THE HEART - CHAPTER 19

• 19.1

  • Cardiovascular system - consists of heart and blood vessels 

    • 2 major divisions:

      • Pulmonary circuit - carries blood to lungs for gas exchange and returns it to heart

        • Right half of heart supplies pulmonary circuit - receives blood that has circulated through the body unloaded oxygen and nutrients, picked up load of co2 and other wastes

        • Pulmonary trunk - right half pumps blood into 

          • Divides into left and right pulmonary arteries - transport blood to lungs, then o2 blood returns back to the heart through pulmonary veins

      • Systemic circuit - supplies blood to every organ of the body, including other parts of the lungs and the wall of the heart itself

        • Blood leaves through aorta, blood goes through aortic arch, passes downward posterior to heart 

        • Aortic arch gives pathways to upper limbs and head 

        • Deoxygenated blood comes through right side of heart mainly by 2 large veins: superior and inferior vena cava

  • Circulatory system - blood and lymphatic system

  • Mediastinum - thick partition between the lungs

    • Extends from broad base to permost end, to the blood vessels, to the apex

  • Enclosed in pericardium - tough, fibrous outer layer of dense irregular connective tissue and thin, deep serous layer

    • Serous layer forms inwards and becomes visceral pericardium (like epicardium, wall of heart)

    • Allows room for heart to expand but resists excessive expansion

  • Pericardial cavity - space between parietal and visceral membranes

  • Pericardial fluid - prevents friction, allows heart to beat w/o problem 

• 19.2

  • Heart wall consists of 3 layers:

    • Epicardium (visceral pericardium) - serous membrane of external heart surface, consists mainly of simple squamous epithelium overlying thin layer of areolar tissue, some places have thick adipose tissue 

    • Endocardium - similar layer, lines interior of heart chambers, has no adipose tissue, covers valve surfaces 

    • Myocardium - between two is composed of cardiac muscle, thickest layer and performs work of the heart 

  • Also has framework of collagenous and elastic fibers that make up fibrous skeleton - concentrated in walls of heart chambers 

    • Functions: 

      • Provides structure to support the heart 

      • Anchors cardiocytes and gives them something to pull against

      • Nonconductor of electricity, electrical insulation between atria and ventricles 

  • 4 chambers

    • Right and left atrium - receive blood returning to the heart 

    • Right and left ventricles - pumps that eject blood into arteries and keep it flowing around body 

      • Right ventricle constitutes most of anterior aspect of heart 

    • Coronary sulcus - encircles heart near the base and separates the atrium above from the ventricles below 

    • Anterior interventricular sulcus - sulci extend obliquely down the heart from the coronary sulcus toward apex (Front)

    • Posterior interventricular sulcus - same as anterior (Back)

    • Interventricular septum - divides right and left ventricles 

    • Interatrial septum - divides right and left atrium 

    • Pectinate muscles - right atrium and both auricles exhibit internal ridges of myocardium 

    • Interventricular septum more muscular, vertical wall between ventricles 

    • Right ventricle pumps blood to lungs and back to left atrium (wall moderately muscular)

    • Left ventricle bears greatest workload so walls are twice as muscular (pumps blood to entire body)

    • Trabeculae carneae - keep ventricular walls from clinging to each other, prevents friction

  • The valves 

    • Atrioventricular (AV) valve regulate openings between atria and ventricles 

    • Right AV - tricuspid

    • Left AV - bicuspid/ mitral valve

    • Tendinous cords (chordae tendineae) connect valve cusps to conical papillary muscles on floor of ventricle

      • Prevent AV valves from flipping inside out or bulging into atria when ventricles come into contact 

      • May distribute mechanical stress

      • Multiple attachments also provide some redundancy that protects AV valve frm complete mechanical failure should one attachment fail 

      • Semilunar valves - regulate flow of blood from ventricles into great arteries 

      • Pulmonary valve - controls opening from right ventricle to pulmonary trunk

      • Aortic valve - controls opening from left ventricle to aorta 

  • Blood flow through chambers 

    • Blood that has been through the systemic circuit returns by ways of superior and inferior vena cava into right atrium 

    • Right atrium through tricuspid valve into right ventricle 

    • Ejects blood through pulmonary trunk into pulmonary valve to exchange CO2 for O2

    • Blood returns by way of pulmonary veins and empties into left atrium 

    • Left atrium through bicuspid valve and into left ventricle 

    • Left ventricle ejects blood to aorta and makes its way into systemic system 

  • Coronary Circulation

    • Myocardium has its own supply of blood vessels to deliver blood to every muscle cell 

    • Coronary circulation - blood vessels of the heart 

    • Arterial supply

      • Myocardial infarction (heart attack) - fatty deposit or blood clot in coronary artery

      • Collateral circulation - alternative route for arterial anastomoses that can supply heart tissue with blood if primary route becomes obstructed  

      • Blood flow peaks when heart relaxes

        • Contraction of myocardium compresses coronary arteries and obstructs blood flow

