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