How Does Volume Make the Blood Go Around?

Abstract

The initial response to the question, “How does volume make the blood go around?”, is usually something like: Volume stretches the left ventricular wall and the greater the stretch the greater the stroke output from the left heart because of an increase in preload as first described by Otto Frank and Ernest Starling. While this is true, it is just part of the role that volume plays in the generation of flow around the circulation. At the beginning of the last century, Ernst Starling appreciated the fundamental point that the heart only can pump out what it receives and factors that affect the return of blood to the heart play a key role in what comes back to the heart [1]. The basic principle in this chapter is that the elastic properties of vascular structures are key determinants of the distribution and movement of the blood volume around the circulation. The discussion will revolve around how the distribution of compliances in the circulation, the concepts of stressed and unstressed volume [2, 3], and Sagawa’s concept of time varying elastance of the ventricles [4] explain how much blood flows around the circulation for a given amount of blood volume. Many of the concepts in this paper are derived from a computerized computational model that we have applied to the circulation [5, 6] as well as measurements in animal studies [7–9]. Although a computational analysis does not indicate exactly what goes on in a living organism, its advantage is that one variable can be changed at a time, which allows an analysis of changes in the mechanical properties of the vasculature in steady states. The real-life situation is a composite of these isolated mechanical changes.