Pulmonary Vein Definition
The pulmonary vein (s) are blood vessels that carry oxygen-rich blood from the lungs towards the left atrium of the heart, which is required for regular breathing. Each lung includes two major pulmonary veins, totaling four major vessels. Because of the critical nature of these veins for breathing, blockages and problems in them may rapidly escalate and lead to more catastrophic illnesses, including pulmonary hypertension and heart failure.
The Circulatory System
Let’s begin with an overview of the system and vessels distributed around the body before moving on to the cardiac and pulmonary systems’ specialised vessels.
All vertebrates (as well as certain invertebrate species) possess a closed circulatory system in which blood flows in a continuous loop throughout the body through a network of channels. The heart facilitates blood travel through the circulatory system by pumping blood at a sufficiently high pressure to maintain a constant flow rate. Arteries, capillaries, and veins are the primary vessels of a closed circulatory system.
Vessels
Arteries are blood channels with strong walls that transport blood away from the heart. In addition to muscle fibres, they are largely made up of elastic fibres. Smooth muscle constricts the arteries, narrowing them and delaying blood flow. On the other hand, the elastic fibres allow the arteries to revert to their resting state, enabling the diameter and blood flow to increase once again. Arterioles are a kind of artery that is very small. These arterioles are essential for reducing blood pressure as blood prepares to pass through capillaries.
Capillaries are extremely tiny capillaries that transport gases and small chemicals from the bloodstream to the tissues. The walls of these capillaries are just a single cell layer thick, indicating that they are exceedingly thin. The capillary diameter is also just large enough for one red blood cell to pass through at a time. The term “capillary bed” refers to a large network of capillaries.After tissues have utilised the oxygen as well as other molecules received, blood is brought back to the heart via venules and veins.
Veins are blood vessels with a thin wall that transport blood to the heart. Blood pressure is often lower at this phase, enabling veins to develop greater diameters and thinner walls than arteries. In most cases, skeletal muscle in the limbs aids blood flow in veins. Large veins, on the other hand, need valves to divide the vessels into different segments and prevent blood from backflowing. Venules are very tiny veins.
Sub-Circulatory Systems
The circulatory system is composed of two components: the pulmonary circuit and the systemic circuit. The pulmonary circuit distributes blood from the heart to the lungs (or, in aquatic species, to the gills), while the systemic circuit transports blood from the heart to the rest of the body. Because gravity plays a little part in maintaining blood flow in fish species, blood pressure is always elevated throughout the body. Gravity, on the other hand, is more important in terrestrial species. While blood travels upwards towards higher parts of the body, blood pressure must rise to fight gravity.
As a result, the separation of the two circulatory subsystems keeps the entire system balanced. Typically, the systemic circuit is high-pressured while the pulmonary circuit is low-pressured. Pulmonary blood pressure, for instance, is only around one-eighth of that of systemic blood pressure.
The Heart
Chambers of the Heart
The heart of all vertebrates is split into at least two chambers, with each species having a single atrium and a single ventricle. The atria (singular atrium) collect blood from the body, whereas the thick-walled ventricles circulate it. The number of atria and ventricles varies by species, with fish possessing one of each, amphibians and many reptiles possessing two atria and one ventricle, and all other reptiles, birds, and mammals (as well as humans) have two atria and two ventricles.
In animals without four completely formed heart chambers, only a segment of the pulmonary and systemic circuits is divided. As a result, blood that is oxygen-rich and blood that is oxygen-poor may mingle. Among animals with four fully developed chambers, such as humans, the two subcircuits are totally divided. This prevents the mixing of oxygen-rich and oxygen-depleted blood, which increases cellular efficiency by allowing tissues to acquire more oxygen. Birds and mammals are classified as endotherms because they need more accessible oxygen to sustain higher body temperatures.
Blood Flow Through the Heart
In animals with four completely formed chambers, such as humans, the overall flow of blood through the heart is as follows:
- Blood that is low in oxygen enters the heart through the superior venae cavae and inferior venae cavae, which flow into the right atrium (both of which are veins).
- The oxygen-depleted blood subsequently goes into the right ventricle via the tricuspid valve.
- The right ventricle pushes poor oxygenated blood through the pulmonary valve and into the pulmonary arteries.
- The right and left pulmonary arteries supply the right and left lungs with oxygen-rich blood, respectively. Gas exchange occurs when blood travels through the capillary beds of the alveoli in the lungs.
- Through the pulmonary veins, oxygen-rich blood returns from the lungs to the heart through the left atrium, where it overflows. Each lung has two enormous pulmonary veins, all of which go to the left atrium.
