Buffering characteristics of eel blood at the extreme conditions in the swimbladder

CO2 binding in whole blood and true plasma of the eel was estimated by measuring CO2 content and pH in blood aliquots equilibrated with various PCO2 values over a wide range expected to occur in the swimbladder. Bicarbonate concentration, [HCO3-], was calculated using the CO2 solubility coefficient, which was measured to average 50 mumols.L-1.Torr-1 (20 degrees C). Buffer lines of non-bicarbonate buffers were obtained in plots of [HCO3-] against pH, and non-bicarbonate buffer values, beta NB, were obtained by curve fitting. In the pH range 6.6-8.2, the buffer line for oxygenated whole blood was sigmoid, while that for deoxygenated blood increased its slope monotonously with increasing pH. The beta NB for oxygenated blood displayed a maximum of about 8.6 mmol.L-1.pH-1 at pH = 7.4 and dropped down to below 1 mmol.L-1.pH-1 at higher and lower pH. Similar shapes of the buffer lines were obtained in true plasma; the [HCO3-] levels and beta NB values were, however, somewhat higher than in whole blood. These data are useful to assess the back-diffusion of CO2 and HCO3- in the rete mirabile of the fish swimbladder and to estimate the effects CO2 back-diffusion exerts on the counter-current enhancement of O2 in the rete.