Veins: Study with Video Lessons, Practice Problems & Examples

Veins are blood vessels that carry deoxygenated blood towards the heart, featuring thinner walls and larger lumens compared to arteries. They contain three tunics: tunica intima, tunica media, and tunica externa, with the tunica externa being the thickest. Unlike arteries, veins have venous valves to prevent backflow due to lower blood pressure. Most blood volume is stored in veins, making them crucial for circulation. Understanding these differences enhances comprehension of the cardiovascular system's structure and function.

Introduction to Veins

In this video, we're going to do just a little bit more of a formal introduction to veins. Recall from our previous lesson videos when we overviewed the major types of blood vessels that we already defined veins as blood vessels that carry blood towards the heart. And so you can think that the v in veins is for the v in visiting the heart. Now, structurally, the walls of veins have all 3 tunics or all three layers that we talked about in our previous lesson videos, including the tunica intima, the tunica media, and the tunica externa. However, these 3 tunics in veins tend to be thinner than the same 3 tunics in arteries. And in general, veins have thinner walls than the walls of arteries, which are thicker, and the reason for this is because veins have lower blood pressure than arteries, which have higher blood pressure. Since the arteries are carrying blood away from the heart, that generates forceful contractions that can create relatively high blood pressure surges. And again, the arteries need to have thicker walls in order to withstand those relatively high blood pressures. Whereas, the veins, which are further away from those forceful contractions of the heart, are going to have relatively low blood pressure, and so the thinner walls are perfectly fine for veins. But because they have a thinner tunica media, which recall has smooth muscle, this means that veins have less smooth muscle and less ability to vasoconstrict and vasodilate in comparison to arteries. And, also, veins tend to have significantly larger lumens in comparison to arteries, which have smaller lumens, which means that if you had a foot long vein and a foot long artery, the foot long vein would be able to store larger volumes of blood in comparison to the artery. And at any given time, most of the blood in our body is going to be stored in veins. Now, it's actually the tunica externa that tends to be the thickest tunic in veins. Now, again, in veins, they have lower blood pressure, so the average blood pressure is lower in veins in comparison to arteries. And so because this is the case, in some veins, particularly the veins that are in our limbs, such as our arms and legs, they have what are known as venous valves, and these venous valves, they prevent the backflow of blood in this low pressure system to ensure that blood is always flowing in the correct direction, which is towards the heart in these veins. Now, recall from our previous lesson videos that venules are going to be relatively small veins, And these venules will collect blood from the capillaries, and they will converge to form larger veins and start to take the blood back towards the heart. Now these venules, they can vary in terms of the number of tunics that they have. They can have, the smallest venules will have just 1 tunic, and larger venules can have up to 3 tunics. But let's take a look at our image down below where we can start to visualize some of these things. Notice on the left hand side, we're showing you the artery, which we already covered in previous lesson videos. We know that it carries blood away from the heart. In this case, it's coming downwards. And then over here what we have is an arteriole, which we'll call is a small artery that we know can feed blood into capillary beds, which is what we're showing you here. The capillary beds is where the exchanges occur between the blood and the surrounding tissues, and then the blood is fed into these venules, which we know are small veins. The smallest venule will be the postcapillary venule that we talked about in previous lesson videos. Now these veins, we know, are going to be carrying blood back towards the heart, and so notice that the direction of blood flow is opposite to that of the arteries. And what you'll notice is that you can see many of the same structures when you closely compare the structures of the artery to the vein. But a few things that you should notice is that the tunics, in general, tend to be thinner in veins in comparison to arteries. Also notice that the lumens of the veins are larger than the lumens of the arteries, allowing them to hold and store more blood. And you'll notice that the tunica media, this layer of smooth muscle here, is thinner in comparison to the tunica media of the arteries, which gives them decreased ability to vasoconstrict and vasodilate. And you'll also notice that it's missing some elastic structures, such as the internal elastic membrane and the external elastic membrane. These Swiss cheese-looking structures essentially are missing from the veins, and that's because, again, veins don't need to have the same elasticity as arteries because the veins have a lower pressure system than the arteries, which have a higher pressure system and need that ability to

Veins Example 1

So here we have an example problem that asks which of the following statements about veins and venules is true? And we've got these 4 potential answer options down below. Now option a says that there are significantly more venules than veins in the body. Now recall that venules are small veins, and these venules will converge or merge in order to form a vein. And so by default, that means that there's going to be more venules than veins. And so option a is looking like a true statement. Let's keep going here. Option b says, venules have a smaller diameter than veins. Now, again, we know that venules are small veins. So, certainly, they have a smaller diameter than veins, which are larger. And so, option b is also looking true, and then option c says that venules always converge into veins. And again, we know that venules are small veins that will merge or converge to form the larger veins, and so c is also true. And so because a, b, and c are all true, we can indicate that option d, all of the above are true, is the best answer to this problem. So d is correct. That concludes this example, and I'll see you all in our next video.

