Teaching Cardiovascular Effects of Exercise Using an Integrated Model of the Cardiovascular System

Abstract

Previously, a teaching model was developed to encompass cardiovascular concepts, including but not limited to venous capacitance, filling pressure (preload), and control of arterial and venous tone (1). The model was designed to articulate integrated cardiovascular responses in multiple physiologic and pathophysiologic conditions. From personal experience, students often assume that since sympathetic stimulation increases during exercise, total peripheral resistance (TPR) must also increase. This reflects a misunderstanding of the magnitude of the role of autoregulatory vasodilation in increasing muscle blood flow during exercise (2). It also reflects a lack of appreciation in the interplay of metabolic demand and the baroreceptor reflex as key factors controlling cardiac output. We use the model to show the following: the initial response to exercise is muscular autoregulatory vasodilation, leading to a decrease in TPR; decreases in TPR may initially decrease mean arterial pressure (MAP) (3); reduction in MAP reduces baroreceptor action potentials, resulting in increased sympathetic activity, including non-muscular vasoconstriction and venoconstriction; venoconstriction mobilizes the blood stored in the venous system (capacitance); blood mobilized by venoconstriction helps to maintain filling pressure (preload) and also satisfies the increased demand for blood by the exercising muscles; increases in sympathetic tone increase the inotropic, chronotropic, and lusitropic states of the heart, and; the consequent increase in cardiac output chiefly results from the interaction of systemic vasodilation in response to metabolic demand and the response of the baroreceptor reflex. This teaching model helps emphasize the importance of autoregulation to meet metabolic needs. It also illustrates the importance of sympathetic venoconstriction in maintaining filling pressure in diverse physiological and pathophysiologic conditions. Finally, it helps students understand that cardiac output is primarily regulated in response to tissue demand, as well as how this may occur. Understanding the normal response to exercise helps the student to recognize abnormal responses in pathophysiologic conditions, such as hypertension (4). References Jawad R., McCabe R. FASEB J 34(Supp1):1 (2020) Laughlin MH. Am J. Physiol 277 (Adv Physiol Educ 22): S244–S259 (1999) Guyton A. Circulation.64 1079-1088 (1981) Kim D., Ha JW. Clin Hypertens 22, 17 (2016)