Cardiovascular System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Cardiovascular System — Key Facts for NEET PG

• Heart: 4 chambers — RA, RV, LA, LV; Left side = systemic, Right side = pulmonary circulation
• Cardiac Output (CO): HR × SV; Normal ~5 L/min; Fick principle: CO = VO₂/(arterial-venous O₂ difference)
• Coronary Circulation: RCA (right coronary artery) supplies SA node, AV node (in 60%), right heart; LCA supplies rest
• Cardiac Action Potential: Pacemaker (slow Ca²⁺ channels) vs. Ventricular (fast Na⁺/K⁺ channels)
• ⚡ Exam tip: Left main coronary bifurcates into LAD (supplies anterior 2/3 of interventricular septum) and LCX

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Cardiovascular System — NEET PG Study Guide

Heart Structure

Chambers:

• Right Atrium: Receives systemic venous blood (SVC, IVC, coronary sinus)
• Right Ventricle: Pumps to pulmonary circulation
• Left Atrium: Receives pulmonary venous blood
• Left Ventricle: Pumps to systemic circulation (thickest wall)

Valves:

• Atrioventricular (AV): Tricuspid (RA→RV), Mitral (LA→LV)
• Semilunar (SL): Pulmonary (RV→PA), Aortic (LV→Aorta)

Blood Flow: Systemic → SVC/IVC → RA → Tricuspid → RV → Pulmonary valve → PA → Lungs → PV → LA → Mitral → LV → Aortic valve → Aorta → Systemic

Cardiac Cycle

Systole: Contraction phase Diastole: Relaxation phase

Phases:

1. Isovolumetric Contraction: AV valves close, SL valves not yet open, pressure rises
2. Ejection: SL valves open, blood ejected
3. Isovolumetric Relaxation: SL valves close, AV valves not yet open, pressure falls
4. Rapid Filling: AV valves open, ventricle fills
5. Atrial Systole (“atrial kick”): 15-25% of filling in late diastole

NCE Exam Pattern

Common question types:

1. Heart structure and blood flow
2. Cardiac conduction system
3. ECG interpretation
4. Coronary circulation
5. Cardiac output regulation

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Cardiovascular System — Comprehensive NEET PG Notes

Detailed Theory

1. Heart Wall Structure

Endocardium: Inner layer — endothelial cells, connective tissue

Myocardium: Middle layer — cardiac muscle, contractile

• Branching, striated, involuntary
• Intercalated discs: Gap junctions (ionic coupling), desmosomes (mechanical)
• Rich capillary supply
• Contraction: ALL or NONE at cellular level (functional syncytium)

Pericardium:

• Fibrous pericardium: Tough outer layer
• Serous pericardium: Parietal (fibrous) and visceral (epicardium)
• Pericardial cavity: Contains 10-30 mL serous fluid
• Protects, anchors, prevents overdistension

2. Cardiac Conduction System

Components (in order of activation):

1. SA Node (Sinoatrial): Primary pacemaker, 60-100 bpm

   • Location: Right atrium, junction of SVC and right atrial appendage
   • “Pacemaker of heart”
   • Rich sympathetic and parasympathetic innervation

2. Internodal Pathways: Anterior, middle, posterior tracts

   • Conduct from SA to AV node

3. AV Node (Atrioventricular): Secondary pacemaker, 40-60 bpm

   • Location: Interatrial septum, near coronary sinus ostium
   • Delays impulse (AV delay ~120 ms) to allow atrial contraction before ventricular contraction

4. Bundle of His: Penetrates fibrous skeleton

   • Only electrical connection between atria and ventricles

5. Bundle Branches: Right and Left

   • Left bundle: Fascicular blocks common

6. Purkinje Fibers: Distribute to ventricular myocardium

   • Fastest conduction (4 m/s)
   • Ensures coordinated ventricular contraction

Intrinsic Rates:

• SA: 100 bpm → normally dominant
• AV: 40-60 bpm
• Purkinje: 20-40 bpm

3. Cardiac Action Potential

Ventricular Myocyte:

• Phase 0: Rapid depolarization — Na⁺ influx (fast Na⁺ channels)
• Phase 1: Initial repolarization — K⁺ efflux, Na⁺ inactivation
• Phase 2: Plateau — Ca²⁺ influx (L-type) balanced by K⁺ efflux
• Phase 3: Rapid repolarization — Ca²⁺ channels close, K⁺ efflux increases
• Phase 4: Resting potential — K⁺ leak, Na⁺/K⁺ ATPase

SA Node Pacemaker Cell:

• Phase 0: Ca²⁺ influx (T-type then L-type Ca²⁺)
• Phase 3: Repolarization — K⁺ efflux
• Phase 4: Slow depolarization (funny current, If) — pacemaker potential
  • Funny current (If): Mixed Na⁺/K⁺ inward current
  • When threshold reached (~-40 mV) → AP
• No stable resting potential (automaticity)

4. Electrocardiogram (ECG)

Lead Systems:

• Limb leads: I, II, III, aVR, aVL, aVF
• Precordial leads: V1-V6
• Standard speed: 25 mm/s (1 small square = 0.04 s, 1 large square = 0.2 s)

Waves and Intervals:

