Transmural pressure and vascular resistance in soft-walled vessels

Abstract

Flow through a segment of soft-walled Penrose tubing was measured as perfusing head, extravascular pressure, and outlet pressure were varied. As long as transmural (intravascular minus extravascular) pressure at the downstream end of the soft-walled segment was positive, the vessel was distended and resistance was relatively low and unaffected by inlet or outlet pressures. When transmural pressure became zero, the vessel was only partially filled and delivery for a given arteriovenous pressure gradient decreased with the reduced cross section of the vessel. When transmural pressure was negative, a rise in venous pressure raised intraluminal pressure, and when this was sufficient to produce a positive transmural pressure, the collapsible segment became distended and resistance was diminished. Further rises in transmural pressure had minimal effects on resistance. Implications of these findings in terms of critical closing pressure, the Starling “resistance,” and related problems of flow are discussed.