Alicia sees a rectangular-looking loop on a graph and memorises the four corners. Tricia memorises arrows showing counterclockwise motion. Kai Kai asks what the shape is actually compressing. The answer is almost an entire ventricular beat: filling, pressure generation, ejection and relaxation, all represented by only two axes.
A ventricular pressure-volume loop plots chamber pressure against chamber volume as the heart moves through one cardiac cycle. The loop shows how a ventricle can contain the same volume at very different pressures, how valves define the phase boundaries, how stroke volume appears as horizontal width, how stroke work appears as enclosed area, and how preload, afterload and contractility alter the loop in different ways.
This article supports How the Heart Works. How the Cardiac Cycle Works owns the time sequence. How the Frank-Starling Mechanism Works owns length-dependent filling response. Here the job is representation: how pressure and volume together reveal pump mechanics.
The loops and numbers below are educational models. They are not instructions for interpreting an individual catheterisation, scan or medical condition.
1. The axes answer two different questions
The horizontal axis is ventricular volume: how much blood is inside the chamber. The vertical axis is ventricular pressure: the mechanical pressure generated within that chamber relative to the chosen reference.
Because pressure and volume are independent coordinates, the same volume can appear at a low pressure during filling and a much higher pressure during systole.
This is the first major lesson of the loop: a ventricle is not a passive container with one fixed pressure for each volume. Its active muscular state changes through time.
2. Time is hidden in the direction around the loop
A pressure-volume graph does not put time on either axis. Time is represented by motion around the path.
In the conventional left-ventricular loop, the path travels counterclockwise: filling moves rightward, isovolumetric contraction moves upward, ejection moves leftward and isovolumetric relaxation moves downward.
Two loops can trace similar shapes at different speeds. The graph therefore preserves the mechanical state trajectory while hiding the duration spent on each segment.
3. Filling forms the lower rightward limb
During ventricular filling, the inlet valve is open and the outlet valve is closed. Volume rises as blood enters from the atrium. Pressure usually rises only modestly because the relaxing ventricle is relatively compliant within its operating range.
The path therefore moves mainly rightward along the lower part of the loop. Its exact curvature reflects the ventricular diastolic pressure-volume relationship and the timing of relaxation and filling.
A perfectly horizontal line would imply no pressure change as volume enters, which is an idealisation rather than real physiology.
4. End-diastole marks the maximum filling state of the cycle
The lower-right corner corresponds approximately to end-diastole. The ventricle has reached its end-diastolic volume and the atrioventricular valve closes as ventricular pressure begins to exceed atrial pressure.
End-diastolic pressure and volume are related to preload, but they are not identical to sarcomere stretch in every condition. Compliance, geometry and external pressure matter.
The corner is therefore a useful state marker, not a universal definition of preload by one number.
5. Isovolumetric contraction creates the vertical right side
After the inlet closes, both main ventricular valves are closed for a brief interval. The myocardium develops active force and ventricular pressure rises sharply while cavity volume changes little.
The loop therefore moves upward at nearly constant volume. This is isovolumetric contraction.
The segment demonstrates why contraction is not identical to ejection. The muscle is already doing mechanical work internally and consuming energy before any blood leaves the chamber.
6. The outlet opens when the pressure condition is met
Ejection begins when ventricular pressure becomes sufficient relative to the pressure in the receiving artery and the semilunar valve opens.
This transition appears near the upper-right corner of the loop. The precise point depends on the arterial load present at that beat.
If aortic pressure is higher, the ventricle must move farther upward before the aortic valve opens. The receiving circulation therefore affects the loop before ejection even begins.
7. Ejection moves the loop leftward while pressure changes
Once the outlet valve opens, blood leaves the ventricle and volume falls. Pressure can continue to rise early in ejection because active myocardial force is still increasing.
Later in systole, ventricular pressure declines while ejection continues and then decelerates. The loop therefore forms a curved upper limb moving from right to left.
The changing pressure during falling volume is another reminder that the ventricle’s active state changes continuously through the beat.
8. End-systole marks the boundary after ejection
The upper-left region of the loop corresponds approximately to end-systole. The semilunar valve closes as the pressure-flow relationship reverses, leaving an end-systolic volume inside the chamber.
The ventricle is not empty. Stroke volume is the difference between end-diastolic volume and end-systolic volume under the usual intact-valve teaching model.
