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Chapter 20, part 3 The Heart
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Contractile Cells Resting membrane potential of approximately –90mV
Action potential Rapid depolarization A plateau phase unique to cardiac muscle Repolarization Refractory period follows the action potential
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Calcium Ion and Cardiac contraction
Cardiac action potentials cause an increase in Ca2+ around myofibrils Ca2+ enters the cell membranes during the plateau phase Additional Ca2+ is released from reserves in the sarcoplasmic reticulum
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Figure 20.15 The Action Potential in Skeletal and Cardiac Muscle
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Figure 20.15 The Action Potential in Skeletal and Cardiac Muscle
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The cardiac cycle The period between the start of one heartbeat and the beginning of the next During a cardiac cycle Each heart chamber goes through systole and diastole Correct pressure relationships are dependent on careful timing of contractions PLAY Animation: Intrinsic Conduction System
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Figure 20.16 Phases of the Cardiac Cycle
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Figure 20.16 Phases of the Cardiac Cycle
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Pressure and volume changes: atrial systole
rising atrial pressure pushes blood into the ventricle atrial systole the end-diastolic volume (EDV) of blood is in the ventricles
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Pressure and volume changes: ventricular systole
Isovolumetric contraction of the ventricles: ventricles are contracting but there is no blood flow Ventricular pressure increases forcing blood through the semilunar valves
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Pressure and volume changes: ventricular diastole
The period of isovolumetric relaxation when all heart valves are closed Atrial pressure forces the AV valves open
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Figure 20.17 Pressure and Volume Relationships in the Cardiac Cycle
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Heart sounds Auscultation – listening to heart sound via stethoscope
Four heart sounds S1 – “lubb” caused by the closing of the AV valves S2 – “dupp” caused by the closing of the semilunar valves S3 – a faint sound associated with blood flowing into the ventricles S4 – another faint sound associated with atrial contraction
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Figure Heart Sounds Figure 20.18a, b
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SECTION 20-4 Cardiodynamics
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Stroke Volume and Cardiac Output
Cardiac output – the amount of blood pumped by each ventricle in one minute Cardiac output equals heart rate times stroke volume CO Cardiac output (ml/min) = HR Heart rate (beats/min) X SV Stroke volume (ml/beat)
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Figure 20.19 A Simple Model of Stroke Volume
Figure 20.19a-d
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