Anatomy and Physiology of the Heart
Cardiac Embryology and Development
The heart develops from two distinct embryonic fields, with the right ventricle and outflow tract originating from the secondary (anterior) heart field, while the left ventricle and atria derive from the primary heart field 1. Beginning in the third week of embryonic development, the primitive heart tube starts beating and undergoes twisting and folding movements. By the fourth week, a muscular ventricular septum emerges to form distinct right and left ventricles. Between weeks 5-8, the aorticopulmonary septum forms and fuses with endocardial cushions to create the membranous septum, establishing distinct pulmonary and systemic circulations by the eighth week 1.
During fetal life, the right ventricle accounts for approximately 60% of total cardiac output, providing systemic perfusion via the foramen ovale and ductus arteriosus. At birth, the left ventricle becomes the dominant systemic ventricle while the right ventricle adapts to provide flow exclusively through the pulmonary circulation 1.
Anatomical Structure
Cardiac Chambers
The heart consists of four chambers with distinct anatomical and functional characteristics 2, 3, 4:
Right Atrium and Ventricle:
- The right ventricle has a thin free wall (2-3 mm) and compliant structure forming a hemi-ellipsoid shape that adheres to the left ventricle 1
- Features a heavily trabeculated interior with a large sinus for venous inflow and tubular outflow tract creating a funnel-like configuration 1
- The crista supraventricularis is a unique muscle bridge separating RV inflow (tricuspid annulus) from outflow tract (pulmonic annulus) 1
- The right ventricle is composed of three functional subunits: inlet (tricuspid valve to proximal infundibulum), apical trabecular body, and outlet (RV outflow tract to pulmonary valve) 5
Left Atrium:
- Consists of three components: venous inflow receiving blood from pulmonary veins, the vestibule surrounding the mitral orifice, and the inter-atrial septum 2
- Smooth endocardium lines the thin muscular walls (superior/roof, posterior, left lateral, septal, and anterior) 2
- The left atrial appendage is a finger-like extension with a well-defined oval orifice, neck region, and lobulated body lined by pectinate muscles 2
Left Ventricle:
- Thicker-walled chamber serving as the dominant systemic ventricle 1
- Shares annulus between aortic and mitral valves 1
Valvular Structures
The heart contains four valves with specific anatomical relationships 3, 6, 4:
- Aortic valve: Composed of a virtual ring (hinge point of cusps), anatomic annulus, sinotubular junction, and crown-like ring from cusps 6
- Mitral valve: Shares annulus with aortic valve
- Tricuspid valve: Separated from pulmonic annulus by crista supraventricularis 1
- Pulmonic valve: Located at RV outflow tract
Coronary Circulation
The epicardial coronary arteries supply blood to specific left ventricular segments based on standardized American Heart Association definitions 4. The distribution follows coronary dominance patterns, with vessels originating from coronary sinuses and following defined trajectories 4.
Cardiac Physiology
Right Ventricular Function
Normal RV function is governed by systemic venous return, pulmonary artery load (afterload), pericardial compliance, and native contractility of the RV free wall and interventricular septum 1. The RV requires one-sixth the energy expenditure of the LV because it maintains forward momentum into a highly compliant, low-resistance pulmonary circulation 1.
Key physiological characteristics:
- The RV pressure-volume loop lacks isovolumic phases of contraction and relaxation, has lower peak systolic pressure, and exists at higher steady-state volume compared to the LV 1
- Peak RV pressure occurs before end of systolic ejection, creating a trapezoid-appearing pressure-volume loop 1
- RV ejection fraction is inversely proportional to pulmonary artery pressure, making RV systolic function highly sensitive to afterload changes 1
- The RV has a shallower end-systolic pressure-volume slope than the LV, resulting in minor afterload increases causing large stroke volume decreases 1
Left Atrial Function
The left atrium demonstrates three distinct functional phases during the cardiac cycle 2:
- Reservoir phase: Pulmonary vein forward flow during ventricular systole
- Conduit phase: Pulmonary vein forward flow during early diastole (passive emptying)
- Contractile phase: Pulmonary vein reverse flow by LA contraction during late diastole (absent in atrial fibrillation)
LA enlargement typically occurs along the superoinferior axis more prominently than anteroposterior axis. Abnormal LA function is characterized by decreased compliance and/or contractile dysfunction, with wall thickening and fibrosis contributing to increased stiffness 2.
Ventricular Mechanics
The heart demonstrates six dynamic actions: narrowing, shortening, lengthening, widening, twisting, and uncoiling 7. These mechanical events are produced by the helical configuration of the ventricular myocardial band, which creates a peristaltic wave starting at the right ventricle and progressing toward the aorta 8.
Cardiac myocytes constitute 70-80% of myocardial mass but are terminally differentiated, non-proliferating cells with minimal capacity for division or replacement in adults 9. Myocyte enlargement (genuine hypertrophy) occurs in response to stress, but complete cell division cycles are rare 9.
Clinical Imaging Perspectives
Standard echocardiographic views for anatomical assessment include 3, 4:
- Subxiphoid four-chamber view: Visualizes right atrium, tricuspid valve, right ventricle, left atrium, left ventricle, and pericardial spaces
- Parasternal long axis: Shows RV, septum, LA, LV, aortic valve, and aortic root
- Parasternal short axis: Demonstrates cross-sections from LV apex to aortic root
- Apical four-chamber: Displays both ventricles, atria, mitral and tricuspid valves with clear view of relative chamber volumes and wall motion
Three-dimensional echocardiography provides more accurate and reproducible measurements of ventricular volumes and ejection fraction compared to 2D techniques, making it the modality of choice for clinical evaluation 5.