Heart Anatomy and Cardiac Cycle

SECTION 1

Historical Context & Motivation

The human heart has captivated physicians, philosophers, and scientists for millennia. Ancient Egyptian physicians recognized its importance during mummification, preserving it as the seat of intelligence and emotion while discarding the brain. The Greek physician Galen of Pergamon (c. 130–210 CE) advanced early cardiac understanding by describing the heart's chambers and vessels, although he incorrectly proposed that blood passed between the ventricles through invisible pores in the interventricular septum. This Galenic model dominated Western medicine for over a thousand years, illustrating how deeply entrenched anatomical misconceptions can persist without rigorous experimental challenge.

The pivotal breakthrough arrived in 1628 when English physician William Harvey published De Motu Cordis, demonstrating through meticulous observation and quantitative reasoning that blood circulates in a closed loop rather than being consumed peripherally. Harvey calculated that the volume of blood ejected per heartbeat, multiplied by the heart rate, produced a quantity far exceeding what the liver could plausibly manufacture de novo—a simple but devastating mathematical argument against the Galenic paradigm. His work established the conceptual foundation for modern cardiovascular physiology and exemplified the power of integrating anatomical observation with quantitative analysis.

c. 130 CE

Galen's Cardiac Anatomy

Galen describes the heart's chambers and major vessels in animal dissections, proposing that blood passes through pores in the interventricular septum—a model that would persist for over 1,400 years.

1628

Harvey's Circulatory Model

William Harvey publishes De Motu Cordis, proving through quantitative reasoning and vivisection that blood circulates in a closed system, with the heart functioning as a mechanical pump.

1816

Invention of the Stethoscope

René Laennec invents the stethoscope, enabling auscultation of heart sounds (S₁ and S₂) and providing clinicians the first non-invasive tool for evaluating valve function and cardiac rhythm.

1903

Einthoven's Electrocardiogram

Willem Einthoven develops the string galvanometer ECG, allowing precise recording of the heart's electrical activity and inaugurating modern cardiac electrophysiology.

1953

First Successful Open-Heart Surgery

John Gibbon performs the first open-heart surgery using cardiopulmonary bypass, demonstrating that surgical intervention in the heart requires deep understanding of both anatomy and the cardiac cycle's hemodynamic phases.

Today, understanding heart anatomy and the cardiac cycle is indispensable across clinical medicine, biomedical engineering, and pharmacology. Every drug that modifies heart rate, contractility, or preload presupposes a detailed knowledge of the structural relationships among chambers, valves, coronary vessels, and conduction pathways. The central question this lesson addresses is: How do the heart's structural features coordinate to produce a rhythmic, directional flow of blood through pulmonary and systemic circuits?

SECTION 2

Core Principles & Definitions

Before examining the heart's detailed anatomy and the phases of the cardiac cycle, it is essential to establish several foundational principles that govern cardiac function. These principles recur throughout cardiovascular physiology and provide the conceptual scaffolding for integrating structure with function. The heart is fundamentally a dual pump—the right side drives deoxygenated blood through the pulmonary circulation while the left side propels oxygenated blood through the systemic circulation. Both sides contract simultaneously, yet the pressures they generate differ dramatically because of differences in vascular resistance.

Dual Circulation

The cardiovascular system comprises two circuits in series: the pulmonary circuit (right ventricle → lungs → left atrium) and the systemic circuit (left ventricle → body → right atrium). This arrangement ensures efficient gas exchange and nutrient delivery.

Unidirectional Flow via Valves

Four cardiac valves—two atrioventricular (tricuspid, mitral) and two semilunar (pulmonary, aortic)—open and close passively in response to pressure gradients, preventing retrograde blood flow.

Intrinsic Rhythmicity

Cardiac muscle cells are autorhythmic: the sinoatrial (SA) node initiates depolarization without neural input. The conduction system (SA node → AV node → bundle of His → Purkinje fibers) coordinates contraction sequence and timing.

