Cardiovascular Physiology: The Heart and Circulation

Master cardiovascular physiology with free flashcards and spaced repetition practice. This lesson covers cardiac anatomy, the cardiac cycle, electrical conduction, hemodynamics, and regulation of blood pressure—essential concepts for USMLE Step 1 success and clinical medicine.

Welcome to Cardiovascular Physiology 🫀

The cardiovascular system is the body's delivery network, transporting oxygen, nutrients, hormones, and immune cells while removing metabolic waste. Understanding cardiac function is fundamental to diagnosing and treating conditions from heart failure to shock. This lesson will guide you through the mechanical, electrical, and regulatory mechanisms that keep blood flowing through approximately 60,000 miles of vessels in your body!

Core Concepts

1. Cardiac Anatomy and Structure 🏗️

The heart is a four-chambered muscular pump located in the mediastinum, consisting of:

Chambers:

• Right atrium (RA): Receives deoxygenated blood from superior and inferior vena cava
• Right ventricle (RV): Pumps blood to lungs via pulmonary artery (low pressure system)
• Left atrium (LA): Receives oxygenated blood from pulmonary veins
• Left ventricle (LV): Pumps blood to systemic circulation via aorta (high pressure system)

Valves (prevent backflow):

• Atrioventricular (AV) valves: Tricuspid (right), Mitral/Bicuspid (left)
• Semilunar valves: Pulmonic (right), Aortic (left)

┌────────────────────────────────────────────┐
│        BLOOD FLOW THROUGH HEART            │
├────────────────────────────────────────────┤
│                                            │
│  Superior/Inferior Vena Cava               │
│            ↓                               │
│       RIGHT ATRIUM                         │
│            ↓                               │
│      (Tricuspid Valve)                     │
│            ↓                               │
│      RIGHT VENTRICLE                       │
│            ↓                               │
│      (Pulmonic Valve)                      │
│            ↓                               │
│      Pulmonary Artery → 🫁 LUNGS          │
│            ↓                               │
│      Pulmonary Veins                       │
│            ↓                               │
│       LEFT ATRIUM                          │
│            ↓                               │
│       (Mitral Valve)                       │
│            ↓                               │
│       LEFT VENTRICLE                       │
│            ↓                               │
│       (Aortic Valve)                       │
│            ↓                               │
│         AORTA → Body 🫀                    │
│                                            │
└────────────────────────────────────────────┘

Wall Layers (from inside out):

1. Endocardium: Smooth inner lining
2. Myocardium: Thick muscular layer (thickest in LV)
3. Epicardium: Visceral pericardium

💡 Tip: Remember valve locations with "Try Pulling My Aorta" = Tricuspid, Pulmonic, Mitral, Aortic (from right to left, base to apex)

2. The Cardiac Cycle ⚙️

The cardiac cycle represents all mechanical events during one heartbeat, divided into systole (contraction) and diastole (relaxation).

Phase	Valves Open	Valves Closed	Key Events
1. Ventricular Filling (Diastole)	AV valves (tricuspid, mitral)	Semilunar valves	70% passive filling, 30% atrial kick (atrial systole)
2. Isovolumetric Contraction	NONE	ALL	Ventricles contract, pressure rises, volume constant
3. Ventricular Ejection (Systole)	Semilunar valves (pulmonic, aortic)	AV valves	Blood ejected into pulmonary artery and aorta
4. Isovolumetric Relaxation	NONE	ALL	Ventricles relax, pressure drops, volume constant

Key Parameters:

• Stroke Volume (SV): Volume ejected per beat (~70 mL)
• Ejection Fraction (EF): (SV/EDV) × 100 = normally 55-70%
• End-Diastolic Volume (EDV): Volume before contraction (~120 mL)
• End-Systolic Volume (ESV): Volume after contraction (~50 mL)
• Cardiac Output (CO): HR × SV = ~5 L/min at rest

PRESSURE-VOLUME LOOP (Left Ventricle)

Pressure
(mmHg)
  120├─────────┐(3)                     
     │         │╲  Ejection            
     │         │ ╲                     
   80│         │  ╲(4)                 
     │         │   ╲                   
     │    (2)  │    ╲                  
   40│    ╱    │     ╲                 
     │   ╱     │      ╲                
     │  ╱      │       ╲               
    0├─╱───────┴────────╲──────        
     │(1)              (1)             
     0    50   100  150   Volume (mL)  
                                        
(1) Filling                             
(2) Isovolumetric Contraction           
(3) Ejection                            
(4) Isovolumetric Relaxation            

Heart Sounds:

• S1 ("lub"): Closure of AV valves (start of systole)
• S2 ("dub"): Closure of semilunar valves (start of diastole)
• S3: Ventricular filling (normal in young, pathologic in adults = volume overload)
• S4: Atrial contraction against stiff ventricle (always pathologic = decreased compliance)

🧠 Mnemonic: "S1 S2 S3 S4" = "Systole, Semilunar, Swishing blood, Stiff ventricle"

3. Cardiac Electrical Conduction System ⚡

The heart generates its own electrical impulses through specialized pacemaker cells and conducting fibers.

