Electrical Activity and Conduction System of the Heart: Key Concepts and ECG Interpretation (2)

What begins atrial systole?

Contraction forces blood into the ventricles.

What occurs during the first phase of ventricular systole?

The AV valves close.

What happens during the second phase of ventricular systole?

Semilunar valves open and blood is ejected.

What characterizes early ventricular diastole?

Relaxation stage where the semilunar valves close and blood flows into atria.

What occurs during late ventricular diastole?

Chambers relax and blood fills ventricles passively.

What is automaticity in the heart?

The heart contracts rhythmically due to action potentials generated by itself.

What percentage of cardiac muscle cells are contractile cells?

99% of cardiac muscle cells do the mechanical work of pumping.

What is the role of autorhythmic cells?

They initiate and conduct the action potentials responsible for contraction of working cells.

How do autorhythmic cells communicate with contractile cells?

Depolarizations spread rapidly through gap junctions.

What is the action potential in cardiac contractile cells characterized by?

A plateau phase where membrane potential is maintained close to peak positive level.

What prevents tetanus in cardiac muscle?

A long refractory period prevents sustained, maximal contraction.

What is the sinoatrial node (SA Node)?

The master pacemaker of the heart.

What are the four components of the cardiac conduction system?

SA node, Atrioventricular Node (AV node), Bundle of His, Purkinje Fibers.

What is the direction of blood flow in the heart?

From the bottom to the top.

What does the ECG record represent?

The overall spread of electrical activity through the heart.

What does the P wave in an ECG indicate?

Atrial depolarization.

What does the QRS complex in an ECG represent?

Ventricular depolarization.

What does the T wave in an ECG signify?

Ventricular repolarization.

What is the PR interval in an ECG?

Conduction through the AV node and AV bundle.

What are the effects of parasympathetic stimulation on the SA node?

Decreases the rate of depolarization to threshold and decreases heart rate.

How does sympathetic stimulation affect the AV node?

Increases excitability and decreases the AV nodal delay.

What happens if the SA node fails?

The AV node can act as a pacemaker, setting the pace at 50 beats per minute.

When are the AV valves closed?

During ventricular systole to prevent backflow.