- [Lecturer] When we start thinking about the heart and blood vessels we have to think of, so we start to look at it you can see that the blood leaves the heart, this major vessel, the aorta, but heads on down to smaller and smaller and smaller arteries. Now these arteries can be compared to veins on the other side. The blue ones are veins. The veins are pretty big in the heart, but get smaller and smaller as you go away from the heart. Right where the arteries and veins meet are a capillary bed. Now you can see at the very bottom here, capillaries are usually only a single layer thick. Very thin, very permeable. This allows the gases oxygen and carbon dioxide to flow back and forth. It allows the nutrients to get into the tissues. But before they get to the capillaries, if you go back up that arterial side, back up that red side a little bit, you get what's called an arteriole. A little thicker than the capillary because we have some extra smooth muscle around it. It's meant to give a little support and help out a little bit with blood pressure. But if you keep going back up towards the heart, on the red arterial side, you'll notice you still have the inner layer, the endothelium. You'll have a thicker smooth muscle layer now and it'll have an outer connective tissue to really give support, strength, and structure. But when you compare those to the other side, the full fledged artery compares to a full fledged vein. But look at the inside of them. The vein is much larger. Still has the same basic three layers, but a much larger inside or much larger lumen. What's happening here is the larger lumen allows for a lower resistance. Helps the blood to get back to the heart easier. venule matches up with arteriole. Again venules are larger, easier for blood flow. Reason being is as you go from your heart all the way down toward the capillaries, your blood pressure steadily declines, declines, declines. Well, down in the tissues there's no way of bringing the pressure back up again. So you have to have a low pressure or a low resistance system to get blood back up to your heart. So one thing we say about all arteries that carry blood away from the heart. They're under higher pressure. You cannot say all arteries are red, or all arteries carry oxygen rich blood. Those don't hold up. So arteries carry blood away from the heart and under a higher pressure system. Capillaries, they're meant to exchange. Water, solutes like salts, gases, are all going back and forth. The bi tissues to the capillaries. Veins, their simple task is returning blood to the heart. They'll handle larger openings, so larger lumens. They're meant to bring blood back from the tissues back towards the heart. Let's go back to these arteries for a second now. We already saw they have three different layers. There's this inner layer, middle, and outer. Well the inner layer is endothelium. That's epithelial tissue. It's meant to allow for a very smooth surface. That way the blood can't stick to it. Middle, smooth muscle. That's giving you the strength to create your blood pressure, assuming you create and maintain blood pressure. While the outer layer is connective tissue, which this is meant to give a little bit of support and structure to the artery. So we said arteries carry blood away from the heart. Simple as that. They're pressurized, and they're typically or mostly oxygen rich. But the key thing is, there's a few arteries in your body which are oxygen poor. So it's not always true to say they're oxygen rich. Arterioles, well they're smaller arteries. The same basic structure, same basic idea, just they're smaller because they're further away from the heart. We have these precapillary sphincters. The idea here is they're gonna be at the end of the arterioles. Now a sphincter is just a musculature ring. It's a ring of muscle that can open and close to either allow more blood through, or less blood through, in this case. And precapillary means before the capillaries. So it's helping to control the blood flow into the capillaries. We could have one of two things occur. We can have vasodilation, or vasoconstriction. Well vaso refers to vessel, in this case, the arterioles, the capillaries. Dilation means opening up, you're dilating something, you're opening up while constriction means closing down, getting smaller. So vasodilation, you open the vessel larger, means more blood flow. Vasoconstriction, well you're making the vessel smaller, you're gonna decrease or cut down on blood flow. So vasodilation, vasoconstriction. In this image here you can see how on the top it's arteriole. Those round discs, they're going around the opening to the capillaries. Those are your precapillary sphincters. If they're opened, they're vasodilated. You can see how blood is flowing through with the capillaries, no problem. Heading down to the venule. But if those precapillary sphincters are closed or constricted, as the two on the right, you'll notice there's really not any blood flowing through. So this is how you can regulate how much blood is passing through the capillaries. Either constrict or relax the precapillary sphincters. Well if we open up the precapillary sphincters, obviously we get blood flowing into the capillaries. These capillaries are the smallest of all blood vessels. One cell layer thick, very thin walled, they're porous. You can have different gases diffusing, different nutrients diffusing, different ions diffusing. There's a lot of different particles going back and forth between the blood in the capillaries and the fluid around the capillaries. So typically we refer to these as capillary beds and there's a whole network, a bunch of different capillaries all woven together. The idea here is by having this whole network or web of capillaries, you've increased the surface area. There's more tissue that can get nutrients now from these capillaries. The main rule, as I said, is selective exchange of substances, allowing certain small particles, gases, liquid, into and out of the capillaries. Can see the diagram here, oxygen, nutrients, are heading into the tissues while the waste products, as well as the metabolic waste, carbon dioxide, is leaving the tissue. Let's start our return trip back up to the heart. The veins, which are larger structures, and the venules, which are smaller, looking at a three layered, thin-walled structure. So you still have the same basic structure as an artery, but now it's much thinner. But even though the walls are thinner, the lumen is much larger. Lumen is the open space the blood travels through. By having a larger lumen, there's less resistance, less pressure. That's why arteries are the pressurized system. Veins are very low pressure. But these veins can also stretch. They also referred to as high distensibility. High distensibility allows the veins to stretch to accommodate blood flow. So what are they doing? They're carrying blood towards the heart. That's the main rule. Carry blood back to the heart. But they also serve as a blood reservoir because veins will contain the majority of blood in your body. More often than not, they have over 60% of your body's blood within your veins. That means the heart's blood, it means the arteries' blood, it means the capillaries' blood. All three of those together do not have as much blood in them as your veins would. So how do you get the venous return to the heart? I mean you don't have a pump. The heart's the pump. Once it goes to the capillaries, they're very low pressure. So your body has different mechanisms to help assist in the return of blood to the heart. We have contraction of skeletal muscle, so for instance look at the picture here of the back leg, calf muscle. You have a major vein right in the middle of that muscle. You can see other valves inside that vein. They're one way valves. Blood can go up toward the body, but cannot come back. So when your muscles contract, they shorten, then get wider or thicker. This widening or thickening ends up pinching the vein. When you pinch a tube the fluid has to go somewhere. Well in this case the fluid cannot go back down. There's a valve preventing it. That means the fluid must go up towards the body, going through that valve, which prevents it from coming back down. So that pinching right there, pushes the blood up through the valve. The valve below that area, or towards the heel, prevents blood from traveling down to the foot. So these one-way valves are pretty important. By using the skeletal muscle contraction, and the one way valves, it helps move the blood up. Now maybe you heard if you're in a long car ride or on an airplane you're supposed to move your legs, get up and move around? This is why. By bouncing your legs or walking around, you're using these muscles to help bring the blood back up from your lower extremities. Moving the blood up through your leg, back up towards your body and your heart. And then your lungs. Whenever you take a breath in, shallow breath, deep breath, doesn't matter. Whenever you breathe in, you're changing the pressure in your thoracic cavity. So pretty much your whole chest is a sealed cavity. When you breathe, you change that pressure. That pressure change allows the blood to kind of sucked up or pulled up a little bit whenever it changes. Kind of think of a drinking straw. How did they get the liquid up the straw? It's a pressure change. You create a lower pressure by sucking on the straw, pulling the liquid up. So there are three different ways that your body helps return blood back to the heart.