┌────────────────────────────────────────────┐
│     ELECTRICAL CONDUCTION PATHWAY          │
└────────────────────────────────────────────┘

    (1) SA NODE (pacemaker) 🎯
         60-100 bpm
            ↓
            ↓ (spreads through atria)
            ↓
    (2) AV NODE ⏸️
         40-60 bpm
         (delays 0.1 sec)
            ↓
    (3) BUNDLE OF HIS
            ↓
       ┌────┴────┐
       ↓         ↓
  (4) LEFT    RIGHT
    BUNDLE   BUNDLE
    BRANCH   BRANCH
       ↓         ↓
    (5) PURKINJE FIBERS
         20-40 bpm
         ↓         ↓
    VENTRICULAR MYOCARDIUM
       contracts apex → base

Key Conduction Properties:

Structure	Intrinsic Rate (bpm)	Function
SA Node	60-100	Primary pacemaker, located in right atrium
AV Node	40-60	Delays impulse (allows atrial kick), backup pacemaker
Bundle of His	20-40	Transmits impulse to ventricles
Bundle Branches	20-40	Right and left pathways down septum
Purkinje Fibers	20-40	Rapid conduction to ventricular muscle

Action Potential Phases (Ventricular Myocyte):

Phase	Ion Movement	Description
0 - Rapid Depolarization	Na⁺ influx (fast channels)	Rapid upstroke
1 - Early Repolarization	K⁺ efflux begins	Brief dip
2 - Plateau	Ca²⁺ influx = K⁺ efflux	Sustained depolarization (250-300 ms)
3 - Repolarization	K⁺ efflux dominates	Return to resting
4 - Resting Potential	Na⁺/K⁺ ATPase active	-90 mV maintained

💡 Clinical Pearl: The long plateau phase (Phase 2) prevents tetanic contractions and ensures the ventricles have time to refill between beats!

4. Hemodynamics and Blood Pressure 🌊

Ohm's Law Applied to Circulation:

Flow (Q) = ΔPressure (ΔP) / Resistance (R)

Cardiac Output Determinants:

CO = HR × SV

Where Stroke Volume depends on:

1. Preload (venous return/EDV) - Frank-Starling mechanism
2. Afterload (aortic pressure/systemic vascular resistance)
3. Contractility (intrinsic strength of contraction)

Frank-Starling Law: ⬆️ venous return → ⬆️ EDV → ⬆️ sarcomere stretch → ⬆️ force of contraction → ⬆️ SV

FRANK-STARLING CURVE

Stroke
Volume
   ↑    Normal
   │      ╱────
   │    ╱
   │  ╱  Decreased
   │╱    Contractility
   │    ╱────
   │  ╱
   │╱
   └──────────────→
      Preload (EDV)

Poiseuille's Law (Resistance):

R = (8 × η × L) / (π × r⁴)

Where:

• η = viscosity
• L = vessel length
• r = radius (most important factor - raised to 4th power!)

💡 Key Insight: Doubling vessel radius decreases resistance by 16-fold (2⁴ = 16)!

Blood Pressure Calculation:

Mean Arterial Pressure (MAP) = CO × Total Peripheral Resistance (TPR)

Or: MAP = Diastolic BP + 1/3(Systolic BP - Diastolic BP)

Normal MAP: 70-100 mmHg (minimum 60 mmHg needed for organ perfusion)

5. Regulation of Cardiovascular Function 🎛️

Autonomic Nervous System Control:

System	Receptors	Heart Rate	Contractility	Vascular Effect
Sympathetic (β₁)	Norepinephrine, Epinephrine	⬆️ Increase	⬆️ Increase	Vasoconstriction (α₁)
Parasympathetic (M₂)	Acetylcholine	⬇️ Decrease	⬇️ Decrease (atria only)	Minimal effect

Baroreceptor Reflex:

┌────────────────────────────────────────────┐
│      BARORECEPTOR REFLEX                   │
└────────────────────────────────────────────┘

