Overview of Cardiac Physiology and Conduction System

Cardiomyocyte

Heart muscle cell with unique structural features.

Cardiac conduction system

Network of cells initiating heart contractions.

Action potentials

Electrical signals triggering muscle contraction.

Electrocardiogram (ECG)

Graphical representation of heart's electrical activity.

Systole

Phase of heart contraction and blood ejection.

Diastole

Phase of heart relaxation and filling with blood.

Cardiac cycle

Sequence of events during one heartbeat.

Intercalated discs

Connections between cardiomyocytes for synchronized contraction.

Desmosomes

Protein structures linking adjacent cardiac cells.

Gap junctions

Channels allowing electrical signal passage between cells.

Sarcoplasmic reticulum (SR)

Organelle storing calcium ions in muscle cells.

T tubules

Extensions of cell membrane facilitating calcium entry.

Calcium ions (Ca2+)

Key regulator of cardiac muscle contraction strength.

Epinephrine

Hormone increasing heart contraction force via calcium.

Isradipine

Drug blocking calcium channels, slowing heart rate.

Mitochondria

Organelles producing ATP through cellular respiration.

Aerobic respiration

Oxygen-dependent ATP production method in cardiomyocytes.

Anaerobic respiration

ATP production without oxygen, less efficient.

Autorhythmic fibers

Self-excitable cells generating heart rhythm.

Sinoatrial (SA) node

Primary pacemaker located in right atrium.

Atrioventricular (AV) node

Node transmitting signals from atria to ventricles.

Bundle of His

Pathway conducting impulses from AV node to ventricles.

Purkinje fibers

Conduct action potentials rapidly in the myocardium.

Apex contraction

Contraction starts at heart's apex via Purkinje fibers.

SA node

Primary pacemaker, initiates action potentials every 0.6 sec.

Normal sinus rhythm

Regular heart rhythm established by SA node.

AV node

Slows action potentials before reaching ventricles.

AV bundle

Transmits action potentials with smaller diameter fibers.

Cardiac action potentials

Initiated by SA node, excite contractile fibers.

Phase 0

Rapid depolarization due to Na+ channel opening.

Phase 1

Slight dip in AP from K+ outflow.

Phase 2

Plateau phase; balance of Ca2+ and K+ channels.

Phase 3

Repolarization phase with K+ channel opening.

Phase 4

Resting membrane potential at -90 mV.

Refractory period

Interval preventing second contraction during phases.

Electrocardiogram (ECG)

Records electrical activity of the heart.

P wave

Depolarization of both atria in ECG.

QRS complex

Depolarization of both ventricles in ECG.

T wave

Repolarization of both ventricles in ECG.

Lead II trace

Shows cardiac cycle events in ECG.

Cardiac cycle duration

One complete heartbeat lasts about 0.8 seconds.

Heart rate at rest

Adult heart beats approximately 75 times per minute.

Calcium's role

Increased Ca2+ triggers contraction in cardiac AP.

Tetanus in cardiac muscle

Would prevent normal rhythmic contractions.

Contractile fibers

Working cardiomyocytes that contract during AP.

Conduction system

Initiates heart contraction sequence.

Atria contraction

First phase of heart contraction.

Ventricles contraction

Follows atrial contraction in sequence.

Volume changes

Alteration in chamber sizes during contraction.

Pressure changes

Result from volume changes and valve actions.

High pressure

Drives blood from ventricles to body.

Systole

Phase of heart contraction.

Diastole

Phase of heart relaxation.

Ventricular filling

Phase where ventricles fill with blood.

Atrial contraction

Phase where atria contract to push blood.

Isovolumetric contraction

Phase with closed valves and no volume change.

Ventricular ejection

Phase where blood is expelled from ventricles.

Isovolumetric relaxation

Phase with closed valves and decreasing ventricular pressure.

End-diastolic volume

Volume in ventricles before contraction, ~130 ml.

Passive ventricular filling

Blood flows into ventricles without atrial contraction.

Atrial depolarization

Electrical activity leading to atrial contraction.

Ventricular depolarization

Electrical activity leading to ventricular contraction.

QRS complex

Represents ventricular depolarization on ECG.

T wave

Represents ventricular repolarization on ECG.

AV valves

Valves between atria and ventricles.

SL valves

Semilunar valves between ventricles and arteries.

Pressure gradient

Difference in pressure driving blood flow.

Atrial systole

Atrial contraction phase in cardiac cycle.

Ventricular systole

Ventricular contraction phase in cardiac cycle.