we have covered the three main blood vessels which are the artery arterials and capillaries in the previous video and in this particular video we are going to be looking at venules and vein well not exactly venues because all we just have to know about venules are they're just smaller versions of veins but in terms of structure and function they're pretty much the same the venules drain the blood into the vein and the vein just has to transport the blood back to the heart that's about it uh before we look at the structure of the veins in detail I want to talk a little bit about the capillaries okay now if you remember the capillaries are just one cell thick and they also have a very narrow Lumen the Lumen is so narrow which is seven micrometers by the way uh the Lumen is so small that it can only fit one the blood cell the red blood cells can only travel in a single line so in that case when the red blood cells are moving uh they will be moving quite slowly because they want to allow exchange to happen but then they will drain into venules now as you can see the capillaries have a smaller volume of blood but when it goes into the venules the volume of blood becomes larger so when the volume of blood becomes larger the wall has to be slightly thicker than the capillaries to accommodate that slightly larger volume of blood because if the wall of the venules we mean in like the capillaries the wall might burst because it cannot withstand the our mass or the pressure that the higher volume of blood is exerting against the wall hence why the venue will actually need to have a um thicker wall so and then from the venues it drains into the vein now in terms of vein structure three-dimensional view you can see that it has a thin wall but the wall is not as thin as capillaries and they also have a relatively large Lumen okay now notice the when we are looking at the longitudinal view the longitudinal view of the vein starts off with the innermost layer and the innermost layer is made up of the one Celtic endothelium and it's the same thing as like in the artery it's to minimize the friction of the blood flow and I'm also labeling the large Lumen like that now the second layer is the middle layer which has the smooth muscles elastic fiber and collagen fibers same as the arteries the only difference is the middle layer is quite thin and they also have a thin outer layer made up of elastic fiber and collagen fiber immediately you go hey wait a second this is the same as the arteries because the artery has the inner layer of endothelium a thin middle layer of smooth muscle elastic fiber and collagen fiber anything outer layer of elastic fiber and collagen fiber the vein is similar the only difference is the middle layer and the outer layer for the veins are quite thin compared to the arteries because they don't need to withstand a very high pressure so they don't have to worry about bursting all right now but they still need to have the middle and outer layer to accommodate the large volume of blood flowing through the blood vessel and of course the cross section when we're when I'm following the cross section notice that the shape are just drawing on the endothelium and the middle layer and also the outer layer it's going to label it the cross section of the vein shows that it has another irregular shape compared to the arteries uh because the arteries have a rather circular cross-section but veins have a more irregular shaped cross-section that is how you would recognize a vein from an artery a vein usually is quite irregular in its shape when it comes to its cross section whereas arteries are circular simple as that now there is a problem I want to talk about when we are covering the vein so as you can see just drawing a simple diagram here and artery capillary and vein now I know they are supposed to be arterials and venules in the middle but I just want to simplify this diagram now as a reminder the pressure in the artery is at its highest and as the pressure goes further away it becomes lower in the capillary and when it enters the vein the pressure is at its lowest now it poses a problem for the vein because why is pressure important pressure is important because it is the force that pushes the blood forward okay when there is a higher pressure the blood gushes through the blood vessels quickly but when the pressure is very low it just trickles through okay with a lot of difficulty what's the problem if you remember the function of the V is to transport blood back to the heart I'm drawing out a leg over here and in this leg uh in this leg you can actually see the veins now are the blood is going into the smaller veins and from the smaller veins it's going upwards into a larger vein now remember pressure in the vein is low so as the pressure is low and it's pushing the blood upwards the blood will have difficulty moving upwards and there is another problem the other problem is there is an opposing force acting against the blood flow because when the blood is going upwards there's a force pushing it downwards and that force that is pushing it downwards is gravity so why is this a problem well because as the blood is slowly trying to go upwards with its low pressure gravity pushes it the opposite way and this causes the blood to be stuck Midway and this is not good because it stops the blood flow in the vein so this is a huge problem so what is the solution how did we evolve to solve this problem this problem can be solved with two ways number one it can be solved with these skeletal muscles now please be I just want to tell you that skeletal muscles are not smooth muscles these muscles are just like you know your leg muscles that causes your legs to move or your feet to move and this muscles can be surrounding the veins not just the veins in your legs but the veins in your hand uh torso and neck and head okay now so these muscles surround the vein and the veins also have something called valves okay let's look at them in detail so how do valves and skeletal muscles solve the problem and drawing a simple vein and the pink color highlight in the vein is for example blood okay and as you can see the blood is just stuck Midway like that now next to the vein there is a skeletal muscle now when the skeletal muscle contracts especially when you move your feet or you move your leg the muscles squeezes the muscle becomes shorter and it becomes thicker when it becomes thicker it presses against the vein and it squeezes the vein now why is this good when it squeezes the vein is because when the vein is squeezed it pushes the blood upwards which is a good thing now the skeletal muscle will then relax but when the skeletal muscle relaxes the vein is no longer squeezed and the blood may move downwards now do we want that to happen we don't want that to happen do we Okay because that's not a good thing okay right so what's the solution to prevent the backflow now go back to the left diagram as you can see I'm drawing out those greenish structures in the vein I hope you can see that over there the diagram is a bit clear that's the valve right there which I've labeled okay and the valve in that case is closed when the skeletal muscle contracts the valve will open and the blood flows through the valve and when the skeletal muscle relaxes the valve closes again and when the blood wants to rush back downwards it's not able to rush back to its original position because the valve is preventing it from doing so so the when the valve prevents backflow this ensures that blood in the vein is able to fight against the force of gravity that is how it's able to solve the problem of the low blood pressure within the veins in the longitudinal view as a reminder it has the thin layer and outer layer but I'm also adding in its large loop I'm also adding in this semilunar valves the semilunar valves are formed from the endothelium and the function of the semolina valves in the vein is to prevent the backflow of blood that's basically it so why is backflow dangerous this is just an extra bit of information you don't need to know this for the exam but if the blood if the backflow of blood happens it will cause the blood to slow down and stop and when blood slows down and stop the wet blood cells will tend to congeal and group up together and when they group together they will form something known as blood clots and this can be quite dangerous because this is a kind of semi-solid mass now blood clots generally when they're formed your body can destroy them by itself but some blood clots become a bit too big to be spontaneously or automatically destroyed so your body has difficulty breaking them down and large blood clots can travel inside your body and when they move into blood vessels again as you can see here the blood the blood clot is moving to the through the artery and it has no problems but when it's stuck in the arterials any blood flow to the adjacent capillaries will be stopped and any cells which are supposed to receive the blood in the capillaries will die because there is no longer any proper blood flow in that area so blood clots can block blood vessels which will lead to body cells dying and this can be quite a serious thing especially if it happens in the heart it may lead to a heart attack and if it happens in the brain it may lead to a stroke because the brain cells will die that is why valves and the skeletal muscle confection in the vein are extremely important because they ensure that the blood is constantly flowing in One Direction and back flow does not happen