1.10 Heart Sounds and Valve Physiology Foundation

Heart Sounds and Valve Physiology Foundation is the body of physiological knowledge concerned with the mechanical behavior of the cardiac valves and the acoustic events these valves generate as they open and close during the cardiac cycle. This foundation integrates the structural function of the atrioventricular and semilunar valves with the pressure and flow relationships that cause them to move, and it explains how these mechanical events produce the audible heart sounds used clinically to assess cardiac function.

The Cardiac Valves

The heart contains four valves, each ensuring unidirectional blood flow through its respective chamber transition.

Atrioventricular Valves

The mitral valve, between the left atrium and left ventricle, and the tricuspid valve, between the right atrium and right ventricle, open during diastole to permit ventricular filling and close at the onset of systole to prevent backflow of blood into the atria.

Semilunar Valves

The aortic valve, between the left ventricle and aorta, and the pulmonary valve, between the right ventricle and pulmonary artery, open during ventricular ejection to permit forward flow into the great vessels and close at the onset of diastole to prevent backflow into the ventricles.

Mechanism of Valve Opening and Closing

Valve motion is governed entirely by the pressure gradients across each valve, not by active muscular control of the valve leaflets themselves.

Pressure-Driven Opening

A valve opens whenever the pressure upstream of it exceeds the pressure downstream of it, causing the leaflets to separate and allow forward flow.

Pressure-Driven Closure

A valve closes whenever the pressure downstream of it exceeds the pressure upstream of it, causing the leaflets to coapt and preventing retrograde flow.

Origin of Heart Sounds

Heart sounds arise from the vibrations generated within the cardiac structures and adjacent blood when the valves close and the resulting flow is abruptly decelerated.

First Heart Sound

The first heart sound (S1) is produced by the closure of the atrioventricular valves at the onset of ventricular systole, marking the beginning of isovolumetric contraction.

Second Heart Sound

The second heart sound (S2) is produced by the closure of the semilunar valves at the end of ventricular ejection, marking the beginning of isovolumetric relaxation.

Additional Heart Sounds

Under certain physiological or pathological conditions, additional sounds may be audible, including a third heart sound (S3) associated with rapid ventricular filling and a fourth heart sound (S4) associated with atrial contraction against a stiffened ventricle.

Relationship to the Cardiac Cycle

Because valve closure defines the transitions between the major phases of the cardiac cycle, heart sounds serve as reliable audible markers of these transitions.

Systolic Interval

The interval between S1 and S2 corresponds to ventricular systole, encompassing isovolumetric contraction and ventricular ejection.

Diastolic Interval

The interval between S2 and the subsequent S1 corresponds to ventricular diastole, encompassing isovolumetric relaxation and ventricular filling.

Diagrammatic Summary

Clinical Significance

Auscultation of heart sounds, together with their timing, intensity, and any accompanying murmurs, provides essential information about valve competence and cardiac chamber function. Abnormalities in valve structure, such as stenosis or regurgitation, alter the normal pattern of heart sounds and produce characteristic murmurs, making the physiology of valve motion and sound generation foundational to the clinical assessment of cardiovascular health.