Why are the pulmonary vein and artery not like the rest of circulatory system?

I'm learning anatomy. What I learnt is that we have arteries that have oxygenated blood which appears red in color, and branch blood to arterioles to deliver blood to cells via capillaries from where de-oxygeneated blood is collected via venules into veins to go back for oxygenation. So:

artery = oxygenated

vein = de-oxygenated

However, in defining the pulmonary vein and pulmonary artery, I see the reverse of these definitions.

The pulmonary artery is for de-oxygenated blood.

Why is it so?

It is true that nearly all arteries carry oxygenated blood and nearly all veins carry de-oxygenated blood, but that is not what defines them. Arteries are blood vessels that carry blood away from the heart, and veins carry blood towards the heart. If you look at the situation in that light, the naming makes sense: the pulmonary artery is carrying de-oxygenated blood away from the heart to the lungs, and the pulmonary vein is carrying the re-oxygenated blood back to the heart, to be pumped through the rest of the body.


Nervous control

The pulmonary circulation is supplied with both sympathetic and parasympathetic innervation. In general, increased sympathetic activity leads to release of catecholamines (e.g., dopamine, norepinephrine, epinephrine, and neuropeptide Y) that cause vasoconstriction and an increase in pulmonary vascular resistance. Pulmonary arteries contain fewer cholinergic than adrenergic nerve fibers. Parasympathetic stimulation causes the release of acetylcholine and vasoactive intestinal polypeptide, which mediate vascular dilation and a decrease in pulmonary vascular resistance. The lung also contains nonadrenergic, noncholinergic nerves that can be excitatory (e-NANC) or inhibitory (i-NANC). Release of vasoactive intestinal peptide, calcitonin gene-related peptide, substance P, and nitric oxide from i-NANC nerves mediates vasodilation, while the e-NANC nerves mediate vasoconstriction, although the neurotransmitter involved remains unclear.

Curiously, in contrast to the systemic vasculature, there appears to be minimal nervous control in the pulmonary circulation with respect to basal vascular caliber. Moreover, while the existence and activity of e-NANC and i-NANC nerves have been demonstrated in vitro, regulation of tone in vivo by these nerves has not been demonstrated. However, stimulation of adrenergic nerves may modulate pulmonary vascular resistance and blood flow during exercise and cold exposure and may increase in regulatory contribution during pathological states, particularly during pulmonary edema and embolism.

Pulmonary Circulation

Regulation of Pulmonary Vasomotor Tone

The pulmonary circulation differs from the systemic in that it is under minimal resting tone and is almost fully dilated under normal conditions ( Hughes and Morrell, 2001 ). Circulating and local production of vasodilators and vasoconstrictors contribute to the resting tone, with the balance tipped in favor of vasodilators. Nitric oxide, produced locally by endothelial cells, and the arachidonic acid metabolite, prostacyclin, are important vasodilators that contribute to the low pulmonary vascular tone. The autonomic nervous system interacts with humoral mediators and hemodynamic forces in the control of pulmonary vascular tone. Autonomic innervation of the lung is via parasympathetic (cholinergic: predominantly vasodilator) and sympathetic (adrenergic: predominantly vasoconstrictor) nerves in the periarterial plexus.

What are the Other Arteries

Other arteries are the blood vessels that carry oxygenated blood away from the heart. Generally, arteries are made up of a thick wall and they do not contain valves as found in veins. Also, the blood pressure inside arteries is higher than inside the veins. The arterial wall is made up of three layers: tunica externa, tunica media, and the tunica intima. The tunica externa is made up of connective tissue, which prevents the overexpansion of blood vessels. The tunica media is made up of smooth muscles. On the other hand, the tunica intima is made up of elastic fibers and an endothelium.

Figure 2: Arterial System

The main artery which leaves the heart is the aorta. It starts from the left ventricle of the heart and gradually branches off into several arteries, which supply blood to different organs in the body. Some branches are the brachiocephalic artery, coronary artery, etc. These small arteries further branch into arterioles and blood capillaries.

Arteries except for pulmonary artery and veins make up the systemic circulatory system in animals. The oxygenated blood travels from the left ventricle to the body through these arteries and veins collect deoxygenated blood from the body and drain into the right atrium.

Tests for Pulmonary Vascular Disease

Based on a person's symptoms, signs, and history, a doctor may begin to suspect the presence of pulmonary vascular disease. The diagnosis of pulmonary vascular disease is usually made using one or more of the following tests:

Computed tomography (CT scan): A CT scanner takes multiple X-rays, and a computer constructs detailed images of the lungs and chest. CT scanning can usually detect a pulmonary embolism in a pulmonary artery. CT scans can also uncover problems affecting the lungs themselves.

