Three levers that set stroke volume
Stroke volume, the amount of blood the heart ejects per beat, is governed by three relatively independent factors. Preload is the degree of stretch on the ventricle just before it contracts, set largely by how much blood has filled it. Afterload is the resistance the ventricle must overcome to eject blood, related closely to arterial pressure. Contractility is the intrinsic strength of the contraction at a given preload and afterload.
By the Frank-Starling relationship, greater filling (preload) stretches the muscle and generally increases the force of the next contraction, up to a point. Higher afterload tends to reduce ejection, while greater contractility increases it.
How they appear on a pressure-volume loop
A pressure-volume loop plots ventricular pressure against volume across one heartbeat, and each lever moves the loop in a characteristic way: changes in preload shift the filling volume, afterload changes the pressure at which the valve opens, and contractility changes the slope of the end-systolic relationship. This makes the loop a useful teaching tool for separating effects that can otherwise look similar.
This is a conceptual physiology model for education, not a clinical assessment tool. Cardiac function in a real patient is evaluated by qualified clinicians using appropriate testing.