Heart Anatomy and Cardiac Cycle

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.

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?

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.

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.

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.

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.

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.

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.

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.

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.

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.