Ch 18 The CV Sytem The Heart Heart Anatomy Heart is no more than a transport system pump the hollow blood vessels are the delivery routes Size Location and Orientation o About the size of a fist o Enclosed within the mediastinum the medial cavity of the thorax o Rests on the superior surface of the diaphragm the heart lies anterior to the vertebral column and posterior to the sternum o Approx 2 3 of its mass lies left of the midsternal line o Base is directed toward right shoulder Apex points inferiorly toward left hip Point of maximal intensity PMI site where you can feel your heart beating where the apex contacts the chest wall Covering of the Heart o Enclosed in a double walled sac called the pericardium o Loosely fitting superficial part of this sac is the fibrous pericardium Tough dense connective tissue layer Protects the heart Anchors it to surrounding structures Prevents overfilling of the heart with blood o Deep to the fibrous pericardium is the serous pericardium thin slippery two layer serous membrane o Parietal layer lines the internal surface of the fibrous pericardium Attaches to the large arteries exciting the heart and then turns inferiorly and continues over the external heart surface as the visceral layer which is an integral part of the heart wall o Between the parietal and visceral layers is the slitlike pericardial cavity which contains a film of serous fluid Glide smoothly past one another during heart activity allowing the mobile heart to work in a relatively friction free environment 1 o Layers of the heart wall o Epicardium is the visceral layer of the serous pericardium Often infiltrated with fat esp in older people o Middle layer myocardium is composed mainly of cardiac muscle forms the bulk of the heart it is the layer that CONTRACTS In this layer the branching cardiac muscles are tethered to one another by crisscrossing connective tissue fibers and arranged in spiral or circular bundles link all parts of the heart together o The connective tissue fibers from a dense network the fibrous skeleteon of the heart which reinforces the myocardium internally and anchors the cardiac muscle fibers Limits the direct spread of Action Potentials across the heart o The third layer is the endocardium a glistening white sheet of endothelium resting on a thin connective tissue layer Chambers Associated Great Vessels Lines the heart chambers and covers the fibrous skeleton of the valves 2 o Heart has 4 chambers 2 superior atria 2 inferior ventricles o Atria The Receiving Chambers Receving chambers for blood returning to heart from circulation Relatively small because don t need that much force Pectinate muscles mark atrial walls Has a protruding auricle small wrinkled protruding appendages Blood enters left atria from 4 pulmonary veins Transport blood from the lungs back to the heart Blood enters right atria via 3 veins Superior vena cava returns blood from body regions superior to the diaphragm Inferior vena cava returns blood from body areas below diaphragm coronary sinus collects blood draining from the myocardium o Ventricles The Discharging Chambers Make up most volume of the heart Marking the internal walls of the ventricular chambers are irregular ridges of muscle called trabeculae carnae Papillary muscles play a role in valve function project into the ventricular cavity Actual pumps of the heart When ventricles contract blood is propelled out of the heart and into the circulation Right ventricle pumps blood into the pulmonary trunk which routes the blood to the lungs where gas exchange occurs Left ventricle ejects blood into the aorta largest artery in body Pathway of Blood Through the Heart o Right atrium tricuspid valve right ventricle Right ventricle pulmonary semilunar valve pulmonary arteries lungs Lungs pulmonary veins left atrium Left atrium bicuspid valve left ventricle Left ventricle aortic semilunar valve aorta Aorta systemic circulation 3 Coronary Circulation o Pulmonary circuit blood vessels that carry blood to and from the lungs gas exchange o Systemic circuit blood vessels that carry the functional blood supply to and from all body tissues o Functional blood supply of the heart is the shortest circulation in the body o Collateral routes ensure blood delivery to heart even if major vessels are occluded o Left coronary artery runs toward the left side of the heart and then divides into its major branches anterior interventricular artery circumflex artery o Right coronary artery courses to the right side of the heart where it divides into its 2 major branches the marginal artery which serves the myocardium of the lateral right side of the heart posterior interventricular artery Heart Valves o Blood flows through the heart in 1 direction from atria to ventricles and out the great arteries leaving the superior aspect of the heart o Atrioventricular Valves AV There are 2 AV one located at each atrial ventricular junction which prevent backflow into the atria when the ventricles are contracting Right AV valve tricuspid valve Left AV valve mitral valve Attached to each AV valve flap are tiny white collagen cords called chordae tendinae which anchor the cusps to the papillary muscles protruding from the ventricular walls Chordae tendinae and papillary muscles anchor valve flaps in their CLOSED position 4 o Semilunar Valves SV Aortic and pulmonary SV valves guard the bases of the large arteries issuing from the ventricles prevent backflow into the associated ventricles When ventricles are contracting and intraventricular pressure rises ABOVE the pressure in the aorta and pulmonary trunk the SV are forced open and their cusps flatten against the arterial walls as the blood rushes past them Properties of Cardiac Muscle Fibers Microscopic Anatomy o Cardiac muscle is striated short fat branched and interconnected o It contracts by the sliding filament mechanism o The connective tissue endomysium acts as both tendon and insertion o Intercalated discs anchor cardiac cells together and allow free passage of ions 5 Contain anchoring desmosomes Contain anchoring Gap junctions Prevent adjacent cells from separating during contraction Allow ions to pass from cell to cell transmitting current across the entire heart o Heart muscle behaves as a functional syncytium Mechanism and Events of Contraction o Heart muscle is self excitable and can initiate their own depolarization automaticity Stimulated by nerves Contracts as a unit Has a long 250 ms absolute refractory
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