2: Cardiac Cycle, Conduction System, and Pacemaker Action Potential

Vocabulary practice flashcards covering the cardiac cycle phases, volumes, pressures, conduction system anatomy, and pacemaker action potential mechanisms.

Cardiac Cycle

A single cycle that encompasses all of the mechanical and electrical events associated with one heartbeat.

Atrial Systole

The phase of the cardiac cycle where atrial contraction forces a small additional amount of blood ($25\,\text{ml}$) into the relaxed ventricles, bringing total ventricular blood volume from $105\,\text{ml}$ to $130\,\text{ml}$.

Isovolumetric Contraction

The first phase of ventricular systole in which ventricular contraction increases intraventricular pressure and closes the AV valves, but does not yet create enough pressure to open the semilunar valves, maintaining ventricular blood volume at $130\,\text{ml}$.

Ventricular Ejection

The second phase of ventricular systole where rising ventricular pressure exceeds arterial pressure, causing the semilunar valves to open and eject blood until ventricular blood volume drops to $60\,\text{ml}$.

Isovolumetric Relaxation

The early phase of ventricular diastole in which ventricles relax, intraventricular pressure drops, blood flows back against the cusps of the semilunar valves forcing them closed, and ventricular blood volume remains unchanged at $60\,\text{ml}$.

Ventricular Filling

The late phase of ventricular diastole where all heart chambers are relaxed, AV valves are open, and blood flows passively into the ventricles until volume reaches $105\,\text{ml}$.

End Diastolic Volume (EDV)

The volume of blood in the ventricle at the end of diastole (reached at the end of ventricular filling and atrial systole), typically equal to $105\,\text{ml} + 25\,\text{ml} = 130\,\text{ml}$.

End Systolic Volume (ESV)

The volume of blood remaining in the ventricle at the end of systole that is not ejected from the ventricles, typically measuring $60\,\text{ml}$.

Stroke Volume (SV)

The volume of blood ejected from the ventricle with each heartbeat, calculated as $\text{SV} = \text{EDV} - \text{ESV}$ (typically $130\,\text{ml} - 60\,\text{ml} = 70\,\text{ml}$).

Ejection Fraction (EF)

The percentage of End Diastolic Volume that is pumped out of the heart during contraction, calculated as $\text{EF} = \frac{\text{SV}}{\text{EDV}}$ (normally $50\%$ to $55\%$).

Cardiac Output (Q)

The total volume of blood ejected from a ventricle per minute, calculated by the formula $Q = \text{SV} \times \text{HR}$ (e.g., $70\,\text{ml/beat} \times 75\,\text{beats/min} = 5.25\,\text{L/min}$).

Chamber Volume–Pressure Relationship

The mechanical principle stating that a decrease in ventricular chamber volume leads to an increase in pressure ($\downarrow\text{Volume} \rightarrow \uparrow\text{Pressure}$), whereas an increase in chamber volume leads to a decrease in pressure ($\uparrow\text{Volume} \rightarrow \downarrow\text{Pressure}$).

First Heart Sound ("Lub" / S1)

The clinical heart sound produced when the atrioventricular (AV) valves close at the beginning of ventricular systole.

Second Heart Sound ("Dub" / S2)

The clinical heart sound produced when the semilunar (SL) valves close at the beginning of ventricular diastole.

Autorhythmic Fibers

Specialized, self-excitable cardiac muscle fibers that repeatedly and spontaneously generate action potentials to trigger heart muscle contractions.

Sinoatrial (SA) Node

The primary pacemaker of the heart located in the right atrium that sets the rate and rhythm of cardiac action potential generation.

Bachmann's Bundle

A specialized interatrial conduction tract that carries action potentials directly from the sinoatrial node across to the left atrium.

Atrioventricular (AV) Node

The component of the cardiac conduction system that receives action potentials from the SA node via internodal tracts and relays them down to the Bundle of His.

Bundle of His (Atrioventricular Bundle)

The tract of conductive fibers located at the top of the interventricular septum that distributes electrical impulses from the AV node into the right and left bundle branches.

Purkinje Fibers

The terminal conductive fibers that spread action potentials rapidly upward throughout the ventricular myocardium to trigger ventricular contraction.

Pacemaker Potential

The slow, spontaneous depolarization of an autorhythmic cell from $-60\,\text{mV}$ up to the threshold of $-40\,\text{mV}$, driven primarily by slow $\text{Na}^+$ inflow.

Pacemaker Depolarization Phase

The rapid upstroke of the pacemaker action potential from the threshold of $-40\,\text{mV}$ to $+10\,\text{mV}$, mediated by rapid $\text{Ca}^{2+}$ inflow.

Pacemaker Repolarization Phase

The falling phase of the pacemaker action potential returning membrane potential from $+10\,\text{mV}$ back down to $-60\,\text{mV}$, driven by $\text{K}^+$ outflow.

Intrinsic Pacemaker Rate

The inherent rate of SA node depolarization in the absence of autonomic nervous system (ANS) control, characterized by a $0.6\,\text{s}$ cycle duration and an intrinsic heart rate of $100\,\text{bpm}$.