Graphical Placement of Transport Segments Along the Nephron From Urine Concentration Pattern Developed With Stop Flow Technique

Abstract

The authors characterize transport within segments at points along the nephron by the concentration pattern developed along the renal tubule during stopped flow (brief ureteral occlusion during osmotic diuresis in dogs). The pattern is caught in serial urine samples on reinstatement of flow. They here develop a system of graphical construction which places active transport segments of proper length and position to account for the concentration patterns developed for the substances: Sodium, glucose, phosphate and para-aminohippurate (PAH). The system is based on mixing phenomena as nephrons of different length deliver their fluid content into mixed samples. Glucose reabsorption, phosphate reabsorption and PAH secretion occur in identical proximal segments, containing 45% of the total stop flow nephron volume. A far distal area for very effective active sodium reabsorption contains about 29% of the occluded nephron volume. The sodium concentration pattern does not delineate its own proximal segment but forms a plateau in that area. This results from the fact that proximal concentrations are not lowered during stop flow but remain the same as in new flow and free flow proximal fluid, no lower than the 30–60 mm concentration of free flow urine itself, considerably higher if distal removal of Na occurs during the new outflow.