Micropuncture study of the proximal and distal tubule in the dog

Abstract

The transfer of water, Na, and K in the proximal and distal tubule of the antidiuretic dog was studied by means of micropuncture techniques. Approximately 45% of the filtered water is reabsorbed in the surface convolutions of the proximal tubule, 25% in the long loops of Henle, 10% in the distal tubule, and 20% in the collecting ducts. As suggested by previous measurements of osmolality, the water permeability of the distal convoluted tubule is low. A limiting concentration gradient for Na is present throughout the distal tubule, and net reabsorption in this segment is negligible. Reabsorption of K in the proximal tubule and loop of Henle is similar despite widely varying excretion rates induced by K depletion or loading. During low excretion rates the collecting ducts are the site of extensive net K reabsorption, whereas during high excretion rates, net addition occurs only in the distal tubule.