The patterns of renal electrolyte excretion in the duck (Anas platyrhynchos) maintained on freshwater and on hypertonic saline.

In mammals and many terrestrial birds the kidney appears to be the major pathway for the excretion of nitrogenous waste products and excess electrolytes. Thus when these animals are presented with varying concentrations of sodium chloride in their drinking water the tonicity of the excreted urine varies commensurately (Barker & Adolph, 1953; Wolf, Prentiss, Douglas & Swett, 1959; Poulson & Bartholomew, 1962a, b; Hudson, 1962; Dawson, Shoemaker, Tordoff & Borut, 1965; Poulson, 1965). The concentration of sodium chloride which these animals can tolerate appears to be largely dependent upon the capacity of the kidney to produce a hypertonic urine. Such renal concentrating properties are believed to reside in the double countercurrent arrangement of the loop of Henle, the collecting duct and the vasa recta (Wirz, Hargitay & Kuhn, 1951). In addition to the kidney, however, many aquatic birds possess functional nasal glands which constitute an extra-renal pathway for the excretion of electrolytes (see reviews by Schmidt-Nielsen, i960; Holmes 1965). Thus, when these birds are presented with a salt load the sodium chloride is excreted both renally and extra-renally. Since the kidneys of these species appear to have a limited ability to produce hypertonic urine (Scothorne, 1959; Schmidt-Nielsen, i960; Holmes & Phillips, 1965) the possession of an extra-renal excretory pathway seems to be necessary for their survival when they are continuously exposed to hypertonic drinking water (Schildmacher, 1932; Holmes, Butler & Phillips, 1961; Ellis, Goertemiller, DeLellis & Kablotsky, 1963; Schmidt-Nielsen & Kim, 1964). Although an abundance of evidence exists to demonstrate the importance of the extra-renal pathway in the excretion of sodium chloride, the role of the kidney in the homeostatic mechanisms of these birds has been largely neglected. In general the reported studies involve salt loads which are largely in excess of those normally encountered by the birds. The present study, therefore, was directed towards gaining some insight into the renal regulation of water and electrolyte excretion by a bird possessing functional nasal glands. To this end the ducks were not subjected to acute