Plot a single heartbeat as pressure against volume and it closes into a loop, traced counter-clockwise: fill, squeeze at constant volume, eject, relax. The loop's width is the stroke volume — the blood ejected on that beat.
Three independent levers set the loop's shape. Preload — how full the ventricle gets — slides the bottom-right corner along the passive filling curve; more filling makes a wider loop. That is the Frank-Starling law: the heart pumps what it receives.
Afterload is the arterial load the ventricle ejects against. Steepen it and the end-systolic corner rides up the contractility line: the ventricle hits a higher pressure sooner, ejects less, and leaves more blood behind.
Contractility is the slope of the end-systolic line itself. The end of ejection always lands where that line meets the arterial-load line — the two-line coupling that sets exactly how far the ventricle empties.
Push a lever far enough — a stiff arterial load, a stenotic valve, or a rate so fast diastole can't fill — and the loop collapses to a thread. That is a failing pump.
The bench stalled mid-beat. The lesson continues without it — everything you have already done is safe.