Chapter 19.4: The Circulatory System — Heart

What is systole?

Contraction of the heart

What is diastole?

Relaxation of the heart

What is sinus rhythm?

Normal heartbeat initiated by the sinuatrial node (SA node; Sinuatrial Node) (60–100 beats per minute [bpm]; typically 70–80 bpm)

What is an ectopic focus?

Any region other than the sinuatrial node (SA node; Sinuatrial Node) that initiates a heartbeat

What is nodal rhythm?

Heartbeat initiated by the atrioventricular node (AV node; Atrioventricular Node) (40–50 beats per minute [bpm])

What happens if both the SA and AV nodes fail?

Other ectopic foci take over (20–40 beats per minute [bpm]); artificial pacemaker may be needed

What causes the pacemaker potential?

Slow sodium ion (Na⁺; Sodium Ion) inflow in sinuatrial node (SA node) cells

At what voltage do calcium ion (Ca²⁺; Calcium Ion) channels open in the SA node?

What causes repolarization in SA node cells?

Potassium ion (K⁺; Potassium Ion) outflow

How often does the pacemaker cycle repeat?

Every ~0.8 seconds (~75 beats per minute [bpm])

How fast does the SA node signal travel through the atria?

1 meter per second (m/s; meters per second)

What causes the delay at the AV node?

Slower conduction (0.05 meters per second [m/s]) for ventricular filling

How fast is conduction in the atrioventricular bundle (AV bundle; Atrioventricular Bundle) and Purkinje fibers (Purkinje Fibers, also called Subendocardial Branches)?

4 meters per second (m/s; meters per second)

How long after the SA node fires does the ventricular myocardium depolarize?

~200 milliseconds (ms; milliseconds)

Where does ventricular systole begin?

At the apex of the heart

What causes the heart’s twisting contraction?

Spiral arrangement of myocardial muscle fibers

What is the resting potential of cardiomyocytes?

What ion causes rapid depolarization in cardiomyocytes?

Sodium ion (Na⁺; Sodium Ion)

What ion maintains the plateau phase in cardiomyocytes?

Calcium ion (Ca²⁺; Calcium Ion)

How long is the plateau in cardiomyocytes?

200–250 milliseconds (ms; milliseconds)

What ends the plateau in cardiomyocytes?

Closure of calcium ion (Ca²⁺; Calcium Ion) channels and potassium ion (K⁺; Potassium Ion) outflow

What role does calcium play in cardiac contraction?

Binds to troponin to initiate contraction

When does peak muscle tension occur in cardiac muscle?

~150 milliseconds (ms; milliseconds) after action potential peak

What is the duration of the cardiac refractory period?

~250 milliseconds (ms; milliseconds)

Why is the long refractory period important?

Prevents tetanus and wave summation

What does an electrocardiogram (ECG; Electrocardiogram) measure?

Electrical currents of the heart

What is the P wave?

Atrial depolarization

What is the PQ segment?

Time for signal to travel from sinuatrial node (SA node) to atrioventricular node (AV node) (~160 milliseconds [ms])

What is the QRS complex?

Ventricular depolarization (atrial repolarization is masked)

What is the ST segment?

Plateau phase of ventricular action potential (ventricular contraction)

What is the T wave?

Ventricular repolarization

What is the PR interval?

Time for signal to conduct through the atrioventricular node (AV node)

What is the QT interval?

Duration of ventricular depolarization

What can changes in electrocardiogram (ECG) shape or timing indicate?

Conduction defects, myocardial infarction, hypertrophy, etc.

What is an arrhythmia?

Any deviation from sinus rhythm

What is ventricular fibrillation (V

Fib; Ventricular Fibrillation)?

What can cause ventricular fibrillation?

Coronary artery disease

What is atrial fibrillation (A

Fib; Atrial Fibrillation)?

Who commonly experiences atrial fibrillation (A

Fib)?

What are risks of atrial fibrillation (A

Fib)?

What is a heart block?

Failure of part of the conduction system

What is a bundle branch block?

Damage to atrioventricular bundle (AV bundle) branches

What is total heart block?

Ventricles beat independently at 20–40 beats per minute (bpm)

What is a premature ventricular contraction (PVC; Premature Ventricular Contraction)?

Ectopic focus in ventricles fires early; misshapen QRS and no P wave

What can trigger premature ventricular contractions (PVCs)?

Stimulants, stress

What is defibrillation?

Emergency electrical shock to reset the heart rhythm

How does defibrillation work?

Depolarizes entire myocardium so the sinuatrial node (SA node) can restore rhythm