Ventilation/perfusion scan (V/Q scan): This nuclear medicine test takes images of how well the lungs fill with air. Those images are compared to pictures of how well blood flows through the pulmonary blood vessels. Unmatched areas may suggest a pulmonary embolism (blood clot) is present.

Echocardiography (echocardiogram): An ultrasound video of the beating heart. Congestive heart failure, heart valve disease, and other conditions contributing to pulmonary vascular disease can be discovered with echocardiogram.

Right heart catheterization: A pressure sensor is inserted through a needle into a vein in the neck or groin. A doctor advances the sensor through the veins, into the right heart, then into the pulmonary artery. Right heart catheterization is the best test to diagnose pulmonary arterial hypertension.

Chest X-ray film: A simple chest X-ray can't diagnose pulmonary vascular disease. However, it may identify contributing lung disease, or show enlarged pulmonary arteries that suggest pulmonary arterial hypertension.

Pulmonary angiography (angiogram): Contrast dye is injected into the blood, and X-ray images of the chest show detailed images of the pulmonary arterial system. Angiography is very good at diagnosing pulmonary embolism but is rarely performed anymore because CT scans are easier, less invasive, and have lower risk.

4. The Pulmonary Loop Only Transports Blood Between the Heart and Lungs

In the pulmonary loop, deoxygenated blood exits the right ventricle of the heart and passes through the pulmonary trunk. The pulmonary trunk splits into the right and left pulmonary arteries. These arteries transport the deoxygenated blood to arterioles and capillary beds in the lungs. There, carbon dioxide is released and oxygen is absorbed. Oxygenated blood then passes from the capillary beds through venules into the pulmonary veins. The pulmonary veins transport it to the left atrium of the heart. The pulmonary arteries are the only arteries that carry deoxygenated blood, and the pulmonary veins are the only veins that carry oxygenated blood.

How is a pulmonary embolism treated?

Treatment choices for pulmonary embolism (PE) include:

Anticoagulants. Also described as blood thinners, these medicines decrease the ability of the blood to clot. This helps stop a clot from getting bigger and keep new clots from forming. Examples include warfarin and heparin.

Fibrinolytic therapy. Also called clot busters, these medicines are given intravenously (IV or into a vein) to break down the clot. These medicines are only used in life-threatening situations.

Vena cava filter. A small metal device placed in the vena cava (the large blood vessel that returns blood from the body to the heart) may be used to keep clots from traveling to the lungs. These filters are generally used when you can't get anticoagulation treatment (for medical reasons), develop more clots even with anticoagulation treatment, or when you have bleeding problems from anticoagulation medicines.

Pulmonary embolectomy. Rarely used, this is surgery done to remove a PE. It is generally done only in severe cases when your PE is very large, you can't get anticoagulation and/or thrombolytic therapy due to other medical problems or you haven't responded well to those treatments, or your condition is unstable.

Percutaneous thrombectomy. A long, thin, hollow tube (catheter) can be threaded through the blood vessel to the site of the embolism guided by X-ray. Once the catheter is in place, it's used to break up the embolism, pull it out, or dissolve it using thrombolytic medicine.

An important aspect of treating a PE is preventive treatment to prevent formation of additional embolisms.

What is the Vascular Resistance: Systemic, Pulmonary and Peripheral

Vascular resistance describes the degree to which the blood vessels of the cardiovascular system – the arteries, capillaries and veins – affect the blood flow of the various organs of the body.

The main characteristics that determine the amount of resistance are the diameter, the length of the vessels and the viscosity or thickness of the blood. Of these three factors, the diameter of the vessel is the most significant.

Vasoconstriction, which is the constriction or narrowing of the diameter of the blood vessels, increases vascular resistance in the same way that the tap nozzle restricts and increases the pressure of water flowing through a tube or hose.

There are three types of vascular resistance: systemic vascular resistance, pulmonary vascular resistance and peripheral pulmonary resistance.

Systemic Vascular Resistance

It is the resistance that blood “sees” as it travels through the body’s circulatory system. It is controlled by three different factors: blood vessel length (l), blood vessel radius (r), and blood viscosity (η). The equation that relates these three factors to the resistance is known as the Poiseuilles equation:
R ≈ (η x L) / R4

The systemic vascular resistance is influenced by the length and radius of the blood vessels, as well as the viscosity of the blood. It is calculated from mean arterial pressure, central venous pressure, and cardiac output.

Systemic vascular resistance plays a vital role in maintaining blood pressure within the established ranges so that organ perfusion is maximized.

Pulmonary Vascular Resistance

It occurs when the pulmonary artery creates resistance against the blood that flows in it from the right ventricle. Resistance is, naturally, created by the disposition of the blood vessels of the lungs and is healthy at low levels.

