Summary. Gases which are customarily viewed as biologically inert may nevertheless be secreted into the swimbladder at high pressure just like their biologically active counterparts, O2 and COg. The late Werner Kuhn proposed that the mechanism for all gases was countercurrent multiplication of small changes in partial pressure initiated by secretions of the gas gland situated near the bend of the multiplier loop. Hitherto this hypothesis has been confirmed only for O2 and CO2: an analysis of swimbladder data on argon/nitrogen ratios presented here gives the required confirmation in the inert gas case. A second part of this paper deals with the nature of the secretory agent relevant to inert gas. Kuhn postulated the secretion of an electrolyte to salt out dissolved gas in addition to lactic acid secretion known to be operative in the release of O2 and CO2. This postulate may be redundant: lactic acid is shown to release N2 from fish red cells perhaps by a pfl-related change in haemoglobin polar groups. Countercurrent multiplication of all gases would thus be under the control of a single agent. Interspecies variations in swimbladder gas content would not be inconsistent with this unitary theory since these may merely reflect interspecies variations in the development of the multiplier apparatus and probable variations in the pH-effect also. INTRODUCTION.The late Werner Kuhn showed the power of the interdiseiplinary approach when he introduced the concept of countercurrent exchange into biology from the field of physical chemistry in order to explain the generation of high sodium concentrations in the hairpin loops of Henle in the renal medulla (Kuhn and Ryffel, 1942). The concept was subsequently applied in several areas of biology and, in particular, enabled Scholander (1954) to end the speculations of six generations of biologists by providing the first satisfactory explanation for the generation of high gas pressures in the fish swimbladder. Scholander (1954) described the exquisite design of the countercurrent apparatus, the rete mirabile, which consists of an array of densely-packed thin-walled capillaries arranged in parallel to give maximal contact area between venous and arterial streams. The
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