A major difference between venules and arterioles is that:

• A. The smallest arterioles consist of only one layer (tunic), while the smallest venules have all 3 layers (tunics).
• B. Some arterioles make direct contact with capillaries, but no venules do.
• C. Arterioles deliver blood to capillaries while venules collect blood from capillaries.
• D. Arterioles collect blood from capillaries while venules deliver blood to capillaries.

Arteries vs. Veins

In this video, we're going to complete the comparison table down below that is showing the key differences between arteries and veins. Notice that the left-hand side of the table is all about arteries, whereas the right-hand side of the table is all about veins. Now when it comes to arteries and veins, the direction of blood flow with respect to the heart is critically important. Arteries will always carry blood away from the heart, whereas veins will always carry blood towards the heart. Recall that you can think that the "a" in artery stands for the "a" in away from the heart, and the "v" in vein for the "v" in visiting the heart. Arteries almost always carry oxygenated blood or blood that has relatively high concentrations of oxygen. The only notable exception to this is the pulmonary artery, which carries deoxygenated blood away from the heart and toward the lungs so that it can be reoxygenated. Veins, on the other hand, almost always carry deoxygenated blood, or blood that has relatively low concentrations of oxygen and higher concentrations of carbon dioxide gas. The only notable exception to this is the pulmonary vein, which carries oxygenated blood towards the heart from the lungs.

Because arteries usually carry oxygenated blood, the blood that they carry is a brighter red color, since oxygenated blood is brighter red. Veins, on the other hand, because they usually carry deoxygenated blood, their blood is darker red in color or purple, due to deoxygenated blood having a darker red or purple color. Because arteries carry blood away from the heart and the heart generates very forceful contractions that eject blood out with relatively high pressure, arteries need to have thick walls to withstand that relatively high pressure. They also need to contain significantly higher proportions of both muscle and elastin. Recall that the muscle facilitates the change in diameter via vasoconstriction and vasodilation. Arteries have a greater ability to vasoconstrict and vasodilate. The elastin imparts more elasticity, giving arteries the ability to stretch and also recoil back to its original shape.

Veins, on the other hand, are further away from the forceful contractions of the heart that eject blood out at high pressure. Because of this, their walls are thinner than those of arteries and contain significantly less muscle and elastin, meaning they have less ability to vasoconstrict, vasodilate, and be elastic. Veins lack internal and external elastic lamina, which, as shown in the image on the left-hand side, are highlighted yellow structures looking somewhat like Swiss cheese, made of elastic connective tissue, giving arteries quite a bit of elasticity.

Arteries typically have a more narrow lumen, meaning the size of their lumen is smaller compared to veins, which have significantly larger lumens. This allows veins to hold larger volumes of blood. If you had a foot-long artery and a foot-long vein, the foot-long vein would be able to hold more volume of blood because of the larger lumen. At any given time, most of the blood in our body is found in veins due to their larger lumens. Because arteries have relatively high pressures from the forceful contractions of the heart that ejects blood out with relatively high pressures, these arteries do not need to have valves. The high blood pressure keeps the blood moving in the right direction away from the heart. On the other hand, veins, having relatively lower pressure, may have venous valves in them to prevent the backflow of blood and ensure that blood is always moving towards the heart. These venous valves are especially found in the limbs, such as our arms and legs, where the blood needs to fight against gravity even more so than other areas of our body. You can see the venous valve on the right, which allows blood to flow in the correct direction towards the heart but prevents backflow due to the relatively low pressure.

Last but not least, the idea of pressure again highlights that arteries, which are closer to the forceful contractions of the heart that eject blood with high pressures, are part of a relatively higher pressure system. They have higher blood pressure because they receive blood directly from the heart. On the other hand, veins have relatively lower blood pressure because they are further away from the forceful contractions of the heart that eject blood out, and these veins are delivering the blood back to the heart. These are the key differences between arteries and veins. Moving forward, we'll be able to apply some of these concepts in practice problems. I'll see you all in our next video.

