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Circulatory System - Heart Physiology

Circulatory System - Heart Physiology - The Circulatory...

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The Circulatory System: Heart Physiology Assigned reading: pages 732-749 in Saladin 5th edition
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CARDIAC RHYTHM Cycle of events in heart: systole – atrial or ventricular contraction diastole – atrial or ventricular relaxation Sinus rhythm - normal heartbeat triggered by the SA (sinoatrial) node set by SA node at rest, suppressed to 70-80 bpm (vagal tone) arrhythmia? 1 2 3 4 5 2 Right atrium Sinoatrial node (pacemaker) Left atrium Right ventricle Left ventricle
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PACEMAKER PHYSIOLOGY SA node cells do NOT have a stable RMP gradually depolarizes from a slow inflow of Na+ = pacemaker potential voltage regulated “fast Ca2+ and Na+ channels” open each depolarization of the SA node sets off an AP that travels over the whole heart
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PACEMAKER PHYSIOLOGY Sympathetic innervation: Norepinephrine binds to β- adrenergic receptors in the heart c-AMP 2nd messenger system: 1. opens Ca2+ channels in plasma membrane increased Ca2+ inflow accelerates depolarization of SA node Increases contraction strength of muscles cells 2. increases Ca2+ uptake by SR Allows faster recovery
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PACEMAKER PHYSIOLOGY Parasympathetic innervation: ACh binds muscarinic receptors K+ gates open: K+ diffuses out of cell, slowing the depolarizaion
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Impulse Conduction to Myocardium 2. signal from SA node travels to two atria almost simultaneously reaches AV node in 50 msec 3. signal slows down through AV node delays signal 100 msec 4. signals travel through AV bundle and (5) Purkinje fibers entire ventricular myocardium depolarizes AP’s lead to contraction Wave of AP’s is followed by wave of contraction 1 2 3 4 5 2 Right atrium Atrioventricular node Left atrium Right ventricle Left ventricle
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Electrical Behavior of Myocardium Note: Long plateau phase Ca2+ channels are slow to close Long absolute refractory period prevents wave summation and tetanus 0 .15 .30 Plateau 1 2 3 4 5 1 2 3 4 5 Membrane potential (mV) +20 0 –20 –40 –60 –80 Voltage-gated Na+ channels open. Na+ inflow depolarizes the membrane and triggers the opening of still more Na+ channels, creating a positive feedback cycle and a rapidly rising membrane voltage. Na+ channels close when the cell depolarizes, and the voltage peaks at nearly +30 mV. Ca2+ entering through slow Ca2+ channels prolongs depolarization of membrane, creating a plateau. Ca2+ channels close and Ca2+ is transported out of cell. K+ channels open, and rapid K+ outflow returns membrane to its resting potential.
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