| Summary: | A perfused and ventilated gill preparation is described in which the pO 2 of the perfusion medium and the irrigating water is controlled. Dorsal aorta effluent pO 2 and flow were measured together with the branchial vascular resistance. Decreasing pO 2 in the perfusion fluid caused increased branchial vascular resistance, probably by constriction of efferent lamellar arterioles. -adrenoceptor stimulation caused constriction of arteriovenous connections and of efferent lamellar arterioles, and enhanced oxygenation of the perfusion fluid. -adrenoceptor stimulation also increased O 2 transfer, but to a lesser extent. It is suggested that both hypoxia and -adrenoceptor stimulation improved O 2 transfer via constriction of efferent lamellar arterioles. Both stimuli may also increase systemic blood flow by constriction of the arterio-venous connections, although such an effect of hypoxia has not been clearly shown. -stimulation probably increased O 2 transfer by dilation of afferent lamellar arterioles, thereby causing recruitment of unperfused lamellae.
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