    ⬆️ Blood Pressure Detected
            ↓
    Baroreceptors (carotid sinus,
     aortic arch) fire MORE
            ↓
    Signal to Medulla (NTS)
            ↓
       ┌────┴────┐
       ↓         ↓
  ⬆️ Vagal    ⬇️ Sympathetic
   Output     Output
       ↓         ↓
    ⬇️ HR    ⬇️ Contractility
            ⬇️ TPR
            ↓
    ⬇️ Blood Pressure (back to normal)

Hormonal Regulation:

1. Renin-Angiotensin-Aldosterone System (RAAS):

   • ⬇️ Renal perfusion → Renin release
   • Angiotensinogen → Angiotensin I (via Renin)
   • Angiotensin I → Angiotensin II (via ACE in lungs)
   • Angiotensin II: vasoconstriction + aldosterone release
   • Aldosterone: ⬆️ Na⁺/H₂O retention → ⬆️ blood volume

2. Atrial Natriuretic Peptide (ANP):

   • Released when atria stretch (⬆️ blood volume)
   • Promotes Na⁺ excretion → ⬇️ blood volume → ⬇️ BP

3. Antidiuretic Hormone (ADH/Vasopressin):

   • Released with ⬇️ blood volume or ⬆️ osmolarity
   • ⬆️ H₂O reabsorption in kidneys + vasoconstriction

🤔 Did you know? Your heart beats approximately 100,000 times per day, pumping about 2,000 gallons (7,500 liters) of blood through your circulatory system!

6. Coronary Circulation 🫀

The heart muscle itself receives blood through coronary arteries:

• Left Main Coronary Artery → branches into:
  • Left Anterior Descending (LAD): Supplies anterior LV and septum
  • Left Circumflex (LCx): Supplies lateral and posterior LV
• Right Coronary Artery (RCA): Supplies RV, inferior LV, SA and AV nodes (usually)

💡 Clinical Pearl: The LAD is nicknamed the "widow maker" because occlusion causes extensive anterior MI with high mortality!

Coronary Blood Flow Timing:

• Occurs primarily during diastole (ventricles relaxed)
• LV subendocardium most vulnerable to ischemia (highest compression during systole)
• Regulated by local metabolic factors (adenosine, CO₂, H⁺)

Examples with Explanations

Example 1: Calculating Cardiac Output 📊

Scenario: A patient has a heart rate of 75 bpm and a stroke volume of 80 mL. What is their cardiac output?

Solution:

Step	Calculation	Result
1	Use formula: CO = HR × SV	-
2	CO = 75 beats/min × 80 mL/beat	6000 mL/min
3	Convert to liters	6.0 L/min

Explanation: Normal resting cardiac output is approximately 5 L/min, so this patient's CO is slightly elevated, which could be normal during mild exercise or stress. CO can increase 4-5 fold during maximal exercise in trained athletes!

Example 2: Understanding the Frank-Starling Mechanism 💪

Scenario: A patient receives IV fluids, increasing their venous return. What happens to stroke volume and why?

Explanation:

⬆️ IV Fluids → ⬆️ Blood Volume → ⬆️ Venous Return → ⬆️ Preload (EDV)

When ventricular filling increases:

1. Sarcomeres stretch to optimal length (2.0-2.4 μm)
2. More actin-myosin cross-bridge formation possible
3. ⬆️ Force of contraction (intrinsic property - NO neural input needed)
4. ⬆️ Stroke Volume → ⬆️ Cardiac Output

This is the Frank-Starling mechanism - the heart's intrinsic ability to adapt output to venous return!

⚠️ Important Limitation: In heart failure, the curve flattens and shifts right. Excessive preload can lead to pulmonary edema without improving CO.

Example 3: Baroreceptor Response to Standing 🧍

Scenario: You stand up quickly from lying down. Trace the cardiovascular response.

Step-by-step Response:

Time	Event	Mechanism
0 sec	Stand up → blood pools in legs	Gravity effect
1-2 sec	⬇️ Venous return → ⬇️ CO → ⬇️ BP	Decreased preload
2-3 sec	Baroreceptors fire LESS	Detect ⬇️ pressure in carotid/aorta
3-5 sec	Medulla responds	⬇️ Vagal, ⬆️ Sympathetic output
5-10 sec	⬆️ HR, ⬆️ contractility, vasoconstriction	Compensatory mechanisms
10+ sec	BP returns to normal	Homeostasis restored

Explanation: This rapid adjustment usually happens unconsciously. If the baroreceptor reflex fails (autonomic dysfunction), orthostatic hypotension occurs - causing dizziness or fainting upon standing.

Example 4: Resistance and Vessel Radius 🔬

Scenario: Atherosclerosis reduces an artery's radius from 2 mm to 1 mm. How does this affect resistance and flow?

