Chapter 19: Cardiac Conduction System and the Cardiac Cycle

Vocabulary terms covering Chapter 19: the cardiac conduction system, electrical action potentials, EKG tracings, cardiac cycle phases, hemodynamics, and autonomic regulation of the heart.

Cardiac conduction system

The involuntary internal signaling network of the heart that coordinates the timing of cardiomyocyte contraction across chambers.

Sinoatrial (SA) node

The internal pacemaker located in the upper superior portion of the right atrium that spontaneously depolarizes roughly every $0.8\,\text{seconds}$ to initiate the heartbeat.

Atrioventricular (AV) node

An electrical relay node in the interatrial septum that slows the conduction velocity, giving the atria time to finish contracting before ventricles are stimulated.

Atrioventricular (AV) bundle

A tract of conducting fibers (historically known as the bundle of His) extending into the interventricular septum before branching into left and right bundle branches.

Purkinje fibers

Also called the subendothelial conducting network; fibers that weave through the ventricular myocardium to deliver electrical impulses from the apex upward.

Gap junctions

Intercellular connections linking adjacent cardiomyocytes that allow rapid ion passage, ensuring the myocardium contracts as a synchronized unit.

Systole

The phase of contraction and chamber squeezing within the heart that generates high pressure to promote forward blood flow.

Diastole

The phase of relaxation of the heart muscle during which chamber pressure drops and refilling occurs.

Sinus rhythm

The normal resting rhythm of the heart, determined by the spontaneous firing of the sinoatrial (SA) node at roughly $70$ to $80\,\text{beats per minute}$.

Pacemaker potential

A gradual, unstable depolarization of the SA node from approximately $-60\,\text{mV}$ toward a threshold of $-40\,\text{mV}$, driven by the inward leak of sodium through slow sodium channels.

Plateau

A prolonged depolarized phase during cardiomyocyte action potential caused by slow calcium channel opening, resulting in sustained muscular contraction.

Electrocardiogram (EKG)

A clinical recording using surface leads that captures electrical conduction vectors, mapping out the timing of myocardial depolarization and repolarization.

P wave

The initial deflection on an EKG tracing produced by SA node firing and the subsequent depolarization and contraction of the atria.

PQ interval

The duration on an EKG tracing reflecting the time required for signals to travel from the SA node through the atria to the AV node.

QRS complex

The sharp set of peaks and valleys on an EKG generated by rapid ventricular depolarization as impulses spread through the bundle branches and Purkinje fibers.

ST segment

An interval on an EKG that corresponds to the plateau phase of ventricular cardiomyocytes during active ventricular contraction.

T wave

The final rounded deflection on an EKG indicating ventricular repolarization as ventricular muscle cells return to their relaxed diastolic state.

Bradycardia

An abnormally slow resting heart rate characterized by an SA node firing rate below $50\,\text{beats per minute}$.

Tachycardia

An abnormally fast heart rate defined as an active or resting rate in excess of $100\,\text{beats per minute}$.

Fibrillation

A severe uncoordinated arrhythmia where electrical signaling becomes chaotic, causing heart chambers to vibrate or quiver without functional pumping.

Ectopic beat

An irregular disturbance or anomalous spike appearing on an EKG produced by spontaneous electrical firing outside the normal conduction pathway.

Asystole

A state with zero detectable electrical activity in the heart displayed as a flat line on an EKG, which cannot be corrected with defibrillation alone.

Sphygmomanometer

The formal medical instrument, commonly called a blood pressure cuff, used to measure arterial pressures in units of $\text{mm}\,\text{Hg}$.

First heart sound ($S_1$)

The audible 'lub' sound resulting from the sudden closure of the atrioventricular (AV) valves at the beginning of ventricular systole.

Second heart sound ($S_2$)

The audible 'dub' sound caused by the snap closure of the semilunar valves at the onset of ventricular diastole.

Heart murmur

An abnormal clicking, gurgling, or regurgitation sound heard between $S_1$ and $S_2$ typically indicating turbulent flow through a defective valve.

Ventricular filling

The stage of the cardiac cycle where relaxed, low-pressure ventricles receive blood through open atrioventricular valves.

Diastasis

The slower, intermediate phase of ventricular filling that precedes the final squeeze provided by atrial contraction.

End-diastolic volume (EDV)

The maximal volume of blood contained within a ventricle at the very end of diastole, typically measuring up to $130\,\text{mL}$.

Isovolumetric contraction

The brief phase at the start of ventricular systole when all four valves are closed and tension builds without a change in ventricular blood volume.

Ventricular ejection

The phase of systole where rising ventricular pressure forces the semilunar valves open, expelling blood into the pulmonary trunk and aorta.

Stroke volume (SV)

The quantity of blood ejected from a single ventricle during one contraction cycle, averaging approximately $70\,\text{mL}$ in a resting adult.

End-systolic volume (ESV)

The leftover volume of blood that remains in a ventricle after ejection concludes, calculated by subtracting stroke volume from end-diastolic volume ($\text{ESV} = \text{EDV} - \text{SV}$).

Isovolumetric relaxation

The early diastolic phase where semilunar valves have snapped closed and AV valves have not yet opened, resulting in unchanged chamber volumes while the ventricles relax.

Cardioaccelerator center

A regulatory nucleus in the medulla oblongata that increases heart rate and contractile force through sympathetic fibers releasing norepinephrine.

Cardioinhibitory center

A nucleus in the medulla oblongata that sends signals via parasympathetic fibers in cranial nerve $10$ (vagus nerve) to slow the heart toward resting rate.

Cardiac output (CO)

The volume of blood pumped by a ventricle in one minute, calculated by the equation $\text{CO} = \text{HR} \times \text{SV}$.

Cardiac reserve

The difference between an individual's maximal cardiac output achieved during peak exercise and their resting cardiac output.

Proprioceptors

Sensory receptors monitoring joint movement and postural positioning that alert the medulla oblongata to adjust cardiac output during changes such as standing up.

Baroreceptors

Sensory stretch receptors situated in major vascular walls that monitor blood pressure and mediate the baroreflex.

Chemoreceptors

Receptors sensitive to arterial levels of carbon dioxide, pH, and oxygen that communicate with the cardiovascular center to adjust blood flow.

Preload

The degree of passive stretch and tension present in the ventricular muscle fibers immediately before contraction begins.

Contractility

The intrinsic force and degree of shortening produced by cardiomyocytes during contraction, independent of preload.

Afterload

The sum of all external forces, including arterial pressure and vascular resistance, that oppose ventricular blood ejection.