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CVS Physiology - NEET MDS Notes

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1. CONDUCTING SYSTEM OF THE HEART

Components & Anatomy

• SA node = normal pacemaker (discharges fastest)
• Right vagus -> mainly SA node; Left vagus -> mainly AV node
• Right sympathetics -> SA node; Left sympathetics -> AV node
• Internodal tracts: Anterior (Bachmann bundle to left atrium), Middle (tract of Wenckebach), Posterior (tract of Thorel)
• Atrial fibers are separated from ventricular fibers by a fibrous ring; the ONLY conduction pathway between atria and ventricles is the Bundle of His
• Purkinje fibers: largest cells, fewest mitochondria and striations, specialized for rapid conduction
• SA node cells: smallest, sparsely striated, highest internal resistance

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2. CARDIAC ACTION POTENTIALS

A. Ventricular Muscle Action Potential (Resting Membrane Potential = -90 mV)

B. Pacemaker (SA Node) Action Potential - Prepotential / Pacemaker Potential

• After each impulse, IK declines
• Channel permeable to both Na+ and K+ activates - called "h" channel (funny/hyperpolarization-activated) - produces If (funny current / Ih)
• This Ih causes initial slow depolarization (first part of prepotential)
• T-type (transient) Ca2+ channels open - complete the prepotential
• L-type (long-lasting) Ca2+ channels produce the actual impulse
• No stable resting potential - spontaneous depolarization is the key feature

• "0-1-2-3-4": Depolarize, Notch, Plateau, Repolarize, Rest
• Plateau (phase 2) = due to Ca2+ (important for cardiac contractility)
• Pacemaker has "funny current" (If) - activated by HYPERpolarization

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3. ECG (Electrocardiogram)

Normal ECG Waves

Normal Cardiac Rate

• Normal: 60-100 beats/min
• Tachycardia: >100/min
• Bradycardia: <60/min

ECG in Electrolyte Imbalances (High-Yield!)

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4. CARDIAC CYCLE

Phases of Ventricular Systole & Diastole

Key Volumes

• End-diastolic volume (EDV): ~130 mL
• End-systolic volume (ESV): ~60 mL
• Stroke volume (SV): EDV - ESV = ~70 mL
• Ejection Fraction (EF): SV/EDV × 100 = ~55-65% (normal ≥55%)

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5. HEART SOUNDS

• During inspiration: increased venous return -> RV takes longer to empty -> pulmonary valve closes later -> A2-P2 split (A2 before P2)
• Fixed splitting: ASD (right bundle branch block pattern)
• Paradoxical/reverse splitting: LBBB, severe aortic stenosis (P2 before A2)

Heart Murmurs (High-Yield Table!)

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6. CARDIAC OUTPUT

Definition & Normal Values

• Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)
• Normal CO = ~5 L/min (at rest)
• Cardiac Index = CO / Body surface area = 3.2 L/min/m²

Measurement Methods

1. Fick's Principle (Gold standard):
   • CO = O2 consumption / (arterial O2 - venous O2)
   • Example: 250 mL/min ÷ 50 mL/L = 5 L/min
   • Venous sample from pulmonary artery (via cardiac catheter)
2. Indicator Dilution Method (Stewart-Hamilton):
   • Known amount of dye/radioactive isotope injected IV
   • CO = Amount of indicator / Average arterial concentration
3. Thermodilution (most common clinically):
   • Cold saline injected into RA; temperature change measured in PA
4. Doppler + Echocardiography (non-invasive)

Factors Controlling Cardiac Output

• Determined by venous return
• Increased by: supine position, exercise (muscle pump), increased blood volume
• Frank-Starling Law: "Energy of contraction is proportional to the initial length of the cardiac muscle fiber"
  • More stretch -> more forceful contraction (up to a limit)
  • Heterometric regulation = changes in CO due to changes in fiber length

• Determined by: aortic pressure, peripheral vascular resistance
• Increased afterload -> reduced stroke volume

• Positive inotropes: sympathetic stimulation (catecholamines), digitalis, Ca2+
• Negative inotropes: parasympathetic stimulation, beta-blockers, Ca2+ channel blockers
• Homometric regulation = changes in CO due to changes in contractility independent of length

• Sympathetic: increases HR (positive chronotropy)
• Parasympathetic: decreases HR (negative chronotropy)
• Increased HR increases CO, but very high HR reduces filling time -> reduced SV

