Teaching venous return without throwing students a curve

Here we develop a comprehensive presentation of venous return. Guyton’s venous return curve describes venous return in terms of right atrial pressure, mean circulatory filling pressure, and sympathetic activity. This work demonstrated the key role of venous return in cardiac function, and it has been part of undergraduate medical education since that time. However, experience shows that venous return curves are difficult to present or understand, and have limited direct clinical application. , Venous return remains of central importance to physiology and the pathophysiology underlying many clinical disorders. , We propose a simple, intuitive model to explain venous return that can be progressively presented to explain both venous filling and circulatory mobilization of blood. The latter concept encompasses venoconstriction as well as other mobilizing factors. The presentation centers around venous return for students familiar with the role of central venous pressure (CVP) in cardiac preload. The model is presented in the following progression: Start with a simple circuit consisting of the heart, arteries, a set of capillaries, and veins at stop‐flow condition with a specified filling pressure. Illustrate the development of normal CVP and mean arterial pressure (MAP), starting from stop‐flow to normal cardiac output. Introduce positive or negative external filling sources that control filling pressure (i.e. gastrointestinal [GI], urinary, respiratory and integumentary systems). Add transcapillary forces as the factors controlling internal filling, via shifting volume between interstitial fluid (ISF) and plasma. Model venules ‐ added in parallel in the diagram ‐ as mobilizers of blood under the control of sympathetic nerves. Add other mobilizing factors (i.e. GI/splenic blood, muscular pumping, muscular filling) as additional parallel elements under appropriate controls. The model is qualitative, while the venous return curve is quantitative. However, we anticipate it being more easily understood. It is also more comprehensive and more easily applied to clinical medicine. Additional regulatory and pathophysiological factors are easily incorporated into this model. We plan to compare students’ understanding of venous return following presentation of this model versus their understanding following presentation of Guyton’s venous return curve.