- The mitral (or bicuspid) valve permits the passage of oxygenated blood into the left ventricle.
- The left ventricle sends oxygen-rich blood into the aorta by pushing it past the aortic valve.
- The oxygen-rich blood subsequently leaves the heart and travels via the body’s arteries.
The pulmonary circulation is handled in stages 1-3, whereas the systemic circulation is handled in steps 5-8. Despite the fact that both arteries include the word “pulmonary” in their names, the pulmonary circulation is comprised only of the pulmonary artery. The pulmonary vein is related to the systemic circulation of the body.
Location of the Pulmonary Veins
Blood must flow effectively via the pulmonary and systemic systems, which necessitates the use of many vessels. As such, it is vital to understand the location of the pulmonary veins in relation to the rest of the body. The anterior (topmost) part of the heart contains all four pulmonary veins.
The right pulmonary veins enter the left atrium from beneath the right atrium and superior vena cava, stemming straight from the root of the right lung. The left pulmonary veins, on the other hand, originate directly from the base of the left lung and pass straight into the left atrium in front of the descending aorta.
Diseases and Dysfunctions
Pulmonary Vein Stenosis
Pulmonary vein stenosis is a very uncommon condition in which one or more of the four pulmonary veins become occluded. This barrier prevents oxygen-rich blood from the lungs from accessing the left atrium of the heart. This may happen when the vein walls thicken, causing the capillaries themselves to constrict. Patients might have their veins widened by surgery.
However, since this is a recurrent issue in patients, this remedy is only temporary. If left untreated, this syndrome may develop into severe lung disorders, including pulmonary hypertension and pulmonary arterial hypertension. Despite its rarity, clinical research is underway to learn more about this disorder and potential remedies.
Pulmonary Vein Thrombosis
Pulmonary vein thrombosis is another uncommon condition that may affect the pulmonary veins. Thrombosis, or the growth of thrombi, or blood clots, inside vessels, may reduce a blood vessel’s accessible diameter, reducing blood flow. When a thrombus is removed from its source, it produces thromboembolism, a kind of embolism (or intravascular mass). These masses may cause pulmonary hypertension, blood clotting, and even death.
Antecedent Heart Diseases
Blood pressure may also be raised by prior cardiac disorders such as mitral stenosis. Because the mitral valve separates the atrium from the ventricle on the left side of the heart, pressure may build up in the left atrium. Increased pressure in the pulmonary veins flowing into the atrium might result from this rise in pressure. This may cause pulmonary hypertension and pulmonary arterial hypertension, just as it did previously. The pathogenesis of mitral stenosis has been compared to that of pulmonary vein thrombosis.
Resulting Conditions
Pulmonary hypertension is a condition characterised by high pulmonary pressure, as indicated in the preceding two illnesses. The pulmonary pressure rises to a quarter or more of the systemic pressure. (As a reminder, pulmonary pressure should be less than 1/8 of systemic pressure.) This disorder is divided into numerous subgroups dependent on the aetiology of the malfunction.
Pulmonary hypertension, on the other hand, usually occurs when the arteries around the heart constrict (or if vascular blood flow increases). The heart has to work harder to push blood through these restricted vessels. This weakens the heart over time, increasing the odds of having heart failure. Pulmonary hypertension may be caused by a variety of factors, including pulmonary vein stenosis and thrombosis. Pulmonary arterial hypertension is a kind of pulmonary hypertension that has its origins in the tiny arteries of the lungs.
Conclusion
Pulmonary veins are important vessels in the pulmonary system that guarantee that appropriate breathing is distributed throughout the body. These veins deliver oxygen-rich blood from the lungs to the heart’s left atrium, where it may be distributed to all the following tissues. Select species have evolved multi-chambered hearts, allowing full separation of the pulmonary and systemic circuits, preventing the mixing of oxygen-rich and oxygen-poor blood. Though pulmonary vein abnormalities are uncommon, they may lead to more serious illnesses and diseases that can rapidly become lethal. Some of these disorders are still being researched in order to better understand them and enhance treatment options.
References
Freeman, S., Quillin, K., Allison, L. A., Black, M., Podgorski, G., Taylor, E., & Carmichael, J. (2017). “Biological Science (Sixth edition).” Boston: Pearson Learning.
Kato H, Fu YY, Zhu J, et al. (2014). Pulmonary vein stenosis and the pathophysiology of “upstream” pulmonary veins. 148 (1), pp. 245–253.
Kumar, V., Abbas, A., and Aster, J. (2013). “Robbins Basic Pathology (Ninth Edition).” Philadelphia, Elsevier, Inc.