Pressure–Volume Relationship

The cardiac cycle is characterized by alternating phases of contraction (systole) and relaxation (diastole). Pressure differentials between chambers and vessels determine when valves open and close, governing the timing of filling and ejection.

Frank–Starling Mechanism

The heart possesses an intrinsic capacity to adjust its stroke volume in response to changes in venous return. Increased preload stretches myocardial sarcomeres toward their optimal length, enhancing contractile force without requiring neural or hormonal input.

✦ KEY TAKEAWAY

Think of the heart as a two-story building with a shared electrical wiring system. The upper floor (atria) collects incoming deliveries (venous blood), and the lower floor (ventricles) ships them out under pressure. The one-way doors between floors (AV valves) and at the exit docks (semilunar valves) ensure that packages never travel backward. The building's wiring—the conduction system—fires in a precise top-to-bottom sequence so the upper floor empties just before the lower floor contracts, maximizing the volume shipped per cycle.

SECTION 3

Visual Explanation — Heart Anatomy

Anterior cross-section of the heart showing the four chambers (RA, RV, LA, LV), the four valves (tricuspid, pulmonary, mitral, aortic), the interventricular septum, and the great vessels. Note the significantly thicker wall of the left ventricle (13–15 mm) compared to the right ventricle (3–5 mm), reflecting the higher systemic pressure the left ventricle must generate.

The diagram above illustrates the heart's gross anatomical organization from an anterior perspective. The right atrium receives deoxygenated blood from the superior and inferior venae cavae and the coronary sinus. Blood then passes through the tricuspid valve (named for its three cusps) into the right ventricle, which pumps it through the pulmonary semilunar valve into the pulmonary trunk en route to the lungs. Oxygenated blood returns via four pulmonary veins to the left atrium, crosses the mitral (bicuspid) valve into the left ventricle, and is ejected through the aortic semilunar valve into the ascending aorta for systemic distribution.

The structural asymmetry between the ventricles merits attention. The left ventricular myocardium is approximately three times thicker than its right counterpart because systemic vascular resistance exceeds pulmonary vascular resistance by a factor of roughly five to seven. This wall-thickness differential is directly predicted by the Law of Laplace, which relates wall tension to intraluminal pressure, radius, and wall thickness. Additionally, the chordae tendineae and papillary muscles anchor the AV valve leaflets, preventing prolapse during the high-pressure phase of ventricular systole. This anatomical feature is a common site of pathology—mitral valve prolapse affects approximately 2–3% of the population and can be appreciated on auscultation as a mid-systolic click.

SECTION 4

The Cardiac Cycle — Mechanical Events

The cardiac cycle encompasses all events from the beginning of one heartbeat to the beginning of the next. At a resting heart rate of 75 beats per minute, each cycle lasts approximately 0.8 seconds. The cycle divides into two major phases—systole (contraction, ~0.3 s) and diastole (relaxation, ~0.5 s)—each of which can be subdivided further based on valve positions and pressure dynamics. Understanding these sub-phases is essential for interpreting heart sounds, pressure tracings, and ECG waveforms in clinical settings.

Phases of the Cardiac Cycle

During ventricular filling (early diastole), the AV valves are open and the semilunar valves are closed. Blood flows passively from the atria into the ventricles, driven by the pressure gradient established during ventricular relaxation. Approximately 70–80% of ventricular filling occurs passively. Near the end of diastole, atrial contraction (the atrial kick) contributes the remaining 20–30%, a contribution that becomes clinically significant in patients with atrial fibrillation who lose this coordinated atrial contraction.

As ventricular contraction begins, intraventricular pressure rapidly rises and exceeds atrial pressure, causing the AV valves to snap shut—producing the first heart sound (S₁). For a brief period, all four valves are closed and ventricular volume remains constant; this is isovolumetric contraction. When ventricular pressure exceeds arterial pressure (≈80 mmHg in the aorta, ≈10 mmHg in the pulmonary trunk), the semilunar valves open and the ventricular ejection phase begins. Ejection is initially rapid, then slows as the pressure gradient diminishes.

