A gill perfusion technique allowing control of flow and pO2 in the ventral aorta and in the irrigating water is described. The procedure includes measurements of flow and pO2 in the dorsal aorta and the inferior jugular vein. Pressure recordings were made in the ventral and dorsal aortas.

Lowering the perfusion fluid pO2 and/or the irrigating water pO2 increased the branchial vascular resistance, without altering flow distribution. This response is probably released by vasoconstriction proximal to the arteriovenous anastomoses in the gill filaments.

Adrenaline acted on receptors both proximal and distal to the arteriovenous anastomoses: branchial vascular resistance decreased, dorsal aortic flow increased and oxygenation of the perfusion fluid increased.

It is suggested that a combination of a direct myogenic response to hypoxia and release of adrenaline serve to increase O2 uptake efficiency when fish are exposed to hypoxic stress.

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