Heart Physiology and Cardiac Dynamics

Comprehensive vocabulary flashcards covering heart anatomy, electrophysiology, cardiac cycle phases, hemodynamic regulation, autonomic control, and fetal circulation.

Cardiac Conduction System

The specialized network of non-contractile autorhythmic cells (including the sinoatrial node, atrioventricular node, bundle of His, bundle branches, and Purkinje fibers) that initiates, coordinates, and distributes electrical action potentials across the heart in series: first the atria, then the ventricles.

Sinoatrial (SA) Node

The natural pacemaker of the heart situated in the right atrial wall that spontaneously initiates action potentials and sets the resting heart rate.

Atrioventricular (AV) Node

A nodal structure located in the inferior interatrial septum that delays the electrical impulse to allow adequate time for complete ventricular filling before ventricular systole.

Atrioventricular (AV) Bundle (Bundle of His)

The only electrical connection between the atria and ventricles that conducts action potentials through the electrically non-conductive fibrous skeleton of the heart.

Purkinje Fibers

Large-diameter conducting fibers extending from the bundle branches throughout the ventricular myocardium that rapidly transmit the action potential to trigger coordinated ventricular contraction.

Pacemaker Potential

The spontaneous, gradual depolarization of cardiac autorhythmic nodal cells from $-60\,\text{mV}$ to threshold ($-40\,\text{mV}$), characterized by the absence of a stable resting membrane potential.

Funny Current ($I_f$)

An inward $\text{Na}^+$ current through hyperpolarization-activated channels that opens at $-60\,\text{mV}$ in nodal pacemaker cells, initiating slow depolarization while outward $\text{K}^+$ permeability declines.

T-Type Calcium Channels

Transient voltage-gated $\text{Ca}^{2+}$ channels in pacemaker cells that open around $-50\,\text{mV}$ to bring the membrane potential to threshold.

L-Type Calcium Channels

Long-lasting voltage-gated $\text{Ca}^{2+}$ channels that open at threshold ($-40\,\text{mV}$) to drive the rapid depolarizing upstroke of nodal pacemaker action potentials, and open during the plateau phase of contractile myocardial cells.

Cardiac Action Potential Plateau

A sustained depolarized phase in contractile cardiac muscle cells maintained by the balance of slow $\text{Ca}^{2+}$ influx through L-type channels and $\text{K}^+$ efflux, which prolongs the absolute refractory period and prevents tetanus.

Absolute Refractory Period (Cardiac Muscle)

A period lasting approximately $200\,\text{ms}$ to $250\,\text{ms}$ during which voltage-gated $\text{Na}^+$ channels are inactivated and the cardiac muscle cell cannot be re-stimulated, preventing tetanic contractions and ensuring alternating contraction and relaxation cycles.

Electrocardiogram (ECG)

A composite recording of electrical potentials generated by heart cells during each cardiac cycle, consisting of characteristic waveforms: P wave, QRS complex, and T wave.

P Wave

The initial positive deflection on an ECG that corresponds to atrial depolarization and precedes atrial contraction.

QRS Complex

The series of sharp deflections on an ECG that represents ventricular depolarization and triggers ventricular contraction, while electrically masking atrial repolarization.

T Wave

The deflection on an ECG following the QRS complex that represents ventricular repolarization.

PR Interval

The ECG segment measured from the beginning of the P wave to the onset of the QRS complex, reflecting the conduction time required for the impulse to travel through the atria and the AV node.

ST Segment

The isoelectric portion of an ECG between the end of the QRS complex and the start of the T wave, corresponding to the plateau phase of the ventricular action potential.

Systole

The phase of the cardiac cycle when the heart chambers contract and pump blood into the adjacent chambers or arterial trunks.

Diastole

The phase of the cardiac cycle when heart muscle relaxes and chamber chambers fill with incoming blood.

Isovolumetric Contraction

The initial period of ventricular systole when both atrioventricular and semilunar valves are closed, causing intraventricular pressure to rise sharply without any change in blood volume.

