Heart Lab Review

Heart Location and Orientation

• Sits within the pericardial cavity of the thoracic cavity.
  • The cavity is enclosed by the double-layered pericardium.
• Lies posterior to the sternum and slightly left of the body’s midsagittal plane.
• Nestled between the lungs in the mediastinum.
  • Clinical relevance: a widened mediastinum on a chest radiograph can indicate aortic injury, pericardial effusion, or lymphadenopathy.
• Apex points inferiorly and anteriorly; base faces posteriorly toward T6–T9 vertebrae and is largely formed by the left atrium.

Pericardium and Heart Wall Layers

• Fibrous pericardium
  • Dense irregular connective tissue.
  • Anchors heart to diaphragm and great vessels; prevents over-distension.
• Serous pericardium (two layers)
  • Parietal layer: lines inner surface of fibrous pericardium.
  • Visceral layer (epicardium): adheres tightly to the heart surface.
• Pericardial cavity
  • Potential space between parietal and visceral layers.
  • Filled with serous fluid → reduces friction during the cardiac cycle.
• Wall layers (superficial → deep)
  • Epicardium: simple squamous epithelium + areolar & adipose CT; houses coronary vessels.
  • Myocardium: cardiac muscle fibers responsible for contraction.
  • Thickness varies: thicker in the ventricles (especially the left) because of systemic pressure requirements.
  • Endocardium: simple squamous endothelium + areolar CT; continuous with the endothelium of blood vessels; prevents turbulent blood flow and thrombosis.

Macroscopic Chambers, Valves, and Great Vessels

• Four chambers
  • Right atrium (RA)
  • Left atrium (LA)
  • Right ventricle (RV)
  • Left ventricle (LV)
• Valves
  • Atrioventricular (AV) valves
  • Tricuspid valve (RA → RV)
  • Bicuspid/mitral valve (LA → LV)
  • Semilunar valves
  • Pulmonary (RV → pulmonary trunk)
  • Aortic (LV → aorta)
  • Function: ensure unidirectional blood flow; closure produces characteristic heart sounds (S1 = AV closure, S2 = semilunar closure).
• Great vessels
  • Superior & inferior venae cavae (systemic venous return to RA)
  • Pulmonary trunk → right & left pulmonary arteries (deoxygenated blood to lungs)
  • Pulmonary veins (oxygenated blood to LA)
  • Aorta (systemic arterial outflow; ascending, arch, descending thoracic segments)

Internal Chamber Features

• Right Atrium
  • Fossa ovalis: remnant of fetal foramen ovale; failure to close → patent foramen ovale.
  • Openings for superior vena cava, inferior vena cava, and coronary sinus.
  • Pectinate muscles line anterior wall and auricle.
• Right Ventricle
  • Trabeculae carneae: irregular muscular ridges that prevent suction.
  • Papillary muscles connect to tricuspid valve cusps via chordae tendineae; prevent valve prolapse during systole.
• Left Atrium
  • Receives four pulmonary veins; walls mostly smooth.
• Left Ventricle
  • Thickest myocardium; generates systemic pressure (~120\,\text{mmHg}).
  • Two robust papillary muscles tether mitral valve via chordae tendineae.
  • Aortic vestibule funnels blood toward the aortic semilunar valve.

Septa

• Interatrial septum: separates atria; houses fossa ovalis.
• Interventricular septum
  • Muscular portion (majority) & membranous portion (thin, fibrous) — common site of congenital VSDs.
• Atrioventricular septum: fibrous skeleton providing electrical insulation and valve anchoring.

External Surface Anatomy

• Anterior (sternocostal) surface
  • Right atrium & ventricle dominate.
  • Visible vessels: right coronary artery (RCA) in coronary sulcus, anterior interventricular artery (LAD) in anterior IV sulcus, circumflex artery, pulmonary trunk, aorta, SVC, auricles.
• Posterior (diaphragmatic) surface
  • Mostly left atrium and left ventricle.
  • Coronary sinus runs in posterior coronary sulcus; opens into RA.
  • Posterior interventricular artery (PDA) and middle cardiac vein travel in posterior IV sulcus.

Histology of Cardiac Muscle

• Cardiac myocytes
  • Short, branched, striated cells with one (occasionally two) central nuclei.
  • Rich in mitochondria (≈25\% of cell volume) → fatigue-resistant.
• Intercalated discs
  • Desmosomes: mechanical coupling.
  • Gap junctions: electrical coupling for rapid impulse spread → functional syncytium.
• Endomysium connects to cardiac skeleton assisting in force transmission.

Coronary Circulation

• Coronary arteries (originate from ascending aorta just above aortic valve cusps)
  • Right Coronary Artery (RCA)
  • SA nodal branch (≈60\% supply to SA node)
  • Right marginal branch (lateral RV)
  • Posterior interventricular artery (PDA) in right-dominant hearts (≈70\% of people)
  • Left Coronary Artery (LCA)
  • Anterior interventricular artery (LAD) — “widow-maker”; supplies anterior IV septum & anterior LV.
  • Circumflex artery → left marginal branches; supplies lateral/posterior LV.
• Coronary veins
  • Great cardiac vein (runs with LAD)
  • Middle cardiac vein (runs with PDA)
  • Small cardiac vein (runs with right marginal a.)
  • Coronary sinus: receives above veins; drains into RA.
• Clinical significance
  • Myocardial infarction results from occlusion; LAD blockage most lethal.

Cardiac Conduction System & Electrocardiography (ECG)

• Intrinsic conduction pathway
  1. Sinoatrial (SA) node (pacemaker, \approx 70\,\text{bpm})
  2. Internodal pathways / Bachmann’s bundle to left atrium
  3. Atrioventricular (AV) node (gatekeeper; intrinsic 40!–!60\,\text{bpm}; introduces \approx 0.1\,\text{s} delay)
  4. AV bundle (Bundle of His) → right & left bundle branches
  5. Purkinje fibers (subendocardial network) → ventricular myocardium.
• ECG components
  • P wave: atrial depolarization.
  • P–Q segment: AV nodal delay.
  • QRS complex: ventricular depolarization (atrial repolarization hidden).
  • S–T segment: ventricles in plateau phase; diagnostic for ischemia (elevated/depressed).
  • T wave: ventricular repolarization.
  • P–R interval: onset of atrial depol → onset of QRS; prolonged in AV block.
  • Q–T interval: beginning of QRS → end of T; reflects ventricular action potential duration.
• Heart rate calculation
  Heart\ Rate = \frac{60}{R\text{–}R\ Interval\,(\text{s})}
• ECG abnormalities
  • Tachycardia: HR >100\,\text{bpm} (physiologic vs. pathologic).
  • Bradycardia: HR <60\,\text{bpm} (athletes vs. nodal disease).
  • Fibrillation: chaotic electrical activity; atrial (A-fib) vs. life-threatening ventricular (V-fib).

Laboratory & Practical Components

• Vernier EKG sensor used to collect Lead II recordings (RA-LL configuration).
• Labeling practice covers:
  • External landmarks (e.g., ligamentum arteriosum, auricles, coronary sulci).
  • Internal structures on dissected hearts & models.
  • Microscopic identification of cardiac tissues (myocardium, endocardium, epicardium, intercalated discs).
• Ethical/clinical relevance
  • Accurate anatomical knowledge guides catheterization, valve replacement, and bypass grafting.
  • Understanding conduction & ECG fundamentals essential for rhythm interpretation in emergency settings.