The Regulation of Cardiac Activity in Fish in a Hypoxic Environment

Abstract

oxygen in the water passing over the gills of the tench results in an increase in ventilation volume and, when the oxygen falls below a critical level, a decrease in heart rate (Randall and Shelton, 1963). The functional significance of these changes was explained in terms of the delivery of oxygen to the respiratory surface and saturation of the blood leaving the gills. There are several ways in which a fish could respond to decreased oxygen levels in the water in order to maintain near saturation of the blood leaving the gills. The ventilation volume could be increased, thus maintaining an adequate delivery of oxygen to the respiratory surface in the face of decreased oxygen content of the water. Ventilation volume can be augmented by an increase in either the rate or the amplitude of the breathing movements. The amount of blood passing through the gills could be decreased by changing either the heart rate or stroke volume; thus blood would flow through the gills more slowly, allowing more time for gas exchange. The amount of oxygen given up by the blood to the tissue could be decreased, raising the oxygen content of the afferent blood and decreasing the amount of oxygen required to insure near saturation of the