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
Table of contents
- 1. Introduction to Anatomy & Physiology5h 40m
- What is Anatomy & Physiology?20m
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- 25. The Urinary System2h 39m
- 26. Fluid and Electrolyte Balance, Acid Base Balance Coming soon
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19. The Blood Vessels
Veins
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