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Transcript
Overview of the Cardiovascular System Topics to be addressed: Blood Anatomy of Blood Vessels Anatomy of the Heart The Conduction System The Cardiac Cycle Cardiodynamics Blood Flow and its Regulation Adaptation and Disorders of the Cardiovascular System 1 The Cardiac Cycle and the ECG Pumping Blood: a mechanical event initiated by electrical events Normally, the SA node generates an action potential, and passes the signal down the conductive system Purkinje fibers distribute the stimulus to the contractile cells, which make up most of the ventricle wall 2 Characteristics of Cardiac Muscle Cells Small size Single, central nucleus Branching interconnections between cells Intercalated discs 3 Structure of Cardiac Muscle Cells: Intercalated Discs 4 Structure of Cardiac Muscle Cells Intercalated discs contain two types of cell-cell junctions: • Desmosomes physically tie cells together • Gap Junctions connect cytoplasm • allow ion flow directly from one cell into another • “electrical coupling” 5 The Action Potential of a single contractile cardiac muscle cell Net gain of + charge Net loss of + charge **The resting membrane potential of contractile cells is stable (unlike that of the conductive cells like those of the SA node) 6 The Role of Calcium Ions in Cardiac Contractions Contraction of a cardiac muscle cell is produced by an increase in calcium ion concentration around myofibrils Actin-Myosin crossbridges form From Silverthorn Human Physiology, 4th ed., Pearson/Benjamin Cummings 2007 Cardiac muscle tissue is very sensitive to extracellular Ca2+ concentrations Calcium channel blockers are a group of powerful medications for heart patients 7 Comparison of the action potential and resulting contraction between skeletal and cardiac muscle In skeletal muscle, the action potential was brief relative to the contraction. A second action potential soon after the first increased cytoplasmic calcium levels and increased the contraction 8 Comparison of the action potential and resulting contraction between skeletal and cardiac muscle The Absolute Refractory Period is very long: cardiac muscle cells cannot be stimulated again until this period is over In cardiac muscle, the action potential lasts longer than the contraction. One contraction is over before another can begin, preventing summation of contraction and tetany. This ensures time for the heart to fill between contractions. 9 Monitoring Heart Activity: the ECG The Electrocardiogram (ECG or EKG) is a recording of electrical events in the heart, representing ALL the action potentials from ALL the cardiac cells – conducting and contractile 10 Features of an ECG P wave: Atria depolarize QRS complex: Ventricles depolarize T wave: Ventricles repolarize 11 Common Clinical ECG Measures P–R interval Time from start of atrial depolarization to start of QRS complex Q–T interval Time from ventricular depolarization to ventricular repolarization 12 Abnormal ECG Recordings Normal P waves absent SA node nonfunctional Slower heart rate now driven by AV node P waves not always followed by QRS wave A form of “heart block” Problem within conduction system between SA node and rest of system Chaotic deflections “Ventricular fibrillation” Bad. Very bad. 13 Defibrillators shock the heart back into a normal rhythm Portable AEDs (automated external defibrillators) are now common 14 The Cardiac Cycle One cycle = from the start of one heart beat to the start of the next heartbeat Two Phases: Systole (contraction) Diastole (relaxation) Begins with initiation of action potential at SA node Produces action potentials in cardiac muscle cells (contractile cardiac cells) of Atria Both Atria begin contracting = Atrial Systole Signal is transmitted through conducting system (conducting cardiac cells of AV node, Bundle branches, Purkinje fibers) Both Ventricles contract, apex to base, pushing out blood = Ventricular Systole atria begin relaxing = Atrial Diastole Ventricles relax, heart refills = Ventricular Diastole 15 The Cardiac Cycle The period between the start of one heartbeat and the beginning of the next What makes blood move? 1. A pressure gradient Blood moves from area of high pressure to area of low pressure 2. Open valve 16 Steps in the Cardiac Cycle Initiated by pacemaker potential at SA node 17 Steps in the Cardiac Cycle Ventricles are contracting and exerting pressure, but valves are closed so blood is unable to move out “Isovolumetric contraction” phase 18 Steps in The Cardiac Cycle Pressure generated by ventricle wall finally great enough to exceed trunk pressure; semilunar valves are pushed open “Ejection” phase 19 Steps in The Cardiac Cycle Ventricle wall begins relaxation, with pressure falling below trunk pressure. Back flow of blood from trunk toward ventricle closes semilunar valve and ends ejection phase 20 Steps in The Cardiac Cycle Ventricle wall relaxed, with pressure falling below that in atria; AV valves open as blood moves from atria to ventricles “Filling” stage 21 The Cardiac Cycle Looking at only the electrical events and resulting contractile events 22 The Cardiac Cycle Plotting pressures generated by atrial and ventricular contraction 23 Comparison of Right and Left Heart Pressures 24 The Cardiac Cycle Changes in blood volume are driven by changes in pressure and state of the valves 25 Heart sounds normally heard during the Cardiac Cycle Heart Murmur : Abnormal sounds produced by regurgitation through faulty valves or by damaged valve flaps “Lub” = S1 Produced by turbulence as AV valves close and blood pushes against them “Dub” = S2 Produced by turbulence as semilunar valves close and blood pushes against them 26 The Cardiac Cycle 27 28