• P wave: Atrial depolarization (≤0.12 s)
• PR interval: AV conduction time (0.12-0.20 s)
• QRS complex: Ventricular depolarization (≤0.12 s)
• ST segment: Ventricular plateau (isoelectric — indicates no difference)
• T wave: Ventricular repolarization
• QT interval: Total ventricular activity (corrected: QTc = QT/√RR)

Mean Electrical Axis:

• Normal: -30° to +90°
• Left axis deviation: < -30° (left anterior fascicular block, left ventricular hypertrophy)
• Right axis deviation: > +90° (right ventricular hypertrophy, LAFB)
• Extreme axis: > +180° or < -90°

5. Cardiac Cycle — Detailed

Late Diastole (0.7 s):

• Atrial and ventricular relaxation
• Rapid filling phase (80% of filling)
• Atrial systole (atrial kick) — 20% of filling
• End-diastolic volume (EDV): ~120 mL

Systole (0.3 s):

• Isovolumetric contraction: All valves closed, pressure rises rapidly
• Ventricular ejection: SL valves open, blood ejected
• End-systolic volume (ESV): ~50 mL
• Stroke volume: EDV - ESV = ~70 mL (ejection fraction 60-70%)

Diastole (0.5 s):

• Isovolumetric relaxation: SL valves close, AV valves still closed
• Rapid filling: AV valves open, passive filling
• Atrial systole: “atrial kick”

6. Cardiac Output

Formula: CO = HR × SV

Normal Values:

• HR: 60-100 bpm
• SV: 60-100 mL/beat
• CO: 4-8 L/min

Regulation:

• Heart Rate: Sympathetic (↑HR), Parasympathetic (↓HR)
• Stroke Volume: Preload, Afterload, Contractility

Frank-Starling Mechanism:

• Increased preload → Increased SV
• Sarcomere length-tension relationship
• Optimized overlap of actin-myosin at EDV ~120 mL

Preload: End-diastolic volume/pressure (venous return) Afterload: Pressure ventricle works against (systemic vascular resistance) Contractility: Inotropic state (sympathetic activation increases)

7. Coronary Circulation

Right Coronary Artery (RCA):

• Supplies: RA, RV, posterior 1/3 of IVS, SA node (60%), AV node (60%), posterior wall
• Dominance: Right dominant (70%), Left dominant (10%), Codominant (20%)
• Travels in right AV groove

Left Coronary Artery (LCA):

• LAD (LAD = Left Anterior Descending):

  • Anterior 2/3 of IVS, anterior wall of LV, apex
  • Travels in anterior interventricular groove
  • “Widow maker” — proximal occlusion

• LCX (Left Circumflex):

  • Lateral and posterior walls of LV
  • Travels in left AV groove

Venous Drainage:

• Coronary sinus → RA (majority)
• Anterior cardiac veins → RA directly
• Thebesian veins → all chambers

8. Systemic and Pulmonary Circulation

Systemic Circulation (Left heart → Body → Right heart):

• High pressure system (100 mmHg)
• Long pathway
• Delivers O₂, nutrients; removes CO₂, waste

Pulmonary Circulation (Right heart → Lungs → Left heart):

• Low pressure system (25/10 mmHg)
• Short pathway
• Gas exchange: O₂ in, CO₂ out

Pressure Gradient: Aorta (120/80) → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava (2-8 mmHg)

9. Cardiac Innervation

Parasympathetic (Vagus):

• Via vagus nerve to SA and AV nodes
• Releases ACh → activates M2 receptors → ↓HR, ↓AV conduction
• Resting tone: Vagal tone sets resting HR ~70-80 bpm (not 100)

Sympathetic:

• Via cardiac nerves from cervical ganglia
• Releases NE → activates β1 receptors → ↑HR, ↑contractility, ↑conduction
• Left cardiac nerve primarily → LV

Reflexes:

• Baroreceptor reflex: ↑ BP → ↑ carotid stretch → ↑ vagal → ↓ HR
• Bainbridge reflex: ↑ Venous return → ↑ RA stretch → ↑ HR
• Chemoreceptor reflex: ↓ O₂, ↑ CO₂, ↓ pH → ↑ HR and ventilation

10. Clinical Correlations

Ischemic Heart Disease:

• Atherosclerosis → stable angina, unstable angina, MI
• MI: ST elevation (STEMI) or non-ST elevation (NSTEMI)
• Troponin I/T: Most specific (rises 3-12 hours, peaks 24 hours, elevated 1-2 weeks)

Heart Failure:

• Systolic (↓EF, dilated): ↓ Contractility
• Diastolic (preserved EF): ↓ Compliance
• Left-sided: Pulmonary edema, dyspnea
• Right-sided: Peripheral edema, JVD

Arrhythmias:

• Bradyarrhythmias: SA node dysfunction, AV block
• Tachyarrhythmias: SVT, VT, VF
• Atrial fibrillation: Irregularly irregular pulse, no P waves

Valve Disorders:

• Stenosis: Obstruction to flow (murmur during flow)
• Regurgitation: Backflow (murmur throughout)

Practice Questions for NEET PG

1. Describe the pathway of blood through the heart.
2. Explain the cardiac conduction system and its functional significance.
3. Correlate ECG waves with the cardiac cycle.
4. Discuss the regulation of cardiac output.
5. Describe the coronary circulation and its clinical significance.

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