Comments on flow through collapsible tubes at low Reynolds numbers.

In a recent paper by Lyon et al. (Flow through Collapsible Tubes at Low Reynolds Numbers. Circ Res 47: 68-73, 1980), the authors contend that waterfall models adequately predict flow through a Starling resistor only for Reynolds numbers <1. Their basic conclusion was that caution should be used when applying waterfall models to the mammalian circulatory system since Reynolds numbers are usually greater than 1 in most blood vessels. This paper points out what I feel has long been a common misconception: that is, failure to distinguish the basic difference between the waterfall model and the Starling resistor. A Starling resistor is a pressure-regulating system which maintains the pressure proximal to a constriction at a constant value. The maintenance pressure is independent of the flow rate (see panel A of Fig. 1). The only equation describing the Starling resistor model is that Ps equals a constant. This system cannot control flow through the tubing system. When a Starling resistor is connected to a flow source, flow will be determined by the source impedance rather than by the Starling resistor.