When ventricular pressure falls below arterial pressure, the semilunar valves close, generating the second heart sound (S₂). There follows another brief interval during which all four valves are closed and ventricular volume does not change: isovolumetric relaxation. Once ventricular pressure drops below atrial pressure, the AV valves reopen and a new filling phase commences.

CARDIAC OUTPUT

CO = HR × SV

Where CO = cardiac output (mL/min), HR = heart rate (beats/min), and SV = stroke volume (mL/beat). A typical resting CO ≈ 75 bpm × 70 mL = 5,250 mL/min ≈ 5.25 L/min.

EJECTION FRACTION

EF = (SV / EDV) × 100%

Where SV = stroke volume (EDV − ESV), EDV = end-diastolic volume (~120 mL), and ESV = end-systolic volume (~50 mL). Normal EF = (70/120) × 100% ≈ 58%. An EF below 40% generally indicates heart failure with reduced ejection fraction (HFrEF).

LAW OF LAPLACE (SIMPLIFIED FOR SPHERE)

T = (P × r) / (2 × h)

Where T = wall tension, P = intraluminal pressure, r = chamber radius, and h = wall thickness. This explains why the thicker left ventricular wall can sustain higher pressures without excessive wall stress, and why ventricular dilation increases myocardial oxygen demand.

SECTION 5

Electrical Conduction & the ECG

The mechanical events of the cardiac cycle are orchestrated by a precisely timed sequence of electrical events originating in the heart's intrinsic conduction system. The sinoatrial (SA) node, located in the superior-posterior wall of the right atrium near the junction of the superior vena cava, serves as the heart's primary pacemaker. Its autorhythmic cells spontaneously depolarize at a rate of 60–100 beats per minute owing to a slow, unstable resting membrane potential driven by funny channels (I_f) that conduct mixed Na⁺/K⁺ currents.

From the SA node, the depolarization wave spreads across both atria via internodal and interatrial pathways, reaching the atrioventricular (AV) node located in the interatrial septum near the coronary sinus ostium. The AV node imposes a critical ~0.1-second delay, allowing atrial contraction to complete before ventricular activation begins. The impulse then travels rapidly through the bundle of His, splits into the left and right bundle branches traversing the interventricular septum, and finally ramifies into the Purkinje fibers that rapidly depolarize the ventricular myocardium from apex to base, ensuring a coordinated, "wringing" contraction that efficiently ejects blood superiorly toward the outflow tracts.

A normal sinus rhythm ECG showing two complete cardiac cycles. The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization (atrial repolarization is hidden within it), and the T wave represents ventricular repolarization. The PR interval (0.12–0.20 s) reflects the AV nodal delay.

The ECG above provides a powerful clinical window into the heart's electrical behavior. Each deflection corresponds to a specific electrical event: the P wave to atrial depolarization, the QRS complex to ventricular depolarization (and simultaneous atrial repolarization, which is obscured by the larger ventricular signal), and the T wave to ventricular repolarization. Importantly, the mechanical events lag slightly behind their electrical triggers—ventricular contraction begins just after the QRS complex, and ventricular relaxation follows the T wave. This electromechanical coupling is mediated by calcium-induced calcium release from the sarcoplasmic reticulum, linking the action potential to the sliding-filament mechanism of sarcomere shortening.

🩺 CLINICAL CONNECTION

A prolonged PR interval (>0.20 s) suggests first-degree AV block, where conduction through the AV node is delayed. A widened QRS (>0.12 s) may indicate bundle branch block or ventricular pre-excitation. These ECG findings directly reflect pathology in the conduction system components depicted above.

SECTION 6

Worked Example — Hemodynamic Calculations

The following example integrates the key quantitative relationships from Section 4 to solve a clinically realistic hemodynamic problem. Approach it systematically, extracting given values, selecting the appropriate equations, and interpreting the physiological significance of each result.

Calculating Cardiac Output and Ejection Fraction

Step 1 — Identify Given ValuesA 68-year-old patient has a resting heart rate (HR) of 82 beats/min. Echocardiography reveals an end-diastolic volume (EDV) of 140 mL and an end-systolic volume (ESV) of 80 mL. We need to determine the stroke volume (SV), ejection fraction (EF), and cardiac output (CO).

