dorsal aortic blood pressure is not seen after a-adrenergic receptor block
ade with phenoxybenzamine. There is a decrease in the resistance to
blood How in both the respiratory and systemic circulations during
exercise and an increase in the transfer factor of the gills for oxygen
(see Section IV, B). The circumstantial evidence cited above indicates
that increased levels of catecholamines (adrenaline and noradrenaline )
alter blood How through the gills in a way that either increases the
functional surface area of the gills or decreases the diffusion distance in
order to increase the rate of gas exchange across the gills.
Acetylcholine increases the resistance to blood How through the gills
and decreases lamellar blood How (Steen and Kruysse, 1964 ). Hypoxia
in the water Howing over the gills of trout increases the resistance to
blood How through the gills (Holeton and Randall, 1OO7a ) but does not
impair the capacity of the gills to transfer gases.
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