Thus the eel can increase the arterial-venous oxygen difference by rais
ing arterial saturation as well as by lowering venous saturation (Steen
and Kruysse, 1964 ). There are probably gill blood shunts in elasmo
branchs (Piiper and Baumgarten-Schumann, 1968a ) and Dipnoi (Johan
sen and Hanson, 1968 ) as well as in teleosts.
Capillaries in the secondary lamellae are close to the water interface
and are undoubtedly more involved in gas transfer than those in the
gill filament. Steen and Kryussc (1964 ) have shown that adrenaline in
creases lamellar blood flow and the percent saturation of the blood
leaving the gills of the eel. Adrenergic receptors are present in fish
gills, and catecholamines are known to cause a marked vasodila
tion of the gills (Keys and Bateman, 1932; Ostlund and Fiinge, 1962 ).
Exercise