Cardiac Physiology and Conductive System

These vocabulary flashcards cover the intrinsic regulation of heart pumping, including cardiac output, the Frank Starling mechanism, the cardiac conductive system, pacemaker potentials, and abnormal sinus rhythms.

Cardiac output (CO)

The volume of blood pumped by each ventricle per minute, calculated as $CO = SV \times HR$. It is approximately $5\,liters/min$ during rest.

Heart rate (H.R.)

The number of heart beats per unit time, specifically contractions of the ventricles. Under basal conditions, it ranges from $60-100\,beat/min$ with an average of $70\,beat/min$.

Stroke volume (SV)

The volume of blood pumped by each ventricle per beat, typically about $70-90\,ml/beat$ during rest.

Preload

The degree of tension on the ventricular myocardium just prior to contraction, representing the volume of blood (End Diastolic Volume) in the ventricle at the end of diastole.

End Diastolic volume (E.D.V.)

The volume of blood remaining in the ventricle at the end of diastole, just before contraction, measuring approximately $110-130\,ml$.

Afterload

The arterial pressure (blood pressure in the aorta and pulmonary trunk) against which the ventricle must contract to exert its contractile force.

Frank Starling law

States that within physiological limits, the force of myocardial contraction is directly proportional to the initial resting length of the myocardial fibers (End Diastolic Volume).

Contractile cells

The working myocardium cells which constitute the bulk of the muscle cells within the atria and the ventricles.

Conductile cells

Also known as autorhythmic cells, these specialized muscle cells are involved in the initiation and propagation of action potentials, including the SAN, AVN, and Purkinje fibres.

Sinoatrial node (SA node)

The normal pacemaker of the human heart located in the posterior wall of the right atrium. It generates the highest frequency of impulses and initiates the cardiac rhythm.

Atrioventricular node (AV node)

A component of the conductive system located between the atrium and ventricle that connects to the SA node via internodal pathways.

Purkinje fibers

Large specialized fibers that form a network in both ventricles and transmit action potentials rapidly at a velocity of $1.5$ to $4\,m/sec$.

Rhythmicity

The ability of the cardiac muscle to initiate its own rhythm and beat regularly without any extrinsic stimulus.

Pacemaker Potential

The resting membrane potential of autorhythmic cells ($-55$ to $-60\,mV$), which is less negative than other cells due to a natural leakiness to sodium and calcium ions.

Phase 1: Prepotential

The initial phase of the SAN action potential characterized by reduced permeability to $K^+$ and increased permeability to $Na^+$ and $Ca^{2+}$.

Phase 2: Depolarization

The phase of the SAN action potential occurring when the firing level ($-40\,mV$) is reached, caused by the entry of $Ca^{2+}$ and $Na^+$.

Phase 3: Repolarization

The phase of the SAN action potential that occurs through an increase in $K^+$ efflux.

Sinus rhythm

The spontaneous rate of firing of the SAN, which is $100\,impulses/minute$ without nerve supply, and $60-90\,impulses/min$ under autonomic control.

Nodal rhythm

The heart rhythm established by the AV node ($40-60\,impulses/min$) if the SA node is damaged or blocked.

Idioventricular rhythm

The heart rhythm established by the Purkinje fibers ($20-40\,beats/min$) if the AV node fails.

Vagal Tone

A continuous inhibitory effect exerted by the vagus nerves during rest that decreases the inherent sinus rhythm from about $120$ to $70\,impulses/min$.

Atrioventricular delay

A slow propagation through the AV node ($0.05\,m/sec$) lasting about $0.1$ seconds, allowing atria to empty into ventricles before ventricular contraction.

Sinus tachycardia

An abnormal sinus rhythm in adults characterized by a fast heart rate of more than $100\,beats/minute$, often caused by exercise or sympathetic stimulation.

Sinus bradycardia

An abnormal sinus rhythm characterized by a slow heart rate of less than $60\,beats/minute$, often caused by parasympathetic stimulation or high blood pressure.