This is the first lecture video about the blood vessels, which corresponds to chapter 22 in your textbook. In lecture I'll be talking about the types of blood vessels, about the tunics or the layers of the different blood vessels, and their functions. We'll be looking at arteries that carry blood away from the heart, veins that carry blood back to the heart, and capillaries where exchange can occur between the blood and the body fluids. The systemic circuit extends to all body regions, while the pulmonary circuit consists of the vessels to and from the lungs. Arteries carry blood away from the heart. Arteries become smaller as they branch, and they lead to capillaries, those tiny vessels that are sites for exchange. Veins are the blood vessels that return blood to the heart. They become progressively larger as they merge and approach the heart. An anastomosis is a convergence of two or more vessels. Anastomoses provide alternate supply routes for an organ or a tissue in case one is blocked. Typically, veins have more of these anastomoses than arteries do. End arteries are the one pathway through which blood can reach one organ. It's the only supply of oxygenated blood to a portion of tissues. An example would be the splenic artery that supplies the spleen, and the renal artery that supplies the kidney. Companion vessels are arteries and veins that lie next to each other. They supply the same body region and they're parallel to each other. The walls of arteries and the walls of veins have three different layers; these are called tunics. The tunica intima is the innermost layer, that's composed of endothelium, a simple squamous lining, and a sub-endothelial layer of areolar connective tissue. The tunica media is the middle tunic and it's composed of circularly arranged smooth muscle. The sympathetic activity of your autonomic nervous system causes smooth muscle to contract, resulting in vasoconstriction, and muscle relaxation results in vasodilation, expansion of the lumen of the blood vessels. That tunica media is really thick in arteries and it's much thinner in veins. The tunica externa is an outer connective tissue that helps to anchor the blood vessels to the surroundings. Very large arteries and large veins require vaso vasorum, which is the vessels of the vessels. This is their own network of small blood vessels on their outer layer, just like how the heart has to have its own coronary vessels, really big blood vessels require their own vaso vasorum, which sounds like a fun spell. We'll look at these tunics. Here we have on the left an artery and on the right a vein and then down at the bottom a capillary. For all of the types of blood vessels the tunica intima is the endothelium, that simple squamous lining and a sub-endothelial layer of areolar connective tissue. That's present in arteries and veins, and in capillaries the lining is just an endothelium with a covering basement membrane. The tunica media for arteries and veins is circularly arranged smooth muscle. It's much thicker in arteries than it is in veins. The tunica externa is that outer connective tissue that helps to anchor blood vessels to their surroundings. Some differences that we can see here are that veins have valves that prevent the backflow of blood in veins. Arteries don't require valves because blood traveling in the artery is under much higher pressure, it wouldn't be flowing backwards. The lumen, the open space inside of a vein, is typically much larger than that of an artery of corresponding size. We'll take a look at some of these companion vessels, vessels that are traveling next to each other and serving the same body region, so arteries and veins that supply the same region are companion vessels. Veins are generally larger in diameter, they carry more blood volume, and they have thinner walls in proportion to their lumen. Arteries tend to be smaller in diameter, they have thicker walls in proportion to their lumen, and they carry blood under much higher pressure than veins. Arteries don't have valves, veins have valves. There's three basic types of each, starting at the largest and then moving to the smallest: large veins and small to medium-sized veins have valves every few centimeters along their length, and the smallest veins, venules don't have any valves. For arteries, we'll identify three basic types: elastic arteries, muscular arteries, and arterioles. In general, we see the theme that as an artery's diameter decreases, as we go from smaller to large..errr....or larger to small, then there's a corresponding decrease in the amount of elastic fibers and a relative increase in the amount of smooth muscle. The largest arteries are the elastic arteries. They contain many elastic fibers in all three tunics, and they're especially abundant in that tunica media. Most examples of these would be near the heart, things like the aorta, the pulmonary arteries, and the brachiocephalic arteries. These elastic fibers allow them to stretch when the heart pumps blood into them. These arteries branch into muscular arteries. These are medium-sized arteries and they have elastic fibers only in two concentric rings: the internal elastic lamina and the external elastic lamina. They have a proportionally thicker tunica media. Most of the arteries that we'll name are muscular arteries, things like the brachial artery. Muscular arteries branch into arterioles. These are the smallest arteries, with a very thin adventitia. Um, let's see, and these lead into capillaries. Capillaries are the smallest blood vessels, their diameter is only slightly larger than an erythrocyte. That forces those red blood cells to form that rouleau to slow passage and to improve gas exchange. For most, their wall is just the tunica intima, a single layer of endothelial cells and a basement membrane, and venules are the smallest veins, originating at capillary beds. They resemble capillaries and the smallest lack a tunica media. Medium-sized veins are similar in size to muscular arteries. That tunica media may be thin and they have valves that prevent backflow and then large veins have thicker walls and bigger diameter lumens, and again valves that prevent backflow. We'll look a little bit more at capillary beds. Capillaries are the smallest blood vessels, only slightly larger than an erythrocyte. This is where we get metabolic exchange between the blood and the tissues. These are the functional