Anatomy M10

Pericardium

The pericardium is a double-walled fibrous sac that encloses, protects, and anchors the heart. It consists of the fibrous pericardium (outer layer), parietal pericardium (lines the fibrous sac), visceral pericardium (epicardium covering the heart), and a pericardial cavity filled with serous fluid to reduce friction during heartbeats.

Heart Wall Layers

The heart wall consists of three layers: epicardium (outer protective layer and visceral pericardium), myocardium (thick cardiac muscle responsible for contraction), and endocardium (smooth inner endothelial lining that also lines blood vessels and helps prevent clot formation).

Epicardium vs. Myocardium vs. Endocardium

Epicardium is the outer covering of the heart, myocardium is the thick muscular pumping layer, and endocardium is the smooth inner lining that reduces friction and prevents blood clotting.

Myocardium

The myocardium is composed of cardiac muscle that contracts to pump blood. It contains abundant mitochondria, relies primarily on aerobic metabolism, has an extensive blood supply, and generates its own action potentials (automaticity) independent of nervous stimulation.

Intercalated Discs

Intercalated discs connect adjacent cardiac muscle cells. Desmosomes provide strong mechanical attachment, while gap junctions allow electrical impulses to spread rapidly so the myocardium contracts as one functional unit.

Heart Chambers & Their Functions

Right atrium receives deoxygenated blood from the body; right ventricle pumps it to the lungs. Left atrium receives oxygenated blood from the lungs; left ventricle pumps it to the body. The left ventricle has the thickest myocardium because it pumps blood through the systemic circulation.

Major Internal Heart Structures

Superior and inferior vena cava return blood to the right atrium; pulmonary trunk leaves the right ventricle; pulmonary veins return oxygenated blood to the left atrium; ascending aorta and aortic arch carry oxygenated blood from the left ventricle to the body.

Interventricular Septum

The interventricular septum is the muscular wall separating the right and left ventricles. It contains the right and left bundle branches, making it essential for both structural support and electrical conduction.

Papillary Muscles & Chordae Tendineae

Papillary muscles contract during ventricular systole, tightening the chordae tendineae to prevent the AV valves from prolapsing into the atria. They do not open or close the valves—they stabilize them during contraction.

Coronary Sinus

The coronary sinus collects deoxygenated blood from the myocardium through the cardiac veins and empties into the right atrium.

Heart Valves

The four valves maintain one-way blood flow through the heart. The tricuspid and mitral valves are atrioventricular (AV) valves located between the atria and ventricles, while the pulmonary and aortic valves are semilunar valves located between the ventricles and their great arteries.

Right-Sided vs. Left-Sided Valves

Right side: Tricuspid valve (RA→RV) and Pulmonary semilunar valve (RV→Pulmonary trunk). Left side: Mitral/Bicuspid valve (LA→LV) and Aortic semilunar valve (LV→Aorta).

AV Valves vs. Semilunar Valves

AV valves (tricuspid and mitral) allow blood to move from atria to ventricles and are supported by chordae tendineae and papillary muscles. Semilunar valves (pulmonary and aortic) regulate blood leaving the ventricles and do not have chordae tendineae.

Blood Flow Through the Heart

Body → Superior/Inferior Vena Cava → Right Atrium → Tricuspid Valve → Right Ventricle → Pulmonary Valve → Pulmonary Trunk → Pulmonary Arteries → Lungs → Pulmonary Veins → Left Atrium → Mitral Valve → Left Ventricle → Aortic Valve → Aorta → Body.Cardiac Conduction System

Cardiac Conduction Pathway

SA Node → AV Node → AV Bundle (Bundle of His) → Right & Left Bundle Branches → Purkinje Fibers → Ventricular myocardium. Remember: the impulse begins in the atria and spreads toward the ventricular apex before moving upward through the ventricles.

Sinoatrial (SA) Node

The heart's natural pacemaker located in the superior wall of the right atrium near the opening of the superior vena cava. It spontaneously generates action potentials that normally establish the heart rate (60–100 bpm).

Atrioventricular (AV) Node

Located in the inferior portion of the interatrial septum near the tricuspid valve. It delays the electrical impulse approximately 0.1 second, allowing complete ventricular filling before ventricular contraction begins.

AV Bundle (Bundle of His)

The only normal electrical connection between the atria and ventricles. It carries the impulse from the AV node into the interventricular septum before dividing into bundle branches.

Right & Left Bundle Branches

Travel through the interventricular septum toward the apex of the heart, carrying electrical impulses to each ventricle.

