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We propose a mechanism of an inner medullary concentrating process in which water extraction is accomplished by a colloid osmotic mechanism and hydrostatic pressure. There are 3 essential features of the proposal: 1. the fluid compartmental structure of the inner medullary interstitium: owing to molecular exclusion, negatively charged macromolecules, i.e. hyaluronan and extravasated plasma albumin form separate compartments (the HA and the EPA compartments); the resulting Gibbs-Donnan effect governs the movements of both ions and water. 2. NaCl, in high concentration in the inner medulla conditioned by the outer medullary countercurrent processes, significantly reduces the equilibrium colloid osmotic pressure between these compartments. 3. Urea, also accumulated here by special transport mechanisms, increases the mobility of water molecules and the flexibility of the HA fibrils by loosening hydrogen bonds. These features suggest that rhythmic, small pressure increases of the pelvic/calyceal muscles squeeze dilute fluid out of the HA compartment and, at the same time, accelerate the outflow of fluid and albumin into the ascending vasa recta from the EPA compartment. Further, they suggest a mechanism for the phenomenon that living organisms utilize hydrostatic pressure generated by muscle contractions in water economy namely, concentrating and diluting body fluids.

An Inner Medullary Concentrating Process Actuated by Renal Pelvic/Calyceal Muscle Contractions: Assessment and Hypothesis