Cardiovascular System Histology (Part 1) PDF

JABIR IBN HAYYAN MEDICAL Lecture By: Aaya Hamid UNIVERSITY COLLEGE OF MEDICINE Lec : 7 DEPARTMENT OF HUMAN ANATOMY Cardiovascular System Histology (part 1) T...

JABIR IBN HAYYAN MEDICAL Lecture By: Aaya Hamid UNIVERSITY COLLEGE OF MEDICINE Lec : 7 DEPARTMENT OF HUMAN ANATOMY Cardiovascular System Histology (part 1) The circulatory system pumps and directs blood cells and substances carried in blood to all tissues of the body. It includes both the blood and lymphatic vascular systems, and in an adult, the total length of its vessels estimated at between 100,000 and 150,000 kilometers. The blood vascular system, or cardiovascular system, consists of the following structures: Heart Arteries Capillaries Veins HEART Cardiac muscle in the four chambers of the heart wall contracts rhythmically, pumping the blood through the circulatory system. The right and left ventricles propel blood to the pulmonary and systemic circulation, respectively; right and left atria receive blood from the body and the pulmonary veins, respectively. 1 The walls of all four-heart chambers consist of three major layers: the internal endocardium; the middle myocardium; and the external epicardium. Endocardium, consists of a very thin inner layer of endothelium and supporting connective tissue, a middle myoelastic layer of smooth muscle fibers and connective tissue, and a deep layer of connective tissue called the subendocardial layer that merges with the myocardium, Branches of the heart’s impulse-conducting system, consisting of modified cardiac muscle fibers, are located in the subendocardial layer. Myocardium, the thickest layer consists mainly of cardiac muscle with its fibers arranged spirally around each heart chamber. Because strong force is required to pump blood through the systemic and pulmonary circulations, the myocardium is much thicker in the walls of the ventricles, particularly the left, than in the atrial walls. Epicardium, is a simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves. The epicardium corresponds to the visceral layer of the pericardium, the membrane surrounding the heart. Where the large vessels enter and leave the heart, the epicardium reflected back as the parietal layer lining the pericardium. During heart movements, deposits of adipose tissue in the epicardium cushion underlying structures and friction within the pericardium prevented by lubricant fluid produced by both layers of serous mesothelial cells. 2 Within these major layers, the heart contains other structures important for its overall function of moving blood. Dense fibrous connective tissue of the cardiac skeleton forms part of the interventricular and interatrial septa, surrounds all valves of the heart, and extends into the valve cusps and the chordate tendinous to which they attached. These regions of dense irregular connective tissue perform the following functions: Anchoring and supporting the heart valves. Providing firm points of insertion for cardiac muscle. Helping coordinate the heart beat by acting as electrical insulation between atria and ventricles. Within the subendocardial layer and adjacent myocardium, modified cardiac muscle cells make up the impulse conducting system of the heart, which generates and propagates waves of depolarization that spread through the myocardium to stimulate rhythmic contractions, this system consists of two nodes of specialized myocardial tissue in the right atrium: 1- Sinoatrial (SA) node (or pacemaker), Located in the right atrial wall near the superior vena cava, the SA node is a 6- to 7-mm3 mass of cardiac muscle cells with smaller size, fewer myofibrils, and fewer typical intercalated disks than the neighboring muscle fibers. 2- Atrioventricular (AV) node. 3- AV bundle (of His). 4- Subendocardial conducting network. At the apex of the heart, the bundles branch further into a subendocardial conducting network of myofibers, usually called Purkinje fibers. 3  TISSUES OF THE VASCULAR WALL Walls of all blood vessels except capillaries contain smooth muscle and connective tissue in addition to the endothelial lining. The amount and arrangement of these tissues in vessels are influenced by mechanical factors, primarily blood pressure , and metabolic factors reflecting the local needs of tissues. The Endothelium is a specialized single layer of a squamous epithelium that acts as a semipermeable barrier between two internal compartments: the blood plasma and the interstitial tissue fluid. Vascular endothelial cells are squamous, polygonal, and elongated with the long axis in the direction of blood flow. Endothelium with its basal lamina is highly differentiated to mediate and actively monitor the bidirectional exchange of molecules by simple and active diffusion, receptor mediated endocytosis, transcytosis, and other mechanisms. Smooth muscle fibers occur in the walls of all vessels larger than capillaries and are arranged helically in layers. In arterioles and small arteries, the smooth muscle cells are connected by many more gap junctions and permit vasoconstriction and vasodilation which are of key importance in regulating the overall blood pressure. Connective tissue, components are present in vascular walls in variable amounts and proportions based on local functional requirements. Collagen fibers are found in the subendothelial layer, between the smooth muscle layers, and in the outer covering. Elastic fibers provide the resiliency required for the vascular wall to expand under pressure. Elastin is a major component in large arteries where it forms parallel lamellae, regularly distributed between the muscle layers. Variations in the amount and composition of ground. 4