Hi everyone, welcome to byite-size med where we talk about quick bite-sized concepts and basic medical sciences for study and rapid review. This video is on the structure of the vessel walls. Circulation starts at the heart with oxygenated blood being pumped into the aorta from the left ventricle. This blood then goes through arteries which branch into arterials and these arterials form capillary beds where exchange takes place with the tissues. The capillaries then form venules which join together to form veins. They unite with veins coming from other organs to ultimately drain through the superior and inferior vennea into the right atrium. So the arteries take oxygenated blood from the heart to the peripheries. Capillaries are where exchange takes place and veins bring back the deoxxygenated blood to the heart. Now, the walls of these vessels are slightly different depending on the function that they perform. And in this video, we're just going to go over the layers in the walls of each of the vessels. Let's start with the artery. There are three layers and they're called tunics. Tunica inima, tuna media, and tunica externa. Inima, I N. So, it's the innermost towards the lumen. Media is M. So it's in the middle and external is well it's external. It's the outermost. It's also called the tunica adventicia cuz it's got adventicial tissue. The lining epithelium of all the vessels is called the endothelium. So that's the first layer of the inima the endothelium. Below the endothelium is a basement membrane or the basal lamina. And beneath that is the subendothelial connective tissue. subendothelial because it's beneath the endothelium. Now, just before the tunica media, there's a layer of elastic tissue that's called the internal elastic lamina. It adds to the elasticity of the artery. So, the inima has the endothelium, the basement membrane, the subendthelial connective tissue, and then there's the internal elastic lamina. So the next layer is the tunica media, the middle layer, and that's got connective tissue, smooth muscle, and elastic fibers. The arteries that are closer to the heart need to be more elastic cuz they've got to handle that pressure. So they have more elastic tissue and are called elastic arteries. As we move farther away from the heart and more towards the organs, the elastic tissue reduces and the smooth muscle increases. More muscle means these are called muscular arteries. Like how there was an internal elastic lamina between the inima and the media, there's an external elastic lamina between the media and the adventicia. Both the internal and external elastic lamina are seen in larger vessels. And that brings us to the last layer, the tunica externa or adventicia which has connective tissue mostly collagen. Vessels are important for nutrition. Yes, but they need nutrition as well and they get that from vessels that supply vessels and those are called the vasa vasaurum which are mostly in the adventicia. So that's our three layers tunica inima, tunica media and tunica externa. Now, all of that was for arteries. The arteries divide into arterials. Arterials are smaller. They have the same layers, but with some differences. They're thinner. They don't have an elastic lamina. The media has lesser smooth muscle layers, and the advantia is thin. Next up is the capillaries. The capillaries have super thin walls, just the endothelium and the basement membrane. Capillaries could be continuous or fenistrated. Continuous capillaries have tight junctions between the endothelial cells. Substances move by diffusion if they're soluble in lipids like oxygen. But water and so water soluble substances would pass through intercellular clfts. The fenistrated capillaries also have pores. That's why they're called fenistrated. These can be seen in some organs like the kidneys and the small intestine. Another special capillary is called a sinosoid. A classic example for an organ with sinosoids is the liver. They have more and wider intercellular cliffs and fenistrations. They also have a basement membrane that's incomplete. Substances like plasma proteins normally can't pass through capillaries but here they can. Capillaries form venules and the three layers are back again. The inima, the media and the adventicia. Now they've got the endothelium and the media is thinner when compared to the arterials. They have just a few smooth muscle cells and elastic fibers in the media and the externa is thin as well. There's no internal or external elastic lamina. Now both venules and capillaries can have cells called parasites outside the basement membrane. The venules join together to form veins. Now compared to the arteries, the veins have larger lumens. Again, no internal or external elastic lamina. They have a thin media which is more connective tissue and lesser smooth muscle and elastic fibers. But they have a thick externa. The external also can have a few smooth muscles but they also have the vasa vasaura. So veins are more connective tissue. The arteries are more muscular and elastic with high pressure. The veins with lower pressure are thinner walled. They can distend easier and are more collapsible. Now, since veins have a low pressure, they can have valves. So, that keeps blood flowing in one direction. And that was the structure of the blood vessels. If you found this video useful, give it a thumbs up, share, and subscribe. Thanks for watching, and I'll see you on the next one.