The problem of pulmonary vascular resistance is created when there is an increase in the amount or viscosity of blood flow to the pulmonary artery and therefore an increase in resistance.

The heart, when healthy, functions as a system for pumping and filtering blood. The deoxygenated blood used enters the right atrium of the inferior and superior vena cava and flows into the right ventricle.

The right ventricle contracts and pumps blood into the pulmonary artery, which carries blood to the heart for filtration and oxygenation. This new blood flows into the left atrium and then into the left ventricle, forcing blood to flow into the aorta and the rest of the body.

There is a certain natural resistance to the blood flow created by gravity, particularly when the veins and arteries flow upward at a vertical angle. The contractions of the ventricles of the heart normally provide enough force for enough blood to flow despite the resistance.

Pulmonary vascular resistance is a particular type of resistance created by the vasculature, or the arrangement of blood vessels in the lungs. The heart faces this resistance in the pulmonary artery, where blood enters the lungs for filtration.

The most common cause of pulmonary vascular resistance are circulatory problems. Changes in blood viscosity, which may be caused by a change in the hematocrit, will affect the level of resistance in the pulmonary vessels.

Another factor that affects resistance is arterioles, which can expand and contract to a limited degree in order to increase or reduce blood flow.

When communication between the left and right sides of the heart is interrupted, usually due to circulatory problems, blood will flow to the area of ​​least resistance. This often results in an increase in blood flow to the pulmonary artery.

The increase in blood flow creates an increase in pulmonary vascular resistance. If left untreated, the increased resistance can cause permanent damage to the blood vessels of the lungs.

Pulmonary vascular resistance is quite difficult to detect, since it involves the internal functioning of the heart and lung cavities. Scientists are working on non-invasive methods to detect this disorder.

One of these methods that has been subjected to limited testing is echocardiographic evaluation. This method is effective in detecting low levels of vascular resistance, but is not as effective when dealing with higher levels.

Total Peripheral Resistance

Total peripheral resistance (RPT) is the amount of resistance to the flow of blood present in the body’s vascular system. It can be considered as the amount of work force against the heart, since it expels blood in the vascular system.

Although total peripheral resistance plays an integral role in determining blood pressure, it is a measure defined exclusively by the cardiovascular system and should not be confused with the pressure against arterial walls, which is a measure of blood pressure.

The vascular system, which is responsible for the flow of blood to and from the heart, can be divided into two components: systemic and pulmonary.

The pulmonary system carries blood to and from the lungs, where it is oxygenated, and the systemic vasculature is responsible for transporting this blood to the cells of the body through the arteries and returning blood to the heart after perfusion.

The total peripheral resistance is calculated by using a specific equation. This equation is RPT = change in the pressure / cardiac output.

Change in pressure is the difference in mean arterial pressure and venous pressure. The mean arterial pressure is equal to the diastolic blood pressure, more than a third of the difference between the systolic and diastolic pressure.

What causes pulmonary hypertension?

Some common underlying causes of pulmonary hypertension include high blood pressure in the lungs&rsquo arteries due to some types of congenital heart disease, connective tissue disease, coronary artery disease, high blood pressure, liver disease (cirrhosis), blood clots to the lungs, and chronic lung diseases like emphysema. Genetics also play a role.

Pulmonary hypertension can happen in association with many other diseases, such as lung disease and heart disease. Heart failure is common in pulmonary hypertension.

What’s the Difference Between Veins and Arteries?

Veins and arteries are major players in the circulatory system of all vertebrates. They work together to transport blood throughout the body, helping to oxygenate and remove waste from every cell with each heartbeat. Arteries carry oxygenated blood from the heart, while veins carry oxygen-depleted blood back to the heart. An easy mnemonic is "A for ‘artery’ and ‘away’ (from the heart)." (The exceptions to this general rule are the pulmonary vessels. The pulmonary veins transport oxygenated blood back to the heart from the lungs, while the pulmonary arteries move deoxygenated blood from the heart to the lungs.)

As the vessels that are closest to the heart, arteries must contend with intense physical pressure from the blood moving forcibly through them. They pulse with each heartbeat (which is why your pulse is taken from an artery) and have thicker walls. Veins experience much less pressure but must contend with the forces of gravity to get blood from the extremities back to the heart. Many veins, especially those in the legs, have valves to prevent the backflow and pooling of blood. Although veins are often depicted as blue in medical diagrams and sometimes appear blue through pale skin, they are not actually blue in color. Light interacts with skin and deoxygenated blood, which is a darker shade of red, to reflect a blue tone. Veins seen during surgery or in cadavers look nearly identical to arteries.

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