Step 2 — Calculate Stroke VolumeStroke volume is the difference between end-diastolic volume and end-systolic volume: SV = EDV − ESV = 140 mL − 80 mL.

SV = 60 mL/beat

Step 3 — Calculate Ejection FractionEF = (SV / EDV) × 100% = (60 mL / 140 mL) × 100%.

EF = 42.9% — This is below the normal range of 55–70%, suggesting reduced systolic function consistent with heart failure with reduced ejection fraction (HFrEF), typically defined as EF < 40%. At 42.9%, this patient falls in the "mildly reduced" category (HFmrEF, EF 41–49%).

Step 4 — Calculate Cardiac OutputCO = HR × SV = 82 beats/min × 60 mL/beat.

CO = 4,920 mL/min ≈ 4.92 L/min — This is slightly below the typical resting value of ~5 L/min, consistent with compensatory tachycardia partially maintaining output despite reduced stroke volume.

Step 5 — Interpret with the Frank–Starling MechanismThe elevated EDV (140 mL vs. normal ~120 mL) suggests the heart is relying on the Frank–Starling mechanism: increased preload stretches the myocardium to augment contractile force. However, the elevated ESV (80 mL vs. normal ~50 mL) indicates that this compensatory mechanism is insufficient—the ventricle cannot empty adequately, confirming systolic dysfunction. Over time, chronic ventricular dilation may push the sarcomeres past their optimal length, further reducing contractile efficiency and creating a vicious cycle of progressive heart failure.

SECTION 7

Heart Sounds, Valve Pathology, and Clinical Correlations

Heart sounds provide essential non-invasive diagnostic information that directly links cardiac anatomy to the mechanical phases of the cycle. The two primary heart sounds—S₁ and S₂—correspond to valve closures and can be auscultated with a stethoscope. Abnormalities in these sounds, as well as the presence of additional sounds (S₃, S₄) or murmurs, reflect specific structural or functional pathologies.

Summary of heart sounds and their clinical correlations

Sound / Finding	Timing	Mechanism	Clinical Significance
S₁ ("lub")	Onset of ventricular systole	Closure of AV valves (mitral + tricuspid)	Loudest at apex; split S₁ may indicate bundle branch block
S₂ ("dub")	Onset of ventricular diastole	Closure of semilunar valves (aortic + pulmonary)	Physiologic splitting during inspiration is normal; fixed splitting suggests ASD
S₃ (ventricular gallop)	Early diastole (rapid filling)	Blood rushing into a volume-overloaded, compliant ventricle	Normal in young adults; pathological in patients >40, suggests CHF
S₄ (atrial gallop)	Late diastole (atrial kick)	Atrial contraction against a stiff, non-compliant ventricle	Suggests ventricular hypertrophy or diastolic dysfunction; absent in atrial fibrillation
Murmur	Variable (systolic or diastolic)	Turbulent flow through stenotic or regurgitant valves	Systolic murmurs: aortic stenosis, mitral regurgitation. Diastolic: mitral stenosis, aortic regurgitation

✦ KEY TAKEAWAY

Heart sounds function like quality control checkpoints on a factory assembly line. S₁ signals that the input gates (AV valves) have sealed, confirming the system is pressurizing. S₂ signals that the output gates (semilunar valves) have sealed, confirming the ejection phase is complete. An extra sound (S₃ or S₄) is like an unexpected noise on the assembly line—it signals that a component (ventricular compliance, atrial function) is operating outside its normal specification, warranting further investigation.

SECTION 8

Connections to Advanced Cardiovascular Physiology

The foundational anatomy and cardiac cycle concepts presented in this lesson serve as the launching point for more advanced topics in cardiovascular physiology and pathophysiology. Understanding how the basic principles scale into clinical and research domains is critical for appreciating the broader integrative framework of cardiovascular medicine.

