Interstitial Hydraulic Gating: interstitial volume as a mechanical gate

If a continuous fluid compartment runs between the body's systems, its volume is a variable those systems are exposed to. The interstitium is a continuous, fluid-filled compartment running between nearly every system of the body, and identifiable stromal cells set the matrix that determines its volume. This paper proposes that the volume is itself a mechanical variable: interstitial hydration gates the patency of the axons, capillaries and perivascular conduits the compartment bounds, so that compaction and edema become opposite failures of one gate rather than two unrelated pathologies. The proposal is stated as a falsifiable mechanism, with the hydration-sensitivity ranking on which it depends identified as a prerequisite to be measured rather than a prediction. Competing cell-autonomous and aquaporin-centered accounts are set out alongside the observations that would separate them from this one, and an openly deposited interactive model renders each prediction so that a reader can contradict it directly. Preprint. Not peer reviewed. This paper and its companion atlas together supersede the earlier combined preprint at doi:10.5281/zenodo.22207857.