Distribution of flow and red cells between the efferent (epibranchial) and venous pathways was examined with an isolated perfused gill adapted to collect the two effluents separately. Gills from two fish species (Ictalurids) with abundant prelamellar arteriovenous anastomoses (AVAs) were compared with those of the trout which contain few AVAs. The gills were perfused with Ringer or Ringer containing 51Cr-tagged red cells (blood).

In Ringer-perfused gills, efferent outflow decreased as efferent pressure increased. Epinephrine prevented the decrease in efferent flow at elevated efferent pressures. In all species around one-third of the control blood perfusing the gill drained via the venous pathway. At constant efferent pressure epinephrine increased and acetylcholine decreased efferent outflow. These results suggest that tonic adrenergic stimulation is necessary for normal branchial perfusion.

The haematocrit of efferent effluent was greater than venous effluent in all species. No consistent effects of epinephrine or acetylcholine on plasma skimming were observed. Comparison of measured microhaematocrit and haematocrit values calculated from 51Cr red cell space suggests that the red cells in the venous effluent are larger than those from the efferent pathway and supports the concept of a nutritive function for the venous pathway.

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