G.. KYALYAN R.. PETROSYAN HUMAN ANATOMY ADAPTEC COURSE FOR FOREIGN STUDENTS Volume IV THE TRANSPORT AND COORDINATION YEREVAN 2002 LITERATURE 1. Human Anatomy M.R. Sapin, Russian Edition, 1993 2. Human Anatomy M.Prives N.Lysenkov V.Bushkovich English Edition, 1985 3. Human Anatomy Robert Carola John P. Harley Charles P. Woback English Edition, 1992 4. Gray's Anatomy Edited By Peter L. Willams English Edition, 1993 2 THE SCIENCE OF THE VESSELS (ANGLOLOGY) FLUID-CONDUCTING PATHWAYS The vascular system consists of a network of tubes or canals through which the body's fluids, blood and lymph, circulate. The vascular system, on the one hand, supplies the cells and tissues of the body with necessary nutrients and, on the other hand, removes and transports waste products produced by the vital activity of the cells to the kidneys, the excretory organs. In the Coelenterate the alimentary canal develops numerous growths to facilitate the conveyance of nutrients to various parts of the body, but in the numerations, a subtype of worms, three separate blood vessels develop. The lancelet has a closed system of blood circulation, devoid, however, of a heart; the circulation of colourless blood in the lancelet is achieved by the pulsation of the vessels themselves. The heart appears in the circulatory system of vertebrates as a pulsating organ, which gradually becomes more complicated in structure in the process of phylogenesis. The heart of fish consists of two chambers. The chamber that receives the blood is called the atrium and is preceded by the infundibulum sinus venosus. A second chamber, called the ventricle, discharges the blood. The ventricle is followed by the infundibulum conus arteriosus. Venous blood circulates through the heart and then through the branchial arteries to the gills, where it is enriched with oxygen in branchial respiration. Pulmonary respiration and, with it pulmonary blood circulation developed in addition to branchial respiration when amphibians emerged from the water onto dryland. The pulmonary artery in amphibians, which develops from the last branchial 3 artery, carries blood from the heart to the lunges where the exchange of gases takes place. In connection with this, the atrium is divided by a septum into two separate atria, right and left, as a result of which the heart becomes three-chambered. Venous blood flows through the right atrium, arterial blood through the left, and mixed blood through the common ventricle. Branchial circulation is typical of the larval stage of development while pulmonary circulation develops in maturity, a fact that reflects the transition from aquatic to terrestrial life. When reptiles finally emerged onto land and pulmonary respiration fully replaced branchial respiration, pulmonary circulation developed further and two types of circulation formed: circulation in the lungs and circulation through the body. Accordingly, in reptiles the ventricle is also divided into two parts, the right and left ventricles, by an incomplete septum. In birds and mammals, man including, the ventricle is divided by a septum into two, completely separate, ventricles corresponding to the two types of circulation. Because of this, the venous and arterial blood are fully separated: venous blood circulates in the right heart and arterial blood in the left. The vascular systems of man and other vertebrates can be classified into two systems, according to the character of the circulating fluid: (1) the blood vascular system, made up of tubes (the arteries, veins, and heart), through which the blood circulates; and (2) the lymphatic system, made up of tubes along which lymph, a colourless fluid, flows. In the process of embryogenesis, the lymphatic system comes into contact with the blood circulation system and becomes an additional channel for the veins. The fluid in the lymph vessels flows in the same direction as blood in the veins, i.e., from the tissues to the heart. There are, however, substantial differences in the character of the 4 removal of substances are absorbed by the blood occurs much faster. When methylene blue is introduced subcutaneously, for example, it appears in the urine earlier than its presence can be detected in the thoracic duct. THE BLOOD VASCULAR SYSTEM The blood vascular system consists of a central organ, the heart, and tubes of various calibre connected to it, which are called blood vessels. The rhythmic contractions of the heart continuously propel the entire mass of blood contained in the vessels. In clinical practice the blood vascular system is called the cardiovascular system. The heart itself is referred to as the central heart. The vessels constitute the peripherial heart. The arteries. The blood vessels passing from the heart to the organs and carrying blood to them are called arteries (GK arterial windpipe). Since the arteries are empty in cadavers, in ancient times they were thought to conduct air. The wall of the artery consists of three coats. The inner coat (tunica intima) is lined with endothelium, under which lies the subenfothelium and an inner elastic membrane. The middle coat (tunica media) is made up of two layers of smooth muscle fibres (en external longitudinal and an internal circular layer), which alternate with elastic fibres. The outer coat (tunica externa s. adventitia) contains connective- tissue fibres. The elastic elements of the arterial wall form a single elastic frame, resilient as a spring, which lends elasticity to the arteries. The further they are form the heart, the more the arteries branch out and the smaller they become. Those nearest the heart (the aorta and its large branches) are primarily concerned with conveying blood. They 5 contract to counteract stretching by the mass of blood expelled by the heart beat. As a result, structures of a mechanical character, i.e., elastic fibres and membranes, are relatively more developed in the arterial wall. Such arteries are called arteries of the elastic type. In arteries of a small and moderate calibre, the inertia of the heart thrust weakens, and contraction of the vascular wall itself is required to circulate the blood furthr. This predominantly contractile function can be properly performed because of the relatively greater development of smooth muscle tissue in the vascular wall. Such arteries are called arteries of the muscular type. Some arteries supply whole organs or parts of organs with blood. Arteries can be classified as extraorganic arteries, which pass outside the organ before entering it, and their continunations,intraorganic arteries, which branch out inside the organ. Lateral branches of a single trunk or branches of different trunk can join one another. Such a junction of vessels befor their division into capilliaries is called an anastomoses (GK anastomoein to provide with a mouth). Most arteries from anstomoses. Arteries which do not anaostomose with neighbouring trunks before they transform into capilliaries(see below) are called end arteries(in the spleen, for instance).End arteries are susceptible to thrombosis, the formation of blood clots, to infarction, the local necrosis of an organ. The final branches of the arteries are very fine and delicate and are, therefore, clssified separately as arterioles. They are directly continuous with the capillarirs and perform a regulatory function with the help of contractile elements. Capillaries are hair-like vessels concerned with metabolism. The capillary wall consists of a single layer of a flat endothelial cells permeable to substances and gases dissolved in liquids. It has not yet been determined whether capillaries have no muscular elements and, 6 therefore, no efferent innervation. The capillaries anastomose widely among themselves and form networks continuous with the veins. The veins carry blood from the organs to the heart, i. e .,in a direction opposite to the flow of blood in the arteries. The walls of veins are formed in the same way as those of arteries, except that they are much thinner and contain less elastic and mascular tissue.As a result, empty veins drop flat while the lumen of an artery in cross section gapes. The initial segments of the venous bed are particularly fine. These are the venules, which form directly from the capillary network and make up the roots of the veins. The venules are continuous with the veins merge to form large venous trunks passing to the heart. The veins anastomose widely among themselves and form venous plexuses. Blood flows through the veins because of the suction action of the heart and the thoracic cavity. Suction is created by the negative pressure produced during inhalation as the result of the difference of pressure in the cavities, the contraction of striated and smooth muscles of the organs, and other factors. The contraction of the muscular coat of the veins, more developed in veins of the lower half of the body, where conditions for venous drainage are more difficult, also plays a role. Venous blood is prevented from flowing in the opposite direction by special valves. in the venous walls. These valves are shaped from the folds of the endothelium with a layer of connective tissue inside. The free edge of the valves is turned toward the heart. Consequently, they do not obstract the flow of blood to the heart but prevent it from flowing back. As an individual grows older, the diameter of the veins and the capacity of the venous bed increase relative to the diameter of the arteries and the volume of the arterial bed. 7 Direct connection between the tiniest arteries and veins exist in many organs. These connections, called arteriovenous anastomoses, form in such a way that the artery divides into arterioles and capillaries, while the smaller merges with the veins, losing the characteristics of an arterial vessel and becoming closer in structure to a vein. As a consequence, an excess of arterial blood flowing at any given moment to the tissues may be diverted, bypassing the capillary network.