The Heart – Comprehensive Bullet-Point Notes

Location & Shape of the Heart

• Situated in the thoracic cavity, specifically within the mediastinum
  • Lies between the lungs, posterior to the sternum, superior to the diaphragm
• Base (superior, broader region)
  • Oriented toward the right shoulder; location of great vessels
• Apex (inferior, pointed tip)
  • Directed anteriorly and leftward, resting on the diaphragm at ~5th intercostal space
• Landmarks (sagittal view)
  • 1st rib, trachea, aortic arch, thymus, esophagus, diaphragm
  • Associated vessels: superior & inferior vena cava, pulmonary trunk, thoracic aorta

Membranes & Layers of the Heart Wall

• Fibrous pericardium
  • Tough, dense connective tissue; anchors heart and prevents over-stretching
• Serous pericardium (double layer)
  • Parietal layer (lines fibrous sac)
  • Pericardial cavity (filled with serous fluid → reduces friction)
  • Visceral layer = epicardium (outer surface of heart)
• Myocardium
  • Thick muscular layer; contains contractile & conductive cells
• Endocardium
  • Smooth endothelial lining; continuous with vessel endothelium; minimizes turbulence

External Structures (Major Surface Anatomy)

• Great vessels emerging from base
  • Ascending aorta → aortic arch → brachiocephalic trunk, left common carotid, left subclavian
  • Pulmonary trunk → right & left pulmonary arteries
  • Superior & inferior vena cava
• Chambers & auricles
  • Right & left atria (with auricles), right & left ventricles
• Coronary vessels (anterior)
  • Right coronary artery, marginal artery, anterior cardiac veins, small cardiac vein
  • Left coronary artery → circumflex & anterior interventricular (LAD) branches; great cardiac vein
• Coronary vessels (posterior)
  • Posterior interventricular artery (from right coronary)
  • Coronary sinus (main venous collector), middle & posterior cardiac veins

Internal Structures & Septa

• Right atrium
  • Receives systemic blood via SVC, IVC; contains fossa ovalis (remnant of fetal foramen ovale)
• Right ventricle
  • Tricuspid valve, chordae tendineae, papillary muscles, trabeculae carneae, moderator band
• Left atrium
  • Receives four pulmonary veins
• Left ventricle
  • Thickest wall; forms apex; contains mitral valve & aortic valve
• Interventricular septum separates ventricles; interatrial septum separates atria

Heart Valves & Alternative Names

• Right atrioventricular (AV) valve = Tricuspid
• Left AV valve = Bicuspid = Mitral
• Pulmonary valve = Right semilunar valve
• Aortic valve = Aortic semilunar valve

Path of Blood Flow (Pulmonary & Systemic Circuits)

1. SVC/IVC → Right atrium
2. Through tricuspid valve → Right ventricle
3. Through pulmonary semilunar valve → Pulmonary trunk → Pulmonary arteries → Pulmonary capillaries (gas exchange)
4. Pulmonary veins → Left atrium
5. Through mitral valve → Left ventricle
6. Through aortic semilunar valve → Aorta → Systemic arteries → Systemic capillaries (upper & lower body)
7. Systemic veins return to SVC/IVC

Heart Sounds

• S1 (“lubb”)
  • Closure of AV valves at onset of ventricular systole; longest/loudest
• S2 (“dubb”)
  • Closure of semilunar valves at onset of ventricular diastole
• Valve status examples
  • During systole: AV valves closed, semilunar valves open; chordae tendineae tight, papillary muscles contracted
  • During diastole: AV valves open, semilunar valves closed

Coronary Circulation (Myocardial Blood Supply)

• Arterial supply
  • Right coronary artery → marginal artery, posterior interventricular artery; supplies RA & RV
  • Left coronary artery → circumflex artery (LA & LV) and anterior interventricular (LAD) artery (IV septum & anterior LV)
• Venous drainage
  • Great, middle, small, posterior cardiac veins → Coronary sinus → Right atrium
• Clinical relevance: blockage → myocardial infarction; LAD nickname “widow-maker”

Cardiac Conduction System & Electrical Impulses

• Tissue types
  • Myocardial contractile cells (99%)
  • Myocardial conductive cells (1%) – self-depolarize
• Intrinsic rhythm: fastest cell dominates (normally SA node)
• Components & sequence
  1. Sinoatrial (SA) node (≈80-100 bpm at rest) – pacemaker
  2. Atrial internodal pathways (anterior, middle, posterior) & Bachman’s bundle to left atrium
  3. Atrioventricular (AV) node (delays impulse ≈0.1 s)
  4. AV bundle (Bundle of His)
  5. Right & left bundle branches
  6. Purkinje fibers → ventricular myocardium
• Cardiac skeleton (fibrous connective tissue)
  • Electrically insulates atria from ventricles; provides valve anchorage

