An inner medullary concentrating process actuated by renal pelvic/calyceal muscle contractions: assessment and hypothesis.

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.

Two fluid compartments in the renal inner medulla: a view through the keyhole of the concentrating process.

Approximately four decades ago, the countercurrent theory became influential in studies on the concentrating process in the mammalian kidney. The theory successfully represented the concentrating process in the outer medulla, but the problem of the concentrating mechanism in the inner medulla, as defined by Homer Smith has remained essentially intractable. In a recent comprehensive review by Knepper and coworkers of various theories and models, attention was refocused on the possible role of hyaluronate (HA) in the inner medullary concentrating process. The authors proposed a hypothesis that HA can convert hydrostatic pressure to concentrating work.Here, we briefly survey the earlier ideas on the role imputed to HA and present a new hypothesis which is different from that of Knepper and coworkers. We estimate that the hydrostatic pressures available in the inner medulla can account only for a very small fraction of the concentrating work. We hypothesize that the role of HA is tied up with extravasated plasma albumin and suggest that owing to the property of HA solutions to exclude other macromolecules, extravasated plasma albumin and HA constitute two fluid compartments in the interstitium in the inner medulla. In this proposed two-compartment model, the Gibbs-Donnan distribution influences the movement of ions and water between the HA and the extravasated albumin compartment. To relate the hypothetical role of HA to the concentrating process, we briefly describe new results obtained by other investigators on the accumulation of urea in the inner medulla. This subject has been critically reviewed recently by Yang & Bankir.Many processes have been identified as contributing to the concentrating process in the mammalian inner medulla. We speculate that among these many processes, the primary responsibility for the final concentration of the excreted urine may be portioned out differently in different mammalian species.

Mode locking in reversed-field pinch experiments.

The MHD mode trajectory in the Madison Symmetric Torus reversed-field pinch has been found to obey the sine-Gordon equation. Corresponding to experiment, a perturbation analysis predicts the locations of mode locking to be at the vacuum chamber poloidal and/or toroidal gaps. The mode's energy dissipates when it locks, as shown by a decaying spiral phase-plane trajectory. Unlocked modes travel around the torus without an abrupt energy loss. By varying key machine parameters obtained by statistical analysis, the probability of locking in accordance with the experimental results can be predicted.

Beta limits for torsatrons.

An ideal magnetohydrodynamic equilibrium and stability code is used to study ballooning modes in torsatrons. The most dangerous modes turn out to be those with low poloidal and toroidal wave numbers. Beta limits for equilibrium and stability are determined for an [unk] = 2 ultimate torsatron with large [unk] = 1 and [unk] = 3 sidebands.