Notes on the revised Starling principle

Many researchers are more accustomed to the traditional Starling principle (SP) interpretation in capillary exchanges, compared to the revised Starling principle (RSP), owing to its friendly formulation and its apparently greater adherence to the reality. However, the simplification holds if and only if SP is limited just to the hydrostatic and osmotic pressures in the vascular compartment, omitting the action of the analogous pressures in the interstitial tissue (simplified traditional Starling principle or sSP). This omission allows the elegant interpretation of a fluid exchange symmetry along the capillary, from the arteriolar to the venular end, which is at the base of the Filtration-Absorption Model of capillary exchanges. However, if the interstitial factors are not negligible instead, even SP capillary fluid balance shifts asymmetrically towards filtration, instead of absorption. This asymmetry is the same of the revised Starling principle (RSP), thus the simpler assumption of SP is a clear misinterpretation, due to an erroneous simplification with no historical basis. Indeed, already in 1968 Zweifach and Intaglietta documented an incomplete absorption at the venular end of the capillaries. The current short review underlines the differences between the traditional and the revised Starling principles, with the aim of providing a simpler and clear comparison between the two models of capillary fluid exchanges.