A revision of the venous return-cardiac output graph using both "cardiac input" and a clinical key

For many medical students, the venous return/cardiac output graph is one of the more challenging concepts in cardiovascular physiology. From some starting point, this graph describes what happens to venous return (VR), cardiac output (CO), and right atrial pressure (RAP) when there are changes from some baseline in either contractility, total peripheral resistance (TPR), and/or either blood volume or venous compliance. With up to 14 intersection points from the starting point in the graph’s center, many preclinical students have difficulty both in reading the graph and in understanding the clinical significance of each change. This can be partly due to the use of “venous return” when students may better understand “cardiac input”. It may also be due in part from the lack of a clinical key for what each of the 14 changes on the graph mean to them as future physicians. As a response to these problems, a new form of this graph is presented. It has two changes from older versions. First, on the Y axis, the term “cardiac input” (CI) is used alongside “venous return” (VR). When students realize that CI must equal CO for the heart pump to function, it becomes easier to understand equilibrium points as where these two match. Second, opposing changes in the three main variables are labeled in pairs on the graph. Effects of increased vs. decreased contractility from baseline are shown as points A vs. B on the graph, TPR as C vs. D, and blood volume as E vs. F. Second, a clinical key is included that briefly describes each of the fifteen cardiac input/output equilibrium points in a clinical context. Finally, a short guide on how to first proceed from normal to some change point A-F, and from there if necessary to compensation/decompensation points G-N is provided so that students can more easily read this graph and interpret the changes to pathophysiology of the cardiovascular system. Validation of the format in collaboration with others would be welcomed. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.