Cardiac Output in Various Conditions

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7. ARTERIAL BLOOD PRESSURE

Key Values

• Systolic BP: ~120 mm Hg
• Diastolic BP: ~70 mm Hg
• Pulse Pressure = Systolic - Diastolic = ~50 mm Hg
• Mean Arterial Pressure (MAP) = Diastolic + 1/3 Pulse Pressure = ~93 mm Hg
• Or: MAP = (Systolic + 2×Diastolic) / 3

Pressure Distribution in Systemic Circulation

• Highest resistance: arterioles (main site of peripheral resistance)
• Mean pressure at end of arterioles: 30-38 mm Hg
• Pulse pressure nearly zero at capillaries

Gravity Effect on BP

• 0.77 mm Hg per cm above/below heart level
• Artery in head (50 cm above heart): MAP = 100 - (0.77 × 50) = 62 mm Hg
• Artery in foot (105 cm below heart): MAP = 100 + (0.77 × 105) = 181 mm Hg

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8. FRANK-STARLING LAW (HIGH-YIELD)

• Starling's Law: Energy of contraction is proportional to the initial length of the cardiac muscle fiber
• Frank-Starling Curve: Ventricular stroke volume vs. end-diastolic volume
• Physiological basis: More overlap of actin-myosin at optimal sarcomere length (~2.2 µm)
• Clinical significance:
  • Explains how both ventricles maintain equal output despite different afterloads
  • Heart failure: curve shifts down and right (less force at same preload)
  • Increased venous return -> increased SV (key mechanism in exercise)

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9. VASCULAR SMOOTH MUSCLE & CIRCULATION

Blood Vessel Types

Regulation of Vascular Tone

• Endothelium secretes:
  • Vasodilators: NO (nitric oxide), prostacyclin (PGI2)
  • Vasoconstrictors: Endothelin-1
• Vascular smooth muscle contraction: primarily via myosin light chain phosphorylation (Ca2+-calmodulin pathway)
• Latch-bridge mechanism: Sustained low-energy contraction in vascular smooth muscle

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10. OXYGEN CONSUMPTION BY THE HEART

• Heart extracts ~70-75% of delivered O2 at rest (vs. skeletal muscle ~25% at rest)
• Cannot increase O2 extraction much more on demand
• Increased O2 demand is met primarily by increased coronary blood flow
• Myocardial O2 consumption increased by: tachycardia, increased contractility, increased wall tension (increased preload/afterload)
• Double product (Rate-Pressure Product) = HR × Systolic BP; correlates with myocardial O2 demand

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QUICK-RECALL TABLE: High-Yield Facts for NEET MDS

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CVS Physiology - NEET MDS MCQs

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SECTION 1: Conducting System & Action Potential

• A) AV node
• B) Bundle of His
• C) SA node ✅
• D) Purkinje fibers

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• A) 20-40/min
• B) 40-60/min ✅
• C) 60-100/min
• D) 100-120/min

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• A) -55 mV
• B) -70 mV
• C) -90 mV ✅
• D) -110 mV

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• A) Na+ influx
• B) K+ efflux
• C) Ca2+ influx through L-type channels ✅
• D) Cl- influx

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• A) Ca2+ only
• B) K+ only
• C) Na+ and K+ together ✅
• D) Na+ only

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• A) SA node
• B) AV node
• C) Atrial muscle
• D) Purkinje fibers ✅

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• A) SA node
• B) AV node ✅
• C) Bundle of His
• D) Purkinje fibers

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• A) Smallest cells in the conduction system
• B) Highest internal resistance
• C) Fewest striations and largest cells ✅
• D) Absent gap junctions

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SECTION 2: ECG

• A) Atrial repolarization
• B) Ventricular depolarization
• C) Atrial depolarization ✅
• D) Ventricular repolarization

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• A) 0.04-0.08 s
• B) 0.12-0.20 s ✅
• C) 0.20-0.30 s
• D) >0.30 s

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• A) Prolonged QT interval
• B) Tall, peaked T waves (early) ✅
• C) Prominent U waves
• D) Short QT interval

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• A) Hyperkalemia
• B) Hypercalcemia
• C) Hypokalemia ✅
• D) Hypomagnesemia

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• A) Hypokalemia
• B) Hypomagnesemia
• C) Hypercalcemia ✅
• D) Hypothyroidism

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• A) P wave
• B) QRS complex
• C) T wave ✅
• D) PR interval

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• A) Ventricular rate equals atrial rate
• B) Ventricular rate is LOWER than atrial rate ✅
• C) Ventricular ECG complexes are always distorted
• D) Cardiac output is increased

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SECTION 3: Cardiac Cycle & Heart Sounds

