Re-Inventing How We Teach Venous Return Rasha Jawad & Richard D. McCabe, Ph.D. CMU College of Medicine

Abstract Introduction Guyton’s venous return curve describes venous return Key points covered by this model: in terms of , mean circulatory 1. The concept of central filling pressure. 1 filling pressure, and sympathetic activity. Guyton’s 2 2. The role of both external (GI, Renal, Other) and work revolutionized the understanding of 1 internal (Starling forces) in maintaining fluid cardiovascular physiology at the time it was pressure. published. However, experience shows that venous 3. The role redistribution of fluids within the circulation return curves are difficult to present or understand, by arterial vasomotion (i.e., constriction or dilation) and have limited direct clinical application.2,3 Venous and venomotion. return remains of central importance to the 4. Major mechanisms and controls regulating venous pathophysiology underlying many clinical filling and redistribution. disorders.4,5 5. The importance of venous return in maintaining and . We propose a simple, intuitive model to explain venous return that can be progressively presented to Presentation explain both venous filling and circulatory mobilization of blood. The latter concept 1. Begin with simple circuit and definitions. Define encompasses venoconstriction as well as other locations of , arteries, and veins. Explain parallel mobilizing factors. components. Explain barometers representing MAP 3 4 and CVP. Define these terms. Explain vital nature of We begin with a heart that stopped beating, so that CVP in heart function. Explain equalization of (CVP) and mean arterial pressures. Explain divide of internal and external pressure (MAP) are equal, and yet are not zero due to environment. circulatory filling pressure. Restarting the heart we 2. Define external filling sources, defined as external systems that control level of . Explain illustrate how CVP and MAP develop, explaining that circulatory filling pressure prevents CVP from falling modulators of these systems. below that needed to maintain a preload sufficient to 3. Define internal filling source, most importantly, the support normal cardiac output. role of Starling forces in modulating capillary volume. 4. Explain impact of cardiac pumping. Describe changes to MAP and CVP upon initiation of central drive. External filling sources that control filling pressure include the urinary, gastrointestinal, respiratory, Explain impact of increased pressure at capillary beds. integumentary systems. Internal filling results from 5. Physiology concept using venous return model: the balance of total body water between plasma and in exercise. Explain physiological interstitial fluid, as controlled by transcapillary forces changes that occur as a result of exercise and the triggering of autoregulation via increase in (Starling forces). 5 6 sympathetic tone. In addition to filling, CVP is controlled by the 6. Continued. Impact of on central drive and redistribution of blood within the . impact on heart and vessels. This can include sympathetic mobilization of blood from systemic venous circulation and the References gastrointestinal tract (including the liver and spleen). 1. Guyton, A. C. (1955). Physiol rev, 35(1), 123-129. Metabolic control of precapillary sphincters, such as 2. Beard, D. A., & Feigl, E. O. (2011). Am J Physiol, the opening of muscular capillary beds during 301(3), H629-H633. exercise, can also contribute to CVP. 3. Henderson, W. R., Griesdale, D. E., Walley, K. R., & Sheel, A. W. W. (2010). Critical care, 14(6), 243. The model is qualitative, while the venous return 4. Bressack, M. A., & Raffin, T. A. (1987). Chest, 92(5), curve is quantitative. However, we anticipate it being 906-912. more easily understood, more comprehensive, and 5. Funk, D. J., Jacobsohn, E., & Kumar, A. (2013). more easily applied to clinical medicine. Critical care med, 41(1), 255-262.