Physiology loops in 3D

Some of the classic graphs of physiology can be unified if you think in three dimensions. The pressure-volume loop, the ventricular pressure tracing, and the volume curve are the same cardiac cycle seen from three perspectives. In a forced exhalation, the flow-volume curve and the spirogram are similarly two views of a breath.

Each figure here shows the three-dimensional curve, whose projections into 2D form the more familiar graphs, from a physiological model. You can turn it until one of the familiar graphs appears, and change the physiological parameters to see how all the views move together.

I first built a 3D version of the pressure-volume loop years ago, after I imagined this while sitting in the second-story atrium of the medical education center at Harvard Medical School and made a series of napkin sketches. I was inspired by looking at curves in a book by Eugene Braunwald, a famous cardiologist, and wondered if I could unify some of them and bring them to life by making them interactive.

• Pressure-volume-time loop: one cardiac cycle, with sliders for preload, afterload, and contractility
• Flow-volume-time loop: one forced exhalation, for normal lungs, and ones representing obstructive and restrictive pulmonary disease