Solution using Poiseuille's Law:

R ∝ 1/r⁴

If radius decreases from 2 mm to 1 mm (50% reduction):

• New radius = 1/2 original
• New resistance = 1/(1/2)⁴ = 1/(1/16) = 16 × original resistance

Effect on Flow:

Q = ΔP/R

If ΔP stays constant and R increases 16-fold:

• New flow = 1/16 of original flow (93.75% reduction!)

Explanation: This dramatic example shows why even modest arterial narrowing severely reduces blood flow. In reality, compensatory mechanisms (increased pressure, collateral vessels) partially offset this, but it illustrates why coronary stenosis >70% typically causes symptoms during exertion.

Common Mistakes ⚠️

Mistake 1: Confusing Systole with Diastole Timing

❌ Wrong: "S1 occurs during diastole" ✅ Right: "S1 marks the START of systole (AV valves close as ventricles contract)"

Why it matters: Understanding timing is critical for interpreting heart sounds, murmurs, and ECG correlations.

Mistake 2: Misunderstanding Ejection Fraction

❌ Wrong: "Ejection fraction is the percentage of blood ejected from the body" ✅ Right: "Ejection fraction is the percentage of EDV ejected per beat (SV/EDV × 100)"

Example: If EDV = 120 mL and SV = 70 mL, then EF = (70/120) × 100 = 58% (normal). The heart NEVER ejects all blood - ESV remains (~50 mL).

Mistake 3: Forgetting the Fourth Power in Poiseuille's Law

❌ Wrong: "Doubling vessel radius doubles blood flow" ✅ Right: "Doubling vessel radius increases flow 16-fold (assuming constant pressure)"

Remember: Small changes in vessel diameter have HUGE effects on resistance and flow!

Mistake 4: Mixing Up Preload and Afterload

❌ Wrong: "Aortic stenosis increases preload" ✅ Right: "Aortic stenosis increases afterload (resistance to ejection)"

Definitions:

• Preload = ventricular stretch before contraction (≈ EDV)
• Afterload = resistance ventricle must overcome to eject blood (≈ aortic pressure for LV)

Mistake 5: Assuming Parasympathetic Control of Contractility

❌ Wrong: "Vagal stimulation decreases ventricular contractility" ✅ Right: "Vagal stimulation mainly affects HR and atrial contractility; minimal ventricular effect"

Why: Ventricular myocardium has few parasympathetic receptors. Sympathetic stimulation (β₁) is the primary controller of ventricular contractility.

Key Takeaways 🎯

1. Cardiac anatomy: Four chambers, four valves, three wall layers - blood flows right side (lungs) → left side (body)

2. Cardiac cycle: Diastole (filling) → Isovolumetric contraction → Systole (ejection) → Isovolumetric relaxation

3. Electrical conduction: SA node (pacemaker) → AV node (delay) → Bundle of His → Bundle branches → Purkinje fibers

4. Cardiac output: CO = HR × SV; SV depends on preload, afterload, and contractility

5. Frank-Starling Law: ⬆️ Preload → ⬆️ Sarcomere stretch → ⬆️ Contractility → ⬆️ SV (intrinsic adaptation)

6. Hemodynamics: Flow = ΔP/R; Resistance ∝ 1/r⁴ (radius is most important factor)

7. Blood pressure: MAP = CO × TPR; regulated by baroreceptors, autonomic nervous system, and hormones (RAAS, ANP, ADH)

8. Autonomic control: Sympathetic (⬆️ HR, contractility, vasoconstriction); Parasympathetic (⬇️ HR)

9. Coronary flow: Occurs mainly during diastole; LAD supplies anterior wall, RCA supplies inferior wall

10. Clinical relevance: Understanding these mechanisms is essential for interpreting ECGs, heart sounds, hemodynamic monitoring, and pharmacological interventions

📚 Further Study

1. Cardiovascular Physiology Concepts - https://www.cvphysiology.com/ (Excellent interactive modules on hemodynamics, cardiac function, and regulation)

2. Khan Academy Medicine - Circulatory System - https://www.khanacademy.org/science/health-and-medicine/circulatory-system (Free video lectures with clear animations of cardiac cycle and electrical conduction)

3. USMLE-Rx Express - Cardiovascular Physiology - https://www.usmle-rx.com/ (High-yield review questions and explanations specifically for Step 1 preparation)

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💪 You've completed Cardiovascular Physiology! This foundation will serve you throughout your medical training - from interpreting ECGs and echocardiograms to managing shock and heart failure. Keep reviewing with spaced repetition, and these concepts will become second nature! 🫀✨