Veins Example 2

So here we have an example problem that wants us to choose one of the 4 potential answer options down below that best fills in the blanks in the sentence. Notice the sentence says that arteries need to have thicker walls than veins to prevent collapse under high pressure. Now, notice that for the first blank, the answer options are going to be either thinner or thicker. Recall from previous lesson videos that arteries have thicker walls than veins because arteries are closer to the forceful contractions of the heart that eject blood out of the heart with relatively high pressure. And so this first blank is going to be thicker, which means that we can eliminate any answer option that suggests it will be thinner, such as answer options a and b. Now, for this next blank, the options are going to be either backflow or collapse. Recall that arteries have relatively high blood pressure and the relatively high blood pressure keeps the blood flowing in the correct direction, which is away from the heart in these arteries. Really, because of the high blood pressure, there's really no concern for the backflow of blood in arteries. So, the thicker walls really have nothing to do with the backflow of blood in the arteries. So answer option c is not going to be correct. Instead, these thicker walls have to do with the structural integrity of the arteries under this high-pressure system. And so, that means that these thicker walls will help to maintain the structural integrity, including helping to prevent the collapse of the artery under high pressure. The correct answer here is going to be collapse. We can indicate that answer option d is the correct answer to this problem. And that concludes this example problem, so I'll see you all in our next video.

Which of the following is not a reason that veins need valves to prevent backflow, but arteries do not?

• A. Arteries have thicker walls than veins, providing a natural resistance to backflow.
• B. Veins require valves to prevent gravity from causing backflow when blood is transported up to the heart.
• C. Arteries have higher blood pressure, reducing the likelihood of backflow when they transport blood upward.
• D. All of the above are correct reasons for veins requiring valves.

Which of the following statements regarding arteries and veins is true?

• A. Arteries have less smooth muscle than veins.
• B. Arteries always carry oxygenated blood, while veins always carry deoxygenated blood.
• C. At any given time, there is more blood in the veins than in the arteries.
• D. There tends to be a lower blood pressure in arteries than in veins.

Here’s what students ask on this topic:

What are the main structural differences between veins and arteries?

Veins and arteries differ significantly in their structure. Veins have thinner walls and larger lumens compared to arteries. This is because veins operate under lower blood pressure, whereas arteries handle higher pressure from the heart's forceful contractions. Veins contain three tunics: tunica intima, tunica media, and tunica externa, with the tunica externa being the thickest. In contrast, arteries have a thicker tunica media, which contains more smooth muscle and elastin, allowing for greater vasoconstriction and vasodilation. Additionally, veins have venous valves to prevent backflow of blood, a feature not found in arteries.

Why do veins have valves and arteries do not?

Veins have valves to prevent the backflow of blood, which is crucial due to the lower blood pressure in veins. These valves ensure that blood flows in one direction towards the heart, especially in the limbs where blood must travel against gravity. Arteries, on the other hand, do not need valves because the high pressure generated by the heart's contractions keeps the blood moving in the correct direction. The high pressure in arteries ensures continuous forward flow, making valves unnecessary.

How do the tunics of veins differ from those of arteries?

Both veins and arteries have three tunics: tunica intima, tunica media, and tunica externa. However, in veins, these tunics are generally thinner. The tunica media in veins has less smooth muscle and elastin, reducing their ability to vasoconstrict and vasodilate compared to arteries. The tunica externa is the thickest layer in veins, providing structural support. In contrast, arteries have a thicker tunica media, which allows for greater elasticity and strength to withstand higher blood pressures. Arteries also have internal and external elastic lamina, which are absent in veins.

What role do veins play in blood storage?

Veins play a crucial role in blood storage due to their larger lumens, which can hold more blood compared to arteries. At any given time, most of the blood in the body is stored in veins. This capacity to store large volumes of blood helps maintain blood pressure and ensures a steady supply of blood to the heart. The large lumens and thinner walls of veins make them well-suited for this storage function, contributing to the overall efficiency of the circulatory system.

Why do veins have thinner walls compared to arteries?

Veins have thinner walls compared to arteries because they operate under lower blood pressure. The heart's forceful contractions generate high pressure in arteries, necessitating thicker walls to withstand this pressure. In contrast, veins are further from the heart and experience much lower pressure. Therefore, thinner walls are sufficient for veins. Additionally, the thinner tunica media in veins means they have less smooth muscle and elastin, reducing their ability to vasoconstrict and vasodilate, which is less critical given their lower pressure environment.