The location of the end-systolic point is strongly influenced by contractile state and afterload.
9. Isovolumetric relaxation creates the vertical left side
After the outlet valve closes, the inlet remains closed until ventricular pressure falls below atrial pressure. Volume changes little while pressure drops.
The loop therefore descends almost vertically at the end-systolic volume. This is isovolumetric relaxation.
When the inlet opens, the next filling phase begins and the loop closes.
10. Loop width is stroke volume
The horizontal distance from end-diastolic volume to end-systolic volume represents the volume ejected during the beat.
If EDV is 140 mL and ESV is 60 mL, loop width is 80 mL. That is the model stroke volume.
A wider loop generally represents a larger cavity-volume change per beat. It does not by itself state the pressure at which that volume was ejected or the energy cost of doing so.
11. Loop height is a pressure range, not work by itself
A taller loop reaches higher ventricular pressures. But peak pressure alone does not determine mechanical work because work also depends on how much volume is displaced at each pressure.
A narrow high-pressure loop can enclose less area than a broader loop operating at a lower peak pressure.
This distinction prevents the intuitive but incorrect rule that a higher systolic pressure must mean more work per beat.
12. Enclosed area represents ventricular stroke work
Mechanical work is the integral of pressure with respect to volume. The net area enclosed by the pressure-volume loop therefore represents external pressure-volume work performed by the ventricle during one cycle.
A rectangular approximation with a net ejection pressure difference of 90 mmHg and stroke volume 70 mL gives 6,300 mmHg·mL. Using 1 mmHg·mL ≈ 0.0001333 J, that is about 0.84 J.
The real loop is curved, so actual work is found by integrating the path rather than multiplying one assumed pressure by one volume. The approximation exists only to show why area has energy units.
13. Stroke work is not total myocardial energy consumption
The heart consumes ATP during calcium cycling, ion pumping, cross-bridge turnover and internal deformation even when cavity volume changes little.
Pressure-volume area and related frameworks can account for more mechanical energy than stroke work alone, but neither should be casually equated with total metabolic expenditure.
The loop is a mechanical representation. Metabolism requires a different measurement boundary.
14. The end-diastolic pressure-volume relationship describes passive filling mechanics
The end-diastolic pressure-volume relationship, or EDPVR, describes the pressure required to hold the ventricle at different volumes when active contraction is minimal.
It is nonlinear. At lower volumes, substantial volume can be added with relatively small pressure increases. At higher volumes, pressure rises more steeply.
A less compliant ventricle has a steeper or left-shifted diastolic relationship under comparable definitions. This is a property of filling mechanics, not contractility.
15. The end-systolic pressure-volume relationship indexes contractile state
If several loops are generated under different loading conditions, their end-systolic points can define an end-systolic pressure-volume relationship, or ESPVR.
The slope, often termed end-systolic elastance under a linear approximation, is relatively insensitive to preload compared with simple output measures and is used as an index related to ventricular contractility.
The NCBI text Pressure Volume Loops Provide a Quantification of Contractility explains why end-systolic states are useful for separating intrinsic contractile behaviour from simple loading changes.
16. Increased preload shifts the loop rightward and usually widens it
Hold afterload and contractility approximately fixed, then increase venous return. End-diastolic volume rises, moving the right side of the loop farther right.
Through the Frank-Starling mechanism, stroke volume tends to increase, so the loop becomes wider.
A recent review of pressure-volume loop interpretation describes this rightward expansion and larger stroke volume under increased preload when afterload and contractility are controlled.
17. Increased afterload makes the ventricle build more pressure before and during ejection
If the receiving arterial load rises while preload and contractility are held comparable, the ventricle must generate more pressure before the outlet opens.
End-systolic volume tends to rise because the ventricle ejects less before contraction ends. The loop becomes taller and narrower.
This is the isolated first-beat effect. In the intact circulation, the larger residual volume changes the next filling state and reflexes can alter contractility and vascular tone.
18. Increased contractility shifts the end-systolic point leftward
Increase myocardial contractile state while keeping preload and afterload comparable. The ventricle can eject more before reaching end-systole, so end-systolic volume falls.
The loop widens leftward and the end-systolic pressure-volume relationship becomes steeper under the usual representation.
