Reinventing How We Teach Venous Return

Abstract

We propose a simple, intuitive model to progressively explain the principles underlying venous return. At stop-flow, mean circulatory filling pressure (MCFP) can be defined by the filling of the cardiovascular system from internal and external environments, blood vessel compliance, and oncotic pressure. The dynamic distribution of blood within a regular cardiac system and the establishment of central venous pressure (CVP) and mean arterial pressure (MAP) can then be explained by cardiac output and factors controlling distribution. Lastly, control of the cardiovascular system can be explained by changes in resistances and compliances of the blood vessels and their effect on CVP and cardiac output while briefly considering how these are controlled.

Keywords: Cardiovascular; Education; Instructional design; Physiology.

Fig. 1

Model for control of venous return. Open arrows represent exchange of fluid between plasma and the external environment (left, the line meant to be a division between the two) and also between plasma and interstitial fluid (bottom). Systemic venous circulation, including venous blood volume reservoirs (represented by dilated areas), enters the right atrium via the right atrioventricular valve. After first transiting the pulmonary circulation, the systemic arterial circuit is pumped from the left ventricle via the aortic valve. Arterial and venous circulations are joined by capillaries. Control of distribution is envisioned as involving the heart (top line on right), pre-capillary sphincters (second line on right), arterioles, and venules (bottom line on right). Control mechanisms are not specified but include neural, hormonal, and metabolic. The pulmonary circuit is only included for completeness