Chapter 20, part 3 The Heart
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
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
Figure 20.15 The Action Potential in Skeletal and Cardiac Muscle
Figure 20.15 The Action Potential in Skeletal and Cardiac Muscle
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
Figure 20.16 Phases of the Cardiac Cycle
Figure 20.16 Phases of the Cardiac Cycle
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
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
Pressure and volume changes: ventricular diastole The period of isovolumetric relaxation when all heart valves are closed Atrial pressure forces the AV valves open
Figure 20.17 Pressure and Volume Relationships in the Cardiac Cycle
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
Figure 20.18 Heart Sounds Figure 20.18a, b
SECTION 20-4 Cardiodynamics
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)
Figure 20.19 A Simple Model of Stroke Volume Figure 20.19a-d