Physiological Perspective:
Flow Limitation
As PRA is lowered relative to Pms, venous return rises. However, this increase is not limitless. Beyond a certain point—represented by the inflection on the Guyton venous return curve (red dot), further reductions in PRA fail to produce additional increases in venous return. To the left of this point, the curve flattens, indicating that the system has reached its maximum capacity for venous return.
And why does this happen? To understand this phenomenon, it is helpful to recall that veins behave as collapsible tubes. Flow through such a tube depends on maintaining a positive transmural pressure (PTM > 0), that is, the pressure inside must exceed the surrounding pressure. When intravascular reflected pressure from the pleura becomes way too negative and thus, transmural pressure falls to zero, the vessel collapses, and flow becomes mechanically limited due to an exponential rise in flow resistance, no longer increasing despite further drops in PRA.
In humans, venous vessels collapse at a PTM of 0 (zero).
When the PTM < or = to 0, the tube collapses
Maximal Venous Return (Flow Limitation)
Remember that venous flow moves from a higher pressure (Pms) to a lower pressure (PRA). As PRA decreases, venous return rises. As long as PRA remains above the surrounding pressure (P outside), and thus transmural pressure stays positive, the driving pressure for flow is simply Pms − PRA.
However, if PRA is lowered below P outside, transmural pressure becomes zero or negative, causing the vein to collapse at that point. This transiently halts flow through the segment. Beyond this point, further reductions in PRA do not increase flow because the maximal effective pressure gradient is now Pmsf − P outside.
This phenomenon is known as the “vascular waterfall” or Starling resistor effect. A Starling resistor is a system in which flow depends not only on the pressure gradient between the inflow and outflow (Pmsf − PRA) but also on the pressure surrounding the tube (P outside). In other words, external pressure can become the critical determinant of flow once it exceeds intraluminal pressure. In a Starling resistor that collapses when PTM = 0, PRA no longer determines flow once it falls below P outside, the system has reached its maximal venous return capacity, and flow is governed by the difference Pms − P outside rather than Pms − PRA.
On a Guyton curve, maximal venous return is represented by the flat segment to the left of the red dot. As shown, further reductions in PRA in this region do not yield additional increases in flow, because PRA is now below P outside and no longer determines venous return. The point of maximal venous return typically occurs where the great veins enter the thorax from the abdomen. Since abdominal pressure (P abdomen) is usually close to 0 mmHg, this point generally corresponds to a PRA slightly below 0 mmHg.
