Blood Vessels: What is the Difference between Arteries and Veins?


Arteries and veins are similar, but they are also different. They both carry blood and are part of the circulatory system. However, there is more to them than that simple fact. Each one of them performs a specific function that is essential to keep our bodies working properly. Even the health issues that arise when there is a problem in any of them are unique. But before you can understand what makes them different, it is imperative to learn what they are.

Arteries and Veins: What are Them?

The arteries and the veins are part of the circulatory system and have similar structures. However, there are a few key differences between them.

Arteries are blood vessels responsible for taking oxygenated blood from the heart and transfer it to the different tissues found in the body. There are a few exceptions to this though. The first one is the pulmonary artery which carries deoxygenated blood from the heart to the lungs. The other one is the umbilical artery which is responsible for taking deoxygenated blood from the fetus and delivering it to the placenta.

On the other hand, veins are blood vessels that transport deoxygenated blood from the body towards the heart.

Similarly, there are two types which are the exceptions to the rule: pulmonary veins and umbilical veins.

Pulmonary veins deliver oxygen-rich blood from the lungs to the heart, and umbilical veins carry oxygen-rich blood from the placenta to the fetus.[1]

Structures

- Arteries

Arteries are comprised of four parts: the tunica externa, the tunica media, the tunica intima and the lumen. The outer layer [2] is the tunica externa which is composed of collagen fibers. The tunica media, which is the middle layer, consists of elastic tissues and muscle cells. Endothelial cells are the main components of the inner layer, the tunica intima. Lumen is the name of the cavity where the blood flows.

- Veins

Veins have the same layers; however, they differ quite a bit. They have the same three primary layers and the hollow cavity where blood flows. But the thickness and the composition of the layers vary, and veins also have valves to avoid backflow.

The Role They Play

Arteries can be divided into two types: systemic arteries and pulmonary arteries. Systemic arteries are responsible for taking oxygen-rich blood that is being pumped by the heart and deliver it to the cells in the body. When the heart pumps blood, it goes directly into the pulmonary artery which supplies blood to the lungs where it gets oxygenated.

Once filled with oxygen, the blood is pushed towards the rest of organs with tremendous force. The pressure inside arteries is higher than the one found in veins and varies according to the heart's rhythm. When the heart contracts, the pressure increases, and when it relaxes, the pressure drops.[3]

Since this pressure is high, arteries need to be thick and flexible to withstand it. The larger arteries, the ones with a diameter greater than 10 mm, need to be even more elastic than the smaller ones to adjust to the heart's pressure.

Artery representation

Smaller arteries (less than 10 mm in diameter) have more muscular walls to contract and expand depending on the blood flow allowing a much better control of the amount of blood received by the different tissues of the body. When blood vessels narrow, the pressure inside increases. When they expand, pressure is reduced.

Arterioles, which are more numerous and smaller than arteries, are under a much lower pressure and branch off from the larger arteries to deliver blood to the capillaries.

Capillaries are the smallest blood vessels found in the body. They can be found in almost every tissue and connect the arterioles with the venules. It is in here where nutrients and waste are exchanged between the blood and the cells.


In the following video, you can see the structures of the blood vessels and how they work:



Now, veins are considered to be the counterpart of arteries.[4] They can be divided into two main types: systemic and pulmonary.

Systemic veins are large blood vessels taking oxygen-depleted blood back from the body to the heart. The pulmonary vein carries oxygenated blood from the lungs to the heart which pumps it to the aorta and the rest of the body.

Veins are closer to the skin and may appear blue due to the reflected blue wavelength as the lower energy frequencies of the light spectrum are absorbed by the fat found under the skin. Unlike arteries, veins are under low pressure resulting in interior walls that aren't as elastic or muscular.

Of course, the blood still needs some pressure to get back to the heart, but it is gravity, inertia, and contractions of the skeletal muscles that give blood that extra push necessary for it to make it back. Furthermore, most of the veins contain valves to prevent reverse blood flow. These valves are essential. Whenever skeletal muscles expand, blood is trapped by the valves until a contraction of the very same muscles occurs to move it closer to the heart.

Venules, which are the veins' version of arterioles, are smaller veins connecting to the capillaries. They pick the deoxygenated blood from the capillaries and transport it to the larger veins which in turn take it back to the heart.

