provide structural support to the larger blood vessels.6
Valves in pulmonary veins
Pulmonary veins do not have valves, however, the right-hand pulmonary veins are usually thicker than the ones on the left. They enter the left atrium near the atrial septum and the left pulmonary veins flow into the side wall.1
Function of pulmonary veins
Role in gas exchange
The function of the circulatory system is to take in oxygen and expel carbon dioxide. As mentioned before, oxygenated blood travels from the lungs through the pulmonary veins, into the left side of the heart. Carbon dioxide-rich blood returns to the right side of the heat and then blood is pumped through the pulmonary artery to the lungs, picking up oxygen and releasing carbon dioxide.7
Relationship with pulmonary arteries
Pulmonary veins and pulmonary arteries have a complementary relationship. For instance, they travel in different directions. Pulmonary veins carry blood from the lungs to the heart while the pulmonary arteries carry blood from the heart to the lungs. They are the core vessels in pulmonary circulation which means they hold the serious responsibility of carrying oxygenated blood to the heart from the lungs and deoxygenated blood from the heart to the lungs.1
Pulmonary veins in disease
There are some diseases connected to the pulmonary veins. Pulmonary veno-occlusive disease (PVOD) is caused by blockage of the blood vessels that carry oxygen-rich blood from the lungs to the heart.8 This is caused by the buildup of abnormal fibrous tissue in the small veins of the lungs. As a result, the vessels narrow and impair blood flow, causing pressure to rise in the blood vessels carrying deoxygenated blood from the heart to the lungs.
This can cause issues including the restriction of delivering oxygenated blood to the rest of the body resulting in shortness of breath, tiredness, dizziness, lethargy, and a cough that doesn’t go away. Signs of the conditioning becoming more serious include the development of cyanosis, chest pains, fainting, and pulmonary oedema.8
Summary
- Pulmonary veins are blood vessels that transfer freshly oxygenated blood from the lung to the left atria of the heart
- The circulatory system serves to carry oxygen, nutrients, and hormones to cells while removing waste products, such as carbon dioxide
- Pulmonary veins consist of three wall layers: tunica adventitia, tunica media, and tunica intima
- The function of the circulatory system is to take in oxygen and expel carbon dioxide
- Pulmonary veins and pulmonary arteries have a complementary relationship
References
- Gao Y, Raj JU. Role of veins in regulation of pulmonary circulation. Am J Physiol Lung Cell Mol Physiol. 2005; 288(2):L213-226.
- Khan YS, Lynch DT. Histology, Lung. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024. Available from: http://www.ncbi.nlm.nih.gov/books/NBK534789/.
- Suresh K, Shimoda LA. Lung Circulation. Compr Physiol. 2016; 6(2):897–943.
- Witter K, Tonar Z, Schöpper H. How many Layers has the Adventitia? – Structure of the Arterial Tunica Externa Revisited. Anat Histol Embryol [Internet]. 2017; 46(2):110–20. Available from: https://onlinelibrary.wiley.com/doi/10.1111/ahe.12239.
- Melnikova NB, Svitenkov AI, Hose DR, Hoekstra AG. A cell-based mechanical model of coronary artery tunica media. J R Soc Interface. 2017; 14(132):20170028.
- Milutinović A, Šuput D, Zorc-Pleskovič R. Pathogenesis of atherosclerosis in the tunica intima, media, and adventitia of coronary arteries: An updated review. Bosn J Basic Med Sci [Internet]. 2020; 20(1):21–30. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7029210/.
- Petersson J, Glenny RW. Gas exchange and ventilation-perfusion relationships in the lung. Eur Respir J. 2014; 44(4):1023–41.
- Montani D, Lau EM, Dorfmüller P, Girerd B, Jaïs X, Savale L, et al. Pulmonary veno-occlusive disease. Eur Respir J. 2016; 47(5):1518–34.