1.9.2 Systole Definition

Systole Definition is the precise characterization of systole as the phase of the cardiac cycle during which the heart muscle contracts, most specifically referring to ventricular contraction, during which pressure within the ventricles rises and blood is ejected into the pulmonary artery and aorta. This definition establishes systole as the contractile phase of the cardiac cycle, distinguished from diastole by the direction of pressure change and the resulting movement of blood out of, rather than into, the ventricular chambers.

Elements of the Definition

Phase of Active Muscular Contraction

Systole is defined as the period of active contraction of cardiac muscle, during which the ventricular walls shorten and thicken, generating the mechanical force required to increase intraventricular pressure.

Rising Intraventricular Pressure

Central to the definition is the characteristic rise in ventricular pressure that occurs during systole, beginning with an isovolumetric phase and continuing as pressure exceeds arterial pressure and ejection commences.

Ejection of Blood into the Great Arteries

The definition specifies that systole culminates in the ejection of blood from the ventricles into the pulmonary artery and aorta, the defining mechanical outcome of ventricular contraction.

Distinguishing Features

Contrast with Diastole

Systole is distinguished from diastole by the direction of both pressure change and blood movement, with systole characterized by rising pressure and outward ejection, and diastole characterized by falling pressure and inward filling.

Division into Isovolumetric and Ejection Phases

A defining feature of systole is its internal division into an initial isovolumetric contraction phase, during which pressure rises without a change in ventricular volume, followed by an ejection phase during which volume decreases as blood is expelled.

Distinction Between Atrial and Ventricular Systole

The definition accounts for the fact that systole can refer specifically to either atrial or ventricular contraction, with ventricular systole being the phase most commonly denoted by the term when used without further qualification.

Purpose of the Definition

Establishing the Contractile Phase of the Cardiac Cycle

A precise definition of systole establishes the specific phase of the cardiac cycle associated with active contraction and blood ejection, distinguishing it clearly from the relaxation and filling phase of diastole.

Foundation for Understanding Pressure and Volume Dynamics

The definition provides the basis for understanding the pressure and volume changes that occur during ventricular contraction, including the isovolumetric and ejection subphases that compose systole.

Clarifying Systole's Role Within the Cardiac Cycle

By specifying that systole is the contractile, ejecting phase of the cycle, the definition delineates its relationship to diastole, together forming the complete repeating sequence of the cardiac cycle.