From foundational to advanced cardiovascular topics

Foundational Concept	Advanced Extension
Cardiac output = HR × SV	Fick principle: CO = VO₂ / (CₐO₂ − CᵥO₂); invasive measurement via pulmonary artery catheterization for critically ill patients
Frank–Starling mechanism (length–tension relationship)	Pressure–volume loops: graphical analysis of ventricular performance, preload, afterload, contractility, and their pharmacological manipulation
Conduction system & ECG waveforms	Cardiac electrophysiology: ion channel kinetics, action potential phases (0–4), antiarrhythmic drug classification (Vaughan-Williams), ablation therapy
Heart valves & unidirectional flow	Valvular hemodynamics: Gorlin formula for valve area, Doppler echocardiography for transvalvular gradients, surgical vs. transcatheter valve replacement
Coronary circulation (brief mention)	Coronary physiology: autoregulation, coronary flow reserve, ischemia-reperfusion injury, myocardial oxygen supply-demand balance, coronary angiography

One particularly instructive extension is the pressure–volume (PV) loop, which plots ventricular pressure against ventricular volume throughout the cardiac cycle to produce a closed loop whose area represents stroke work. Changes in preload shift the loop along the end-diastolic pressure–volume relationship (EDPVR), while changes in contractility alter the slope of the end-systolic pressure–volume relationship (ESPVR). These concepts provide a rigorous, quantitative framework for understanding how drugs, disease, and physiological states modify cardiac performance—a framework that builds directly on the chamber anatomy, valve mechanics, and hemodynamic equations introduced in this lesson.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

Explain why the left ventricular wall is substantially thicker than the right ventricular wall, despite both ventricles ejecting approximately the same stroke volume per beat. In your answer, reference the relevant vascular circuits and the Law of Laplace.

PROBLEM 2 — BASIC CALCULATION

A patient has a heart rate of 90 beats/min, an end-diastolic volume of 130 mL, and an end-systolic volume of 55 mL. Calculate the stroke volume, ejection fraction, and cardiac output.

PROBLEM 3 — INTERMEDIATE

During the cardiac cycle, there are two intervals in which all four valves are simultaneously closed. Identify these intervals, explain the pressure conditions that create each one, and describe what is happening to ventricular volume during each.

PROBLEM 4 — APPLIED

A patient with chronic atrial fibrillation has lost organized atrial contraction. Explain how this loss affects ventricular filling, stroke volume, and cardiac output. Under what physiological circumstances would this patient be most symptomatic, and why?

PROBLEM 5 — CRITICAL THINKING

A biomedical engineer is designing an artificial heart valve. Using your knowledge of native valve anatomy and the cardiac cycle, discuss at least three design requirements the prosthetic valve must satisfy. For each requirement, explain the anatomical or physiological principle that motivates it and the consequence of failure.

SUMMARY

Lesson Summary

The human heart is a four-chambered, dual pump that simultaneously drives blood through the pulmonary circuit (right side) and the systemic circuit (left side). Unidirectional flow is ensured by four cardiac valves—the tricuspid and mitral (AV valves) and the pulmonary and aortic (semilunar valves)—which open and close passively in response to pressure gradients. The cardiac cycle encompasses systole (contraction) and diastole (relaxation), with sub-phases including isovolumetric contraction, ventricular ejection, isovolumetric relaxation, and ventricular filling. Key quantitative relationships—CO = HR × SV and EF = (SV / EDV) × 100%—enable clinicians to assess cardiac performance and diagnose pathology.

The heart's intrinsic conduction system (SA node → AV node → bundle of His → Purkinje fibers) initiates and coordinates the electrical events recorded on the ECG as the P wave, QRS complex, and T wave. Heart sounds S₁ and S₂ correspond to AV and semilunar valve closures, respectively, while extra sounds (S₃, S₄) and murmurs indicate structural or functional abnormalities. The Frank–Starling mechanism and the Law of Laplace provide the physiological and biophysical frameworks for understanding how the heart adapts to varying hemodynamic demands. Together, these anatomical structures and physiological principles form the essential foundation for advanced study of cardiovascular pathophysiology, pharmacology, and clinical medicine.