units of the cardiovascular system. A capillary bed, a group of capillaries, originates from a very small, very small blood vessel called a metarteriole, and then capillaries branch off of the metarteriole. There are several true capillaries that branch from the metarteriole to form the bulk of the capillary bed. The origin of each capillary is a pre-capillary sphincter, that is smooth muscle that controls blood flow into the capillaries. Centrally the metarteriole continues as the thoroughfare channel that then...connects to a post-capillary venule, now carrying blood back toward the heart. These pre-capillary sphincters can contract to prevent blood from entering into the capillaries or they can be relaxed to allow blood to enter into those capillary beds. When an organ needs more oxygen, for instance if you're using your muscles, then you'd expect those pre-capillary sphincters traveling through your muscles to allow for more blood flow into those particular capillaries. These pre-capillary sphincters are really important because if all of the capillaries in the body were to open simultaneously, they would collectively hold every drop of blood in the body, and there would be no more to be traveling through the arteries, arterioles, venules, veins, or the heart itself. So for most capillary beds, the pre-capillary sphincters are closed, and then when the surrounding tissues need oxygen and have excess waste, then those pre-capillary sphincters will open and allowing for blood flow and for exchange to occur before they close once more. If all of the pre-capillary sphincters in a particular capillary bed are closed, then blood would flow directly from the metarteriole through the thoroughfare channel without any exchange occurring and out to that post-capillary venule, bypassing the capillary bed entirely. There are three types of capillaries. The first one is a continuous capillary. In a continuous capillary, the endothelial cells form a complete lining. This is aided by tight junctions. These capillaries are found in conventional tissue types, most common types, things like the muscles and the brain. Here, materials pass through the cell membranes of the endothelial cells via simple diffusion or via pinocytosis, cell drinking. The second type is called the fenestrated capillary. In this type, the endothelial cells contain pores that allow for fluid exchange between the blood and the interstitial fluid. These are areas of high exchange between the nutrients and the fluid. You will find them in places like the small intestine and in the glomerulus of the kidney, anywhere there's a great deal of fluid transport. The last type is called the sinusoid capillary, and these have large gaps between the endothelial cells and a discontinuous or completely absent basement membrane that allows for the transport of large molecules and cells to and from the blood. These would be found in areas of cell entry and exit, for example sites in the red blood cell life cycle, like in the bone marrow, in the liver, and in the spleen. Veins drain capillaries and return blood to the heart. Because the blood has slowed all the way down to pass through those capillaries, now pressure is much lower than it is in arteries. So valves you know, so veins contain valves, to prevent backflow of the blood. At rest veins hold about 60 percent of the body's blood, so they function as blood reservoirs. On the bottom here I have an image of an artery and a vein. The orange is indicating an artery, relatively smaller lumen, relatively muscular thick wall, and then on the left here is a vein, showing this bigger lumen, the thinner tunics, and it sort of lost its shape now that it doesn't have blood traveling through. Because blood pressure in the veins is so low, it requires some assistance to get back to the heart. Some factors that help push that blood along include the valves that are present in veins, and that prevents backflow of blood, but in order to move that blood through the veins, we need something to squeeze on the veins, to exert pressure on the veins to move that blood along. One of these mechanisms is the skeletal muscle pump. The skeletal muscle pump is your skeletal muscles moving, squeezing, contracting on the veins, and that pushes the blood flow through the vein. It exerts pressure on the vein and that blood pressure opens the valve and pushes the blood along. This is one of the reasons that you're supposed to stand up and walk around if you're on a really long plane flight, because inactivity, sitting and not contracting your skeletal muscles for a long time, can lead to blood pooling in the leg veins. Another thing that helps with venous return is the respiratory pump. The respiratory pump decreases pressure in the thorax as the lungs expand at inhalation, and that helps to increase the intra abdominal pressure and move blood up from the abdomen. Then at exhalation, there's increased intrathoracic pressure as the diaphragm relaxes and the thorax is losing size, and that helps to push blood along in the thoracic cavity. So these two factors, the skeletal muscle pump and the respiratory pump, help to overcome the pressure gradient within veins and return blood back to the heart. The varicose veins is a clinical application here. Here on the left I got normal veins, with these valves and blood that's moving one way through the veins. However, if somebody gets varicose veins, these are large, enlarged, swollen, twisting veins. They often appear blue or dark purple, and this happens when the incompetent or faulty valves in the veins allow blood to flow in the wrong direction or to pool. When they're found around the anus, this is hemorrhoids. So these are superficial veins that have become enlarged and twisted and typically they occur just under the skin in the legs. They're more common in women than in men, and they're linked with heredity. Other related factors are pregnancy, obesity, menopause, aging, prolonged standing that allows for blood to pool, leg injury, and abdominal straining. Usually they result in few symptoms, but some people experience fullness or pain in the area. Treatment could be lifestyle changes or medical procedures, with the goal of improving symptoms and appearance. They're very common, affecting about 30 percent of people at some time, and they've been described throughout history and have been treated with surgery since at least AD 400.