Valves, Cardiac Conducting System, and ECG Basics

A comprehensive set of practice flashcards covering heart valve mechanics, the cardiac conducting system, cardiac action potentials, and ECG fundamentals.

What determines the opening of the atrioventricular (AV) valves?

The atrial pressure versus ventricular pressure; AV valves open when atrial pressure exceeds ventricular pressure.

What determines the opening of semilunar valves?

Ventricular pressure exceeding the pressure in the outflow vessel (pulmonary trunk or aorta).

What is the role of the chordae tendineae in AV valve function?

They act as anchors to prevent valve prolapse, keeping the valve from inverting into the atrium during ventricular contraction.

What causes AV valves to close during the cardiac cycle?

Ventricular pressure rises above atrial pressure during ventricular contraction, pushing the valve shut.

How do the semilunar valves close when backflow occurs?

The cusps fill with blood from backflow and seal the opening when pressure in the outflow vessel is higher than in the ventricle.

What is endomysium and its role in cardiac muscle?

A loose, highly vascular connective tissue surrounding cardiac muscle fibers that weaves between cells to supply capillaries.

What are intercalated discs and their components?

Cell-to-cell junctions that connect cardiac muscle cells, containing desmosomes (mechanical) and gap junctions (electrical communication).

What percent of myocardial cells are conducting (pacemaker) cells, and what is their job?

About 1%; they self-depolarize and spread excitation but do not contract.

Where is the sinoatrial (SA) node located?

Superior part of the right atrium.

Why is the SA node called the pacemaker of the heart?

It has the fastest rate of depolarization (60–100 BPM) and sets the resting heart rate.

Where is the atrioventricular (AV) node located and what is its function?

Located at the inferior edge of the interatrial septum; it delays conduction briefly to allow atrial contraction before ventricular excitation.

What is the intrinsic rate of the AV node?

About 40–60 depolarizations per minute.

What is the AV bundle (bundle of His) and what does it do?

The electrical connection between the atria and ventricles; propagates depolarization to the ventricles; intrinsic rate ~30 BPM.

Where are the right and left bundle branches located?

In the interventricular septum, branching to the right and left ventricles.

Where are Purkinje fibers located and what is their function?

In the walls of the ventricles; rapidly conduct impulses to ventricular myocytes; intrinsic rate ~15–20 BPM.

What is the normal sequence of conduction in the heart’s conducting system?

SA node → AV node → AV bundle (His) → right and left bundle branches → Purkinje fibers.

What causes the pacemaker potential in conducting cells?

Slow, continuous Na+ leakage leading to threshold and depolarization.

What ions flow during the depolarization of conducting cells and in what order?

Slow Na+ influx to reach threshold, then fast Ca2+ influx to depolarize, followed by K+ efflux to repolarize.

How do conducting cells differ from contractile cells in resting potential and contraction?

Conducting cells have unstable resting potentials and do not contract; contractile cells have stable resting potentials and contract when stimulated via gap junctions.

What is the role of gap junctions between conducting and contractile cells?

Allow ions to pass from conducting cells to adjacent contractile cells, triggering depolarization (excitation) in the contractile cells.

What is the plateau phase in cardiac contractile cell action potentials and why does it occur?

A sustained depolarization caused by a balance between slow Ca2+ influx and K+ efflux, preventing rapid repolarization and tetanus.

What are the sources of calcium for cardiac contraction and why is extracellular calcium important?

Calcium comes from both the sarcoplasmic reticulum and the extracellular fluid; extracellular calcium is essential for contraction in addition to SR calcium.

What is the absolute refractory period and how does it compare to skeletal muscle?

The period during which a cell cannot be restimulated; it is longer in cardiac muscle than in skeletal muscle to allow heart filling between contractions.

What does the ECG measure and what information can it provide?

Electrical activity of the heart (action potentials); it reveals timing of depolarization and repolarization but does not directly measure mechanical events.

What does the P wave represent on an ECG?

Atrial depolarization (SA/AV node activity).

What does the QRS complex represent on an ECG?

Ventricular depolarization; atrial repolarization occurs within the QRS but is masked by ventricular activity.

What does the T wave represent on an ECG?

Ventricular repolarization.

Why is atrial repolarization not seen as a separate wave on the ECG?

It occurs during the QRS complex and is masked by the larger ventricular depolarization.

What is the isoelectric line on an ECG?

Baseline with no electrical activity; 'unchanging' electricity.

What is the difference between an ECG segment and an interval?

A segment is a flat portion on the isoelectric line between waves; an interval includes at least one segment plus at least one wave (e.g., PR interval).

Why is the PR interval sometimes called PR rather than PQ in ECG terminology?

Because depending on the lead, a Q wave may be absent; measurement is from the start of the P wave to the start of the QRS complex, leading to PR or PQ naming depending on presence of Q wave.

Is the cardiac conducting system part of the nervous system?

No. It consists of specialized cardiac muscle cells that self-depolarize, not neural tissue.