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 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 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 and veins tend to be thinner than the same 3 tunics and 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 be elastic. And then, last but not least, notice that we're labeling 1 of the venous valves here in the structure of the vein. And the venous valves allow blood to flow in one direction, but they prevent the backflow of blood. So even though it's a low pressure system, it ensures that blood is always going to be continuously flowing toward the heart and that there's never backflow of blood. And, again, the tunica externa in veins is generally going to be the thickest of the tunics in veins. And so this here concludes our introduction to veins, And moving forward, we'll be able to do more of a direct comparison between arteries and veins. So I'll see you all in our next video.
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example
Veins Example 1
Video duration:
1m
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Video transcript
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.
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Problem
Problem
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.
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concept
Arteries vs. Veins
Video duration:
7m
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Video transcript
In this video, we're going to complete the comparison table down below that is showing the key differences between arteries and veins. And so 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. And so arteries will always carry blood away from the heart, whereas veins will always carry blood towards the heart. And so recall that you can think that the a in artery is for the a in away from the heart, and you can think that the v in vein is for the v in visiting the heart. Now arteries almost always carry oxygenated blood or blood that has relatively high concentrations of oxygen, And really the only notable exception to this is the pulmonary artery, which carries deoxygenated blood away from the heart and towards the lungs so that it can be reoxygenated. And 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. And really the only notable exception to this is going to be the pulmonary vein, which carries oxygenated blood towards the heart from the lungs. Now, because arteries usually carry oxygenated blood, usually the blood that they carry is going to be a brighter red color since oxygenated blood is brighter red. And veins on the other hand, because they usually carry deoxygenated blood, usually their blood is going to be darker red in color or purple, and this is because deoxygenated blood has a darker red or purple color to it. Now because arteries carry blood away from the heart and the heart generates very forceful contractions that ejects blood out of it with relatively high pressure, the arteries need to have thick walls in order to withstand that relatively high pressure in the arteries. And so, they also need to contain significantly higher proportions of both muscle and elastin. And so recall that the muscle is going to allow for the change in the diameter via vasoconstriction and vasodilation, so arteries have a greater ability to vasoconstrict and vasodilate. And recall that the elastin gives these arteries more elasticity, or the ability to stretch and also recoil back to its original shape. Now veins on the other hand, they are further away from the forceful contractions of the heart that eject blood out with high pressure. And because that's the case, they are perfectly fine with thin walls, and so their walls are thinner than the walls of arteries. And they contain significantly less muscle, so they have less ability to vasoconstrict and vasodilate, and they also have significantly less elastin. And that means that they will not have the same ability to, be elastic, stretch and then recoil back to its original shape. Now recall that these arteries may have internal and external elastic lamina, which if you take a look at the image over here on the left hand side, the internal and external elastic lamina are these yellow structures that you can see highlighted here, and they kind of look a little bit like Swiss cheese, but they're made of elastic connective tissue, which gives these which can give these arteries quite a bit of elasticity, again, the ability to stretch and recoil back to its original shape. Now, veins on the other hand, they always lack these internal and external elastic lamina. And so notice over here on the right hand side, those, yellow Swiss cheese looking structures are not found in these veins. Now arteries typically have a, more narrow lumen and so, the size of their lumen is smaller, with respect to, veins, which have significantly, larger, lumens. And so, you can see here that the lumen is larger, and this allows veins to be able to hold larger volumes of blood. And so 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. And at any given time, most of the blood in our body is found in veins because they have those larger lumens. Now because arteries have relatively high pressures from the forceful contractions of the heart that ejects blood out with relatively high pressures, the high pressure means that these arteries don't really need to have valves because the high blood pressure keeps the blood moving in the right direction away from the heart. But on the other hand, veins, because they have relatively lower pressure, they may have what are called venous valves in them, in order to prevent the backflow of blood and ensure that blood is always going to be moving towards the heart. And so these venous valves are especially going to be found in the limbs, such as our arms and legs, which, the blood in those limbs needs to be able to fight against gravity even more so than, other areas of our body. And, what you'll notice is that over here on the right, you can see the venous valve, which will allow blood to flow through it, in the correct direction towards the heart, but it will not allow for the backflow of blood. So blood will not be able to go back despite having this relatively low pressure. And, last but not least here is just this idea of the pressure. Again, arteries which are closer to the forceful contractions of the heart that are, ejecting relatively high blood pressure, or ejecting blood with high pressures, they are going to be relatively higher, pressure system. They have higher blood pressure, and again, this is because they receive blood directly from the heart. And on the other hand, veins are going to have relatively lower blood pressure, and this is because they are, further away from the, forceful contractions of the heart that are ejecting blood out, and, these veins are delivering the blood back to the heart. And so, really, these are the key differences between arteries and veins, and, moving forward, we'll be able to apply some of these concepts and practice problems. So I'll see you all in our next video.
5
example
Veins Example 2
Video duration:
2m
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Video transcript
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. And notice the sentence says that arteries need to have blank walls then veins to prevent blank under high pressure. Now, notice that for this first blank, the answer options are going to be either thinner or thicker. And so 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. Now, 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. And really, because of the high blood pressure, there's really no concern for the backflow of blood in arteries. And so, the thicker walls really have nothing to do with the back flow 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. And so the correct answer here is going to be, collapse. And so 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.
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Problem
Problem
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.
7
Problem
Problem
Which of the following statements regarding arteries and veins is true?