Neural & Hormonal Control of Heart Rate

• Without extrinsic input: SA node ≈ 80!\text{–}!100\;\text{bpm}
• Medullary centers
  • Cardioacceleratory center (sympathetic) → norepinephrine → ↑ HR & contractility
  • Cardioinhibitory center (parasympathetic via Vagus) → acetylcholine → ↓ HR
• Receptor inputs
  • Baroreceptors: ↑ stretch → ↑ parasymp / ↓ symp → ↓ HR (and vice-versa)
  • Proprioreceptors: ↑ movement → ↑ symp → ↑ HR
  • Chemoreceptors: ↓ O2, ↑ CO2, ↑ H^+ → ↑ symp → ↑ HR
  • Limbic system: stress/emotion anticipates ↑ HR
• Hormonal influences
  • Adrenal catecholamines (E, NE), thyroid hormones (T3, T4) → ↑ HR & force

Electrocardiogram (ECG/EKG)

• Waves & intervals
  • P wave: atrial depolarization
  • QRS complex: ventricular depolarization (atrial repolarization hidden)
  • T wave: ventricular repolarization
  • PR interval: SA → AV conduction time
  • QT interval: total ventricular depolarization & repolarization duration
  • Segments: P-R (isoelectric), S-T (plateau phase)
• Clinical use: arrhythmia, ischemia, electrolyte imbalance detection

Cardiac Cycle (Mechanical Events)

• One cycle = atrial systole & diastole + ventricular systole & diastole
• Phases
  1. Ventricular filling (late diastole) – AV valves open, 70-80 % passive
  2. Atrial systole – tops off ventricles (~20-30 %)
  3. Isovolumetric (isovolumic) contraction – all valves closed; pressure rises; S1
  4. Ventricular ejection – semilunar valves open; stroke volume expelled
  5. Isovolumetric relaxation – all valves closed; S2
  6. Ventricular filling begins again when ventricular pressure < atrial pressure

Cardiac Output (CO)

• Definition: volume of blood pumped by each ventricle per minute
• Formula: CO = HR \times SV
  • Typical resting values: HR \approx 75\;\text{bpm},\ SV \approx 70\;\text{mL} → CO \approx 5\;\text{L/min}

Stroke Volume (SV) & Influencing Variables

• Formula: SV = EDV - ESV
  • End-diastolic volume (EDV) = preload (ventricular filling)
  • End-systolic volume (ESV) = blood left after ejection; affected by afterload & contractility
• Preload
  • ↑ venous return / fast filling → ↑ EDV → ↑ SV (Frank-Starling law)
  • ↓ thyroid hormones, hypoxia, abnormal ions, low temp → ↓ EDV → ↓ SV
• Contractility (inotropy)
  • ↑ sympathetic stimulation, E/NE, high Ca^{2+}, thyroid hormones, glucagon → ↓ ESV → ↑ SV
  • Parasymp, hypoxia, hyperkalemia, Ca-channel blockers → ↑ ESV → ↓ SV
• Afterload
  • ↑ vascular resistance or semilunar valve damage → ↑ ESV → ↓ SV
  • ↓ resistance → ↓ ESV → ↑ SV
• Relationship to HR
  • Extremely high HR shortens filling time → ↓ SV; optimal HR maximizes CO

Factors Modifying Heart Rate & Contractile Force

• Increasing factors (Table 19.1 highlights)
  • Sympathetic nerves (NE), E/NE from adrenal medulla
  • High firing of propioreceptors (exercise), chemoreceptor signals (↓ O2 / ↑ CO2 / ↑ H^+), limbic anticipation
  • ↓ baroreceptor firing (low BP), ↑ body temperature, stimulants (nicotine, caffeine)
  • Hormones: thyroid (T3, T4), catecholamines
  • Electrolytes: ↓ Ca^{2+}, ↓ K^+, ↓ Na^+ (note: certain ion shifts alter excitability)
• Decreasing factors
  • Parasympathetic (vagus, ACh)
  • Resting proprioreceptors, chemoreceptor signals (↑ O2 / ↓ CO2 / ↓ H^+)
  • ↑ baroreceptor firing (high BP), low body temperature
  • Hormonal: ↓ thyroid, ↓ catecholamines
  • Electrolyte excess: ↑ K^+, ↑ Na^+, ↓ Ca^{2+}
  • Drugs: opiates, tranquilizers, depressants

Clinical & Real-World Connections

• Pericardial effusion → cardiac tamponade (fluid in pericardial cavity restricts filling)
• Valve stenosis or insufficiency can alter heart sounds (murmurs)
• Coronary artery disease leads to ischemia; importance of lifestyle & lipid control
• Monitoring ECG vital in emergency medicine, athletics, anesthesia
• Cardiac output measurement guides fluid therapy & drug titration in critical care