Pressure-volume-time loop
One heartbeat of the left ventricle, drawn in three dimensions: pressure up, time from left to right, and volume from front to back. Turn it by dragging, or use the buttons to go straight to a 2-D view. From the side you see the pressure-volume loop. From the front you see the ventricular pressure tracing, with aortic and left atrial pressure behind it, and from above, the volume curve.
The sliders change preload, afterload, and contractility. The normal cardiac cycle stays in place as a faded gray loop, with its shadows dashed on the walls, so you can compare the new cardiac cycle with it. On the pressure-volume wall, the ESPVR (end-systolic pressure-volume relation) is the line whose slope is contractility, and the EDPVR (end-diastolic pressure-volume relation) is the curve the ventricle fills along.
Questions this figure helps answer
The figure shows a model of a healthy adult heart, not any one person's.
- What happens inside the heart during one heartbeat? The figure shows the four phases of the cycle as pressure and volume change over time.
- What does ejection fraction mean? The readout gives the share of the blood in the ventricle that each beat pumps out, 59% at the normal settings.
- How does high blood pressure change the heart's work? Raise afterload: the loop grows taller and narrower, and less blood leaves with each beat.
The model behind this figure
The curves come from a model of both sides of the heart and the circulation. Each ventricle and atrium is a chamber whose stiffness rises and falls with every cardiac cycle (a time-varying elastance), and the valves let blood flow one way. Preload scales the volume of blood available to fill the heart, afterload scales systemic vascular resistance, and contractility scales the stiffness of the left ventricle at the end of systole. The circulation is closed, so one slider changes more than one thing: more filling also raises aortic pressure, for example. The model leaves out reflexes and the slow rise in contractility that follows an increase in afterload.
At its normal settings the model gives an end-diastolic volume of 125 mL, an end-systolic volume of 51 mL, an ejection fraction of 59%, and aortic pressure of 121/78 mmHg at 75 beats per minute.
The ventricle follows the time-varying elastance model of Suga and Sagawa (Circulation Research, 1974), with the elastance curve described by Stergiopulos and colleagues (American Journal of Physiology, 1996).
Using this figure with an AI assistant
If you use an AI assistant that can see this page, whether it is built into your browser or opens the page in its own browser, you can ask it about this figure, ask it to show you what happens when a setting changes, or ask it to take you through one of the tours. It can move the sliders, turn the figure, and explain what you see. The rest of this section is for the assistant.
An AI assistant can change this figure in three ways. In a browser with WebMCP, the page offers its actions as tools. In the page's scripts, the same actions are in window.physiologyFigures, with a description of the controls and what they mean, the views, the values shown, the model's limits, and the Show Me tours. Both move the sliders the way a reader would, wait for the figure to redraw, and return the exact values. An assistant can also type into the boxes beside the sliders, but then it has to read the results off the screen. The page address always holds a link to the current settings and view.