The Distinctions Between Them

Arteries Veins
Function Blood vessels carrying blood away from the heart Blood vessels carrying blood to the heart
Pressure High Low
Blood flow direction Begins at the heart and ends at the different tissues of the body Begins at the different tissues of the body and ends at the heart
Location Deep inside the body Close to the skin
Blood constitution Abundance of oxygen in blood (except the pulmonary and umbilical arteries) Low amount of oxygen in blood (except the pulmonary and umbilical veins)
Structure of the blood vessel Thin outer layer, thick middle layer (mostly smooth muscle and elastic fibers in large arteries), thick inner elastic membrane, and small lumen Thick outer layer (plenty of elastic fiber in large veins), thin middle layer, very thin inner layer (mostly smooth muscle), and large lumen
Valves No valves Valves to prevent reverse blood flow are present (most of them in limbs)
Appearance Round with thick walls Irregular with thin walls
Common problems Atherosclerosis, hypertension, blood squirt, and aneurysm Venous insufficiency, deep vein thrombosis, and portal hypertension


What Conditions Affect Arteries and Veins?

Like every other part of the human body, blood vessels may also experience conditions that can affect their performance. Some of them are cosmetic, but others are life-threatening and need immediate medical intervention.

- Atherosclerosis

Arteries may get hard and lose flexibility over time. Debris containing lipids such as cholesterol, fatty acids, calcium, and other fibrous materials can accumulate in their walls forming a plaque that may obstruct partially or entirely the blood flow. This condition is known as atherosclerosis, and it is very dangerous.

- Hypertension

The contractions of the heart generate systemic arterial blood pressures that should be around 120/80 mmHg, which is the accepted standard for healthy individuals. If your blood pressure is consistently above that number, you might have high blood pressure also commonly known as hypertension. Hypertension may be life-threatening and could lead to heart attacks or strokes if left untreated for a long time.

- Blood Squirt

When an artery is cut due to high blood pressure, a blood squirt occurs. Blood loss may be profuse and can be fatal.

- Aneurysm

When the tissues of an artery weaken, an enlargement of the area may occur resulting in an aneurysm. This condition may or may not be life-threatening depending on the location. If it occurs in the aorta (the main artery of the heart) or a brain's artery, it may require surgical intervention. According to the CDC, it was responsible for over 9000 deaths in the United States in 2014.[5]

- Venous Insufficiency

If the blood in your veins is having problems to reach the heart, you may have venous insufficiency. In this condition, the legs' valves are malfunctioning causing the blood flowing from the limbs to pool in the legs. Most of the time, they manifest as varicose veins.[6] Although varicose veins aren't serious, it is always a good idea to check with your doctor to rule out a blood clot.

- Deep Vein Thrombosis

Blood clots that appear in deep veins may cause a condition called deep vein thrombosis. Swelling, pain and skin rash are a few of the symptoms associated with the condition. In most cases, it isn't dangerous. However, if it manages to get to the lungs, pulmonary embolism might develop.

- Portal Hypertension

Another problem that can appear in veins is portal hypertension. The pressure inside the portal veins (which conduct blood from the intestines, pancreas, spleen and gallbladder to the liver) may increase as a result of cirrhosis, blood clots or other conditions. Swollen veins in the esophagus, stomach, rectum and umbilical area may appear and could be life-threatening if they rupture.

Both arteries and veins are essential for the body to work properly. If any of them is having problems, you can be certain you will know. Visit your doctor as soon as you noticed anything out of the ordinary even if it is just minor swelling in your leg; you never know when a routine check can save your life.




Sources:

1. http://www.innerbody.com/image/cardov.html
2. http://inside.ucumberlands.edu/academics/biology/faculty/kuss
   /courses/CirculatorySystem/ArteriesVeins.htm
3. http://www2.highlands.edu/academics/divisions/scipe/biology/faculty/harnden
   /2122/notes/cvbv.htm
4. http://www.diffen.com/difference/Arteries_vs_Veins
5. https://www.cdc.gov/dhdsp/data_statistics/fact_sheets/fs_aortic_aneurysm.htm
6. https://www.newsmax.com/FastFeatures/blood-pressure-
   varicose-veins/2016/07/14/id/738744/










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