Ventricular Ejection Pressure Volume Segment

Ventricular Ejection Pressure Volume Segment is the portion of the cardiac cycle, represented graphically as a distinct segment within the ventricular pressure-volume relationship, during which blood is actively expelled from the ventricle into the arterial circulation, characterized by simultaneously falling ventricular volume and changing ventricular pressure.

Locating This Segment Within the Cardiac Cycle

Beginning at the Opening of the Outflow Valve

This segment begins at the moment ventricular pressure rises sufficiently to exceed arterial pressure, causing the valve separating the ventricle from the artery to open and allowing ejection to begin.

Ending at the Closure of the Outflow Valve

The segment concludes when ventricular pressure falls below arterial pressure as relaxation begins, causing the outflow valve to close and marking the end of active ejection for that cardiac cycle.

The Shape of This Segment on the Pressure-Volume Relationship

A Declining Volume Trajectory

Throughout this segment, ventricular volume declines steadily as blood is expelled into the arterial system, moving from the volume present at the start of ejection toward the smaller volume remaining once ejection concludes.

The Accompanying Pressure Pattern

Ventricular pressure during this segment typically rises briefly to a peak before gradually declining as ejection proceeds, reflecting the changing balance between the ventricle's contractile force and the resistance it faces from the arterial system.

Representing Stroke Volume

The horizontal extent of this segment across the volume axis directly represents the stroke volume ejected during that particular cardiac cycle.

Physiological Factors Shaping This Segment

The Influence of Afterload

The resistance faced during ejection, reflecting arterial pressure and vascular resistance, shapes both the peak pressure reached and the extent to which the ventricle is able to reduce its volume during this segment.

The Influence of Contractility

The intrinsic strength of ventricular contraction determines how effectively the ventricle can continue ejecting blood against the resistance it faces, influencing how far volume is able to decline before ejection ends.

The Influence of Preload

The volume present in the ventricle at the start of this segment, determined by preceding ventricular filling, establishes the starting point from which this portion of the pressure-volume relationship begins.

The Upper Boundary of the Ejection Segment

Relationship to Maximal Contractile Capability

The point marking the end of this ejection segment, corresponding to the smallest volume achieved during that cycle, lies along a boundary reflecting the ventricle's maximal pressure-generating capability at that particular volume, a boundary that shifts outward with increased contractility.

Sensitivity to Changing Contractile State

Because this end point reflects the ventricle's contractile capability, changes in contractility directly alter where this segment concludes, with enhanced contractility generally allowing the ventricle to reach a smaller end volume for a given afterload.

Comparing This Segment Across Different Physiological Conditions

Effects of Increased Afterload

A rise in the resistance faced during ejection tends to shift this segment's endpoint toward a larger remaining volume, since the ventricle must work against greater opposition and cannot eject as completely.

Effects of Increased Contractility

An increase in contractility tends to shift this segment's endpoint toward a smaller remaining volume, reflecting the ventricle's enhanced ability to overcome a given level of afterload.

Effects of Increased Preload

An increase in the volume present at the start of ejection extends the starting point of this segment further along the volume axis, generally resulting in a greater stroke volume assuming other factors remain unchanged.

The Significance of Examining This Segment in Isolation

Isolating the Active Contractile Phase

By focusing specifically on the portion of the cycle during which active ejection occurs, this segment allows the ventricle's contractile performance to be examined separately from the distinct processes of filling and relaxation occurring during other phases of the cardiac cycle.

A Foundation for Understanding the Complete Pressure-Volume Relationship

Understanding this ejection segment provides an essential building block toward a complete understanding of the full ventricular pressure-volume relationship, which incorporates this segment alongside the distinct phases of filling and isovolumic pressure change occurring elsewhere in the cardiac cycle.

Summary of Function

Ventricular Ejection Pressure Volume Segment functions as the specific portion of the cardiac cycle representing active blood ejection, shaped by the combined influence of preload, afterload, and contractility, and providing a focused window into the ventricle's contractile performance during the phase in which it actively transfers blood into the arterial circulation.