This distinguishes true inotropic change from a mere increase in preload. Both can increase stroke volume, but they move different parts of the loop for different reasons.
19. Reduced compliance raises filling pressure at a given volume
Suppose the passive ventricular wall becomes less compliant in a model while systolic contractility is unchanged. At the same end-diastolic volume, end-diastolic pressure is higher.
The diastolic limb therefore shifts upward or leftward depending on how the relationship is drawn and compared.
This is why filling pressure cannot be used as a universal substitute for filling volume. Diastolic mechanics change the conversion between them.
20. Arterial elastance compresses afterload into one useful number
Effective arterial elastance is commonly approximated as end-systolic pressure divided by stroke volume. It provides a lumped representation of the arterial load seen by the ventricle.
It is not identical to systemic vascular resistance. It incorporates aspects of resistance, arterial compliance, characteristic impedance and timing into one effective load measure.
A straight line from the volume-axis intercept toward the end-systolic point can graphically represent this effective arterial load on a pressure-volume diagram.
21. Ventricular-arterial coupling compares the pump with its load
Comparing end-systolic ventricular elastance with effective arterial elastance provides one way to describe how the ventricle and arterial system are matched.
The heart does not operate against a passive fixed resistance. Its performance depends on the relationship between intrinsic ventricular mechanics and the load presented by the arterial tree.
This coupling perspective explains why the same ventricle can produce different stroke volume and efficiency when the vascular system changes.
22. Time-varying elastance is a model of changing ventricular state
One influential model treats ventricular elastance—pressure relative to volume above a reference volume—as changing through the cardiac cycle.
Elastance is low during diastole, rises during systole and reaches a maximum near end-systole. This model explains how the same chamber volume can support different pressures depending on activation state.
No model is exact under every condition, but time-varying elastance provides a compact bridge between muscle activation and pressure-volume behaviour.
23. Worked problem: preload versus contractility
Loop A has EDV 120 mL and ESV 60 mL, giving stroke volume 60 mL. Loop B has EDV 150 and ESV 65, giving 85 mL. The larger EDV suggests increased preload contributed to the wider loop.
Now compare Loop C with EDV 120 and ESV 35, giving 85 mL. C reaches the same stroke volume as B without increased EDV. A change in contractility or afterload must therefore be considered.
Equal stroke volume does not imply equal mechanism. The loop shape preserves information that one output number deletes.
24. Worked problem: peak pressure, work and power can rank differently
Model A ejects 50 mL across an average net pressure of 100 mmHg. Model B ejects 80 mL across 75 mmHg. Their rough rectangular work estimates are 5,000 and 6,000 mmHg·mL.
A has the higher pressure but B performs more pressure-volume work per beat. If A beats twice as fast, however, its mechanical power could exceed B’s because power is work per time.
The graph therefore teaches three different quantities: height, area and traversal rate. They answer different questions.
25. The pressure-volume loop in one causal chain
The ventricle fills and moves rightward along its diastolic relationship. The inlet closes at end-diastole. Muscle activation raises pressure at nearly fixed volume. The outlet opens and volume falls during ejection. End-systole is reached according to contractile state and arterial load. The outlet closes. Pressure falls at nearly fixed volume until the inlet opens again. The loop’s width records stroke volume, its enclosed area records pressure-volume work, its diastolic boundary records filling mechanics and its end-systolic relationship records contractile behaviour across changing loads.
Alicia no longer memorises four corners without meaning. Tricia keeps the counterclockwise arrows but labels every valve transition. Kai Kai changes one variable at a time and watches the loop respond. The drawing has become a simulator for causal reasoning.
The deeper lesson is that a good scientific graph does more than display data. It preserves the relationships needed to distinguish mechanisms that would look identical if we observed only one number.
Evidence trail and connected reading
For an accessible modern review, see Looking Back, Going Forward: Understanding Cardiac Pathophysiology from Pressure–Volume Loops. For the contractility framework, see NCBI Bookshelf: Pressure Volume Loops Provide a Quantification of Contractility. The foundational interaction of inotropy and lusitropy with loop shape is also described in Influence of altered inotropy and lusitropy on ventricular pressure-volume loops.
Return to the parent: How the Heart Works. Continue to How the Cardiac Cycle Works for the time sequence, How the Frank-Starling Mechanism Works for preload and How Cardiac Output Works for flow per minute.