        • Aortic valves forced to open when ventricles contract, blocking blood from flowing into them 

        • Blood in aorta briefly surfers back to heart and fills aortic valves when heart is relaxed 

      • After aorta leaves left ventricle, there is a right and left coronary artery (RCA and LCA)

      • LCA has 2 branches

        • Anterior interventricular branch - also known as left anterior descending branch, supplies blood to both the ventricles and anterior two thirds of the interventricular system

        • Circumflex branch - gives off a left marginal branch passes down left margin of heart, furnishes blood to left ventricle, supplies blood to left atrium and posterior wall of left ventricle 

      • RCA has 2 branches

        • Right marginal branch     - supplies lateral aspect of right atrium and ventricle 

        • Posterior interventricular branch - travels down corresponding sulcus supplies the posterior walls of both ventricles as well as the posterior portions of the interventricular septum

    • Venous drainage

      • Venous drainage - route by which blood leaves an organ 

      • After flowing through capillaries of heart wall, a small percent of blood empties directly from small cardiac veins into the heart chambers. The rest returns to the right atrium by:

        • Great cardiac vein

        • Posterior interventricular (middle cardiac) vein

        • Left marginal vein

        • Coronary sulcus 

• 19.3

  • Heart is autorhythmic bc it doesn’t depend on the nervous system for its rhythm (has its own pacemaker function and electrical system)

  • Structure of cardiac muscle

    • Striated 

    • Cardiocytes - relatively short, thick, branched cells

      • Through these branches, each cardiocyte contracts several others, so in total they form a large network throughout each pair of heart chambers, one in atrium and one in ventricles

      • Usually has one, centrally placed nuclei often surrounded by light=staining mass of  glycogen

      • During excitation, T tubules release calcium ions from extracellular fluid to activate muscle contraction - reason why cardiomyocytes have large mitochondria 

      • Joined end to end by intercalated discs - appear as dark lines thicker than the striations

        • 3 distinct features not found in skeletal muscle

          • Interdigitating folds - increase surface area of intercellular contact

          • Mechanical junctions

            • Fascia adherens - most extensive, broadband in actin thin myofilaments

            • Desmosomes

          • Electrical junctions - also contains gap junctions

      • Cardiac muscle lacks satellite cells that can repair dead muscle fiber (scarring)

  • Metabolism of cardiac muscle

    • Depends almost exclusively on aerobic respiration to make ATP 

    • Rich in hemoglobin and glycogen 

    • Huge mitochondria 

    • More vulnerable to oxygen deficiency than any other specific fuel deficiency bc it makes little use of anaerobic fermentation (fatigue)

  • Conduction system

    • Cardiac conduction system - composed of internal pacemaker and nervelike conduction pathways through the myocardium 

    • Generates and conducts rhythmic electrical signals in the following order:

      • Sinoatrial (SA) node - pacemaker that initiates the heartbeat and determines heart rate

      • Signals from SA node spread throughout atria

      • AV node - acts as electrical gateway to the ventricles, fibrous skeleton acts as an insulator to prevent currents from getting to the ventricles by any other route

      • AV bundle (bundle of His) - pathway by which signals leave AV node, forks into left and right branches, which enter IV septum and descend toward apex 

      • Purkinje fibers - distribute electrical excitation to cardiocytes of ventricles

      • Cardiocytes get these signals and pass it from cell to cell through gap junctions

  • Nerve supply to the heart

    • Sympathetic pathway

      • Preganglionic nerve fibers extend from lower cervical to upper thoracic segments of spinal cord to adjacent sympathetic chain ganglia

      • Postganglionic fibers arise in cervical ganglia through cardiac plexus and continue by way of cardiac nerves to the heart 

      • Increases heart rate and contraction strength 

    • Parasympathetic pathway

      • Nuclei of vagus nerves - preganglionic fibers extend through vagus nerves to cardiac plexus and continue to heart by cardiac nerves 

      • Sometimes intervenes with sympathetic pathway and target cells 

      • Little to no sympathetic innervation of myocardium

      • Reduces heart rate 

• 19.4

  • Systole - contraction

  • Diastole - relaxation

  • Cardiac rhythm 

    • Sinus rhythm - normal heartbeat triggered by SA node, typically around 70 to 80 bpm in adults (resting)

    • Ectopic focus - any region of spontaneous firing other than SA node 

      • If SA node is damage, this may take over 

      • Nodal rhythm - AV node that produces slower heartbeat of 40 to 50 bpm

        • Sufficient to sustain life but 20 to 40 is too low to for brain to survive

  • Pacemaker physiology

    • Pacemaker potential - membrane potential starts at -60mV and drifts upward, showing a gradual depolarization

      • Slow inflow of sodium without a compensation outflow of potassium 

    • Every depolarization of SA node sets off one heartbeat 

    • When SA node fires, it excites the other components in the conduction system, making the SA node the pacemaker