Purkinje Fibers

Large conducting fibers that rapidly distribute the electrical impulse throughout the ventricular myocardium, producing a coordinated contraction from the apex upward.

Why Ventricles Contract from the Apex Upward

Beginning contraction at the apex pushes blood upward through the pulmonary trunk and aorta, maximizing the efficiency of ventricular ejection.

Automaticity

The intrinsic ability of cardiac pacemaker cells to spontaneously depolarize without nervous stimulation. Although the autonomic nervous system modifies heart rate, it does not initiate the heartbeat.

Conducting vs. Contractile Cells

Conducting cells generate and rapidly transmit electrical impulses, while contractile cells make up most of the myocardium and produce the force needed to eject blood.

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart, not the mechanical contraction. Each wave corresponds to depolarization or repolarization of specific cardiac chambers.

ECG Components

P wave = atrial depolarization; PR interval = AV node delay and atrial-to-ventricular conduction; QRS complex = ventricular depolarization (atrial repolarization occurs simultaneously but is hidden); ST segment = ventricles remain depolarized during contraction; T wave = ventricular repolarization.

Relationship Between the ECG & Contraction

Electrical depolarization always occurs before mechanical contraction. The ECG shows when electrical events occur, allowing clinicians to predict when the myocardium contracts and relaxes.

Conduction System Summary

The SA node initiates the heartbeat, the AV node briefly delays conduction, the Bundle of His and bundle branches rapidly transmit the impulse through the septum, and Purkinje fibers stimulate coordinated ventricular contraction.

Clinical Importance of the Conduction System

Damage to the SA node, AV node, bundle branches, or Purkinje fibers can produce arrhythmias or conduction blocks, disrupting normal cardiac rhythm and pumping efficiency.Cardiac Conduction System

Cardiac Conduction Pathway

SA Node → AV Node → AV Bundle (Bundle of His) → Right & Left Bundle Branches → Purkinje Fibers → Ventricular myocardium. Remember: the impulse begins in the atria and spreads toward the ventricular apex before moving upward through the ventricles.

Sinoatrial (SA) Node

The heart's natural pacemaker located in the superior wall of the right atrium near the opening of the superior vena cava. It spontaneously generates action potentials that normally establish the heart rate (60–100 bpm).

Atrioventricular (AV) Node

Located in the inferior portion of the interatrial septum near the tricuspid valve. It delays the electrical impulse approximately 0.1 second, allowing complete ventricular filling before ventricular contraction begins.

AV Bundle (Bundle of His)

The only normal electrical connection between the atria and ventricles. It carries the impulse from the AV node into the interventricular septum before dividing into bundle branches.

Right & Left Bundle Branches

Travel through the interventricular septum toward the apex of the heart, carrying electrical impulses to each ventricle.

Purkinje Fibers

Large conducting fibers that rapidly distribute the electrical impulse throughout the ventricular myocardium, producing a coordinated contraction from the apex upward.

Why Ventricles Contract from the Apex Upward

Beginning contraction at the apex pushes blood upward through the pulmonary trunk and aorta, maximizing the efficiency of ventricular ejection.

Automaticity

The intrinsic ability of cardiac pacemaker cells to spontaneously depolarize without nervous stimulation. Although the autonomic nervous system modifies heart rate, it does not initiate the heartbeat.

Conducting vs. Contractile Cells

Conducting cells generate and rapidly transmit electrical impulses, while contractile cells make up most of the myocardium and produce the force needed to eject blood.

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart, not the mechanical contraction. Each wave corresponds to depolarization or repolarization of specific cardiac chambers.

ECG Components

P wave = atrial depolarization; PR interval = AV node delay and atrial-to-ventricular conduction; QRS complex = ventricular depolarization (atrial repolarization occurs simultaneously but is hidden); ST segment = ventricles remain depolarized during contraction; T wave = ventricular repolarization.

Relationship Between the ECG & Contraction

Electrical depolarization always occurs before mechanical contraction. The ECG shows when electrical events occur, allowing clinicians to predict when the myocardium contracts and relaxes.

Conduction System Summary

The SA node initiates the heartbeat, the AV node briefly delays conduction, the Bundle of His and bundle branches rapidly transmit the impulse through the septum, and Purkinje fibers stimulate coordinated ventricular contraction.

Clinical Importance of the Conduction System

Damage to the SA node, AV node, bundle branches, or Purkinje fibers can produce arrhythmias or conduction blocks, disrupting normal cardiac rhythm and pumping efficiency.