Isovolumetric Relaxation

The early phase of ventricular diastole when all valves are closed following semilunar valve closure, during which ventricular pressure plunges without an alteration in blood volume.

Stroke Volume ($\text{SV}$)

The volume of blood ejected by each ventricle during a single contraction, defined mathematically as $\text{SV} = \text{EDV} - \text{ESV}$, averaging roughly $70\,\text{mL/beat}$ in a resting individual.

Cardiac Output ($\text{CO}$)

The total volume of blood pumped by a ventricle per minute, calculated as the product of heart rate and stroke volume ($\text{CO} = \text{HR} \times \text{SV}$).

End-Diastolic Volume ($\text{EDV}$)

The volume of blood present in a ventricle at the very end of diastole just prior to contraction, typically around $130\,\text{mL}$ at rest.

End-Systolic Volume ($\text{ESV}$)

The volume of blood left in a ventricle following ejection at the end of systole, typically about $60\,\text{mL}$ at rest.

Preload

The stretch on the ventricular muscle fibers immediately before systole, determined directly by the end-diastolic volume ($\text{EDV}$).

Frank-Starling Law of the Heart

The intrinsic physiological relationship wherein greater ventricular end-diastolic volume stretches sarcomeres closer to optimal length, producing more cross-bridges and increasing the force of contraction and stroke volume.

Afterload

The arterial pressure and vascular resistance that the ventricles must overcome to open the semilunar valves and eject blood, which increases in conditions like hypertension and atherosclerosis.

Positive Chronotropic Agents

Factors and substances that increase heart rate by enhancing SA and AV nodal cell depolarization, including sympathetic nerve stimulation (norepinephrine/epinephrine acting on $\beta_1$-adrenergic receptors), thyroid hormone, caffeine, and nicotine.

Negative Chronotropic Agents

Agents that decrease heart rate, notably acetylcholine released by the parasympathetic vagus nerve (CN X) acting on muscarinic receptors to cause hyperpolarization, as well as beta-blocker medications.

Inotropic Agents

Chemical substances that alter the contractility (force of contraction) of the myocardium independently of preload, with positive inotropes increasing sarcoplasmic $\text{Ca}^{2+}$ levels and negative inotropes (such as calcium channel blockers) decreasing them.

Atrial Reflex (Bainbridge Reflex)

An autonomic homeostatic response wherein increased venous return stretches right atrial stretch receptors, signaling medullary centers to inhibit parasympathetic vagal tone and elevate heart rate.

Foramen Ovale

An interatrial septum opening in the fetal cardiovascular system that shunts blood directly from the right atrium to the left atrium to bypass the nonfunctional fetal lungs, closing postnatally to form the fossa ovalis.

Ductus Arteriosus

A fetal vascular shunt that diverts blood from the pulmonary trunk directly into the aorta, bypassing pulmonary circulation, which closes after birth to become the ligamentum arteriosum.

Ductus Venosus

A fetal blood vessel that directs oxygenated blood from the umbilical vein directly into the inferior vena cava, bypassing the fetal liver, and converts postnatally into the ligamentum venosum.

Umbilical Vein

The single fetal vessel that carries oxygenated and nutrient-rich blood from the placenta to the fetus, which obliterates after birth to become the round ligament of the liver (ligamentum teres).

Umbilical Arteries

The paired fetal vessels that transport deoxygenated blood and metabolic wastes from the internal iliac arteries to the placenta, forming the medial umbilical ligaments postnatally.

Atherosclerosis

A cardiovascular condition marked by lipid plaque deposition (cholesterol, foam cells, dead cells, and cellular debris) within the arterial intima, narrowing arterial lumen diameter and increasing vascular resistance and afterload.

Congestive Heart Failure

A progressive cardiac condition categorized into diastolic heart failure (stiff, thickened ventricles unable to fill adequately) and systolic heart failure (thin, weakened ventricles unable to eject blood effectively).