The Control of Respiration and Circulation in Fish during Exercise and Hypoxia

Gas exchange across fish gills is reviewed and the respiratory and cardiovascular changes associated with exercise and hypoxia described. Heart rate is controlled by inhibitory vagal cholinergic activity which increases during hypoxia and decreases during exercise. Stimulation of receptors on the first gill arch during hypoxia initiates bradycardia. The increase in stroke volume during hypoxia in dogfish appears to be related to cardiac slowing rather than /9-adrenergic stimulation of the heart. Stimulation of cardiac /?-adrenergic receptors causes positive inotropic and chronotopic responses in many fish, whether these are operative during exercise and hypoxia is not clear. Gill water flow is inversely related to arterial oxygen content in resting fish and there is probably an arterial oxygen content receptor coupled to gill ventilation. Little is known of the control of breathing during exercise, the switch from rhythmic to ram ventilation at high water velocities may be initiated by mechanoreceptors on the gill surface.