• A) Closure of aortic and pulmonary valves
• B) Closure of mitral and tricuspid valves ✅
• C) Opening of mitral valve
• D) Rapid ventricular filling

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• A) AV valve closure
• B) Aortic and pulmonary valve closure ✅
• C) Rapid ventricular filling
• D) Atrial contraction

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• A) Atrial systole
• B) Isovolumetric contraction
• C) Rapid ventricular filling ✅
• D) Isovolumetric relaxation

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• A) Turbulent flow across a stenotic valve
• B) Atrial contraction into a stiff/hypertrophied ventricle ✅
• C) Rapid ventricular filling
• D) Mitral valve closure

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• A) Mitral stenosis produces a systolic murmur
• B) Aortic regurgitation produces a systolic murmur
• C) Aortic stenosis produces a systolic murmur ✅
• D) Mitral regurgitation produces a diastolic murmur

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• A) Mitral valve closes before tricuspid
• B) Pulmonary valve closes later than aortic during inspiration ✅
• C) Aortic valve closes before pulmonary during inspiration
• D) Both valves close simultaneously

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• A) All four valves are open
• B) AV valves open, semilunar valves closed
• C) All four valves are CLOSED ✅
• D) Semilunar valves open, AV valves closed

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SECTION 4: Cardiac Output & Frank-Starling

• A) 2-3 L/min
• B) 5 L/min ✅
• C) 8-10 L/min
• D) 15 L/min

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• A) CO × body weight
• B) CO / body surface area ✅
• C) SV × HR
• D) CO × peripheral resistance

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• A) CO = O2 consumption / arteriovenous O2 difference ✅
• B) CO = HR × peripheral resistance
• C) CO = EDV - ESV
• D) CO = Indicator concentration × time

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• A) Cardiac output is inversely proportional to preload
• B) Energy of contraction is proportional to initial fiber length ✅
• C) Heart rate determines stroke volume
• D) Afterload increases stroke volume

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• A) Changes in CO due to changes in heart rate
• B) Changes in CO due to changes in fiber LENGTH (preload) ✅
• C) Changes in CO due to changes in contractility
• D) Changes in CO due to afterload changes

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• A) 30-40%
• B) 40-50%
• C) 55-65% ✅
• D) 70-80%

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• A) Exercise
• B) Sympathetic stimulation
• C) Increased venous return
• D) Parasympathetic stimulation ✅

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• A) Aortic pressure
• B) Peripheral vascular resistance
• C) End-diastolic volume ✅
• D) Systolic blood pressure

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SECTION 5: Blood Pressure & Vascular Physiology

• A) Systolic + Diastolic / 2
• B) Diastolic + 1/3 pulse pressure ✅
• C) Systolic - Diastolic
• D) Diastolic + 1/2 pulse pressure

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• A) Capillaries
• B) Arterioles ✅
• C) Large arteries
• D) Veins

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• A) Arteries
• B) Arterioles
• C) Capillaries
• D) Veins ✅

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• A) Mean arterial pressure
• B) Systolic BP - Diastolic BP ✅
• C) Diastolic BP + 1/3 (SBP-DBP)
• D) SBP + DBP / 2

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• A) 0.5 mmHg per cm above/below heart
• B) 0.77 mmHg per cm above/below heart ✅
• C) 1.0 mmHg per cm above/below heart
• D) 1.5 mmHg per cm above/below heart

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SECTION 6: Special Topics (High-Yield for NEET MDS)

• A) SA node
• B) Bachmann bundle
• C) Bundle of His ✅
• D) Thorel tract

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• A) Decreased heart rate
• B) Increased preload
• C) Tachycardia, increased contractility, increased wall tension ✅
• D) Decreased afterload

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• A) Beta-blockers
• B) Calcium channel blockers
• C) Digitalis ✅
• D) Verapamil

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• A) SA node to AV node
• B) Right and left atria ✅
• C) AV node to Bundle of His
• D) Bundle of His to Purkinje fibers

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• A) Atrial rate is higher than ventricular rate
• B) Stokes-Adams attacks (syncope) may occur
• C) Ventricular rate is 20-40/min (Purkinje escape)
• D) Ventricular fibrillation is common ✅

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QUICK ANSWER KEY

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• Questions on SA node rate, action potential phases, heart sounds are almost always asked
• Fick's principle calculation-type questions are common
• Heart murmur timing (systolic vs. diastolic) is a guaranteed 1-2 marks topic
• Frank-Starling / heterometric vs. homometric distinction is a frequent trap question
• Electrolyte-ECG correlations (especially K+ and Ca2+) appear frequently