## 2 Topics and Results
### Changes in the Endothelial Cells of the Vascular Walls  
Endothelial and vasodilation disorders are the first signs of cardiovascular disease. They are proven risk factors preceding cardiovascular problems [90]. Because of this, many researchers and physicians have focused their efforts on the study and evaluation of blood vessels. The main goal is to identify vulnerable groups of people susceptible to the corresponding cardiovascular disease in order to implement timely prevention. Contraction of the smooth muscles in the vessel wall is an indication of increased vascular tone. The endothelial layer that lines the arteries and arterioles plays a significant role in regulating smooth muscle contraction and relaxation [91]. Various hormones in the body cause vasoconstriction or vasodilitation and through them regulate the intensity of the blood flow. In addition, factors affecting vessel diameter may also alter the biomechanical properties and tone of vessel walls and help regulate blood pressure. Large arteries in the cardiovascular system, according to the structure of their vascular walls, are defined as arteries of elastic type. Examples of this group of arterial vessels are the pulmonary artery, the aorta and its immediate branches with a wide lumen, etc. Their wall consists of predominantly dense elastic elements and smooth muscle cells. The three main layers of the vessel wall are the intima, media and adventitia. The intima consists of a well-defined endothelial layer and an internal elastic membrane (elastica interna). The media is made up of numerous concentrically arranged, interconnecting smooth muscle cells and elastic membranes that form a complexly organized elastic network. The adventitia is represented by a thin layer of connective tissue containing elastic fibers and fibroblast and pericyte cells. Medium lumen size arteries are defined as muscular type arteries. They are characterized by their active vasomotor function, which is expressed in changing the dimensions of their lumen during vasoconstriction and vasodilitation. This function of theirs determines the regulation of the amount of blood passing through them, which is why they are also known as \"distributive-type\" arteries. Some arteries of the muscular type are: arterial vessels of the limbs, intercostal arteries, organ arteries in the abdominal cavity, etc. Their intima structure resembles that of the large arteries, and they are characterized by a highly pronounced internal elastic membrane. The media of these vessels is made up of circularly arranged smooth muscle fibers forming multiple layers, between which a small amount of elastic, collagen and reticular fibers are visualized. The adventitia in the arteries of the muscular type is of considerable thickness, composed of collagen and elastic fibers. Small arterial vessels are distinguished by a simpler wall structure, with a gradual decrease in their lumen when passing to the capillaries. A change in the structure of the vascular wall is found in arteries with a diameter of 0.4 mm and is expressed in the following: a gradual disappearance of the subendothelial connective tissue, the internal elastic membrane turns into a fine network of elastic fibers, and the muscle layer located in the media decreases in volume ([[https://medpedia.framar.bg/anatomy/arterial-department-of-the-cardiovascular-system](https://medpedia.framar.bg/anatomy/arterial-department-of-the-cardiovascular-system)]([https://medpedia.framar.bg/anatomy/arterial-department-of-the-cardiovascular-system](https://medpedia.framar.bg/anatomy/arterial-department-of-the-cardiovascular-system)), accessed on 3 July 2021). Vascular endothelial cells play an important role in regulating vascular activity by releasing vasoactive substances, such as endothelin-1 (ET-1) and nitric oxide (NO) [91]. ET-1 is a peptide vasoconstritor that is synthesized in endothelial cells (the innermost layer of cells lining the vascular lumen and interacting directly with blood flow) [92].