Welcome to the second lesson of the first module which is about blood vessels and coronary arteries. My name is Patrick Siegrist, I work at the University Hospital of Zurich in the Department of Cardiology as an invasive cardiologist. First, I want to talk about the function of blood vessels. And to know how this function is achieved, we also need to talk about the types of blood vessels, the architecture of the blood vessels and the histology. And once we understand that, we will also understand the physiology which is behind it. First to the function of the blood vessels which is to carry the blood, which then delivers oxygen, nutrition, hormones, and other cells to the organs. At the same time, they carry away waste products such as carbon dioxide and cellular waste. To make this work in a circuit, we have different types of blood vessels: First, after the heart comes the aorta, which then separates into arteries, which bring the blood to our organs. After the arteries comes the arterioles and then they spread out into millions of capillaries where the exchange of delivery and waste product actually happens. After the capillaries, the blood flows into the venules and then into the veins, which brings the blood back to the heart. All of our vessels basically are built in three layers. And if we have a close look at those layers, we see in the innermost part the tunica intima, which consists of endothelium, and it's then separated from the next layer by the internal elastic membrane. The next layer is the tunica media which mainly consists of smooth muscle cells. Then follows the external elastic membrane which separates the tunica media from the tunica adventitia. If you have a closer look at this, you see that the tunica intima is built in the innermost part of endothelial cells, followed by a subendothelial layer of connective tissue and then followed by the internal elastic Lamina, which separates the tunica intima from the tunica media. The tunica media mainly consists of smooth muscle cells in a spiral arrangement. It also contains elastic collagen fibers, connective tissue and it is separated from the next layer by the external elastic lamina. The next layer is the tunica adventitia, the outermost part responsible to connect the vessels to the organs. It mostly consists of elastic and collagenous fibers. And larger vessels also contain dedicated blood vessels, nerves and lymphatics. Now we saw that the blood vessel contain a lot of muscle cells. And therefore, we can understand that they are not just rigid pipes such as pipes in our buildings, but actually moving organs which can adjust to the needs of our body, of our organs, by narrowing and widening up. There are different blood vessels also concerning the location where they are within our coronary circuit. We always have the three layers, but they are built slightly differently: First, just after the heart we have the elastic arteries, such as for example, the aorta. The elastic arteries are followed by the muscular arteries and then by the arterioles and the capillaries. The elastic arteries are closest to the heart, such as the aorta, for example. They contain a lot of collagen and elastic filaments which give them the ability to stretch. This is very important because they can store about 50% of the stroke volume which is ejected by the heart in every systole. In diastole, these arteries can recoil and guarantee a continuous flow to the distal part of our bodies. This is very important because if there would just be rigid pipes, the blood pressure peaks in every heart beat would be too high and eventually cause damage to the distal vessels which are tiny. This ability to stretch is also known as the Windkessel effect. After the elastic arteries come the muscle arteries, which are also called the distributing arteries, because they go to the different organs in our body. They're medium sized and are rich in smooth muscle cells to regulate the blood flow to these organs. After that come the small vascular resistance arteries, also known as the arterioles. There the most control of the blood pressure is happening as they can regulate the flow into the capillary bed. If the capillary bed is in need of blood and oxygen supply such as, for example, in a working muscle, then this precapillary sphincter gets loose and the capillary bed is more perfused. If the precapillary sphincter is closed, then more of the blood will be shunted directly from the arteriole to the venules, such as, for example, in resting muscles. The capillaries are the smallest of our blood vessels, consist of only one layer of endothelium, and is the site of exchange of water, oxygen, carbon dioxide, and nutrition. Now to make the circuit a loop, we also shortly look at the venous side which consists of venules, medium-sized veins, and large veins which bring the blood back to the heart where the circuit starts all over. Our heart is an organ itself, which also needs to be supplied by oxygen and nutrition. This is done by the coronary arteries, which originate directly above the aortic valve in the aortic root. Typically, we have two coronary arteries on each side: The left coronary artery and the right coronary artery. The left coronary artery begins with the left main stem, which then separates into the left anterior descending artery, which supplies the anterior wall of our heart, and the circumflex artery, which goes to the lateral wall of the heart. On the right side, we find the right coronary artery, which gives rise to the posterior descending artery, supplying the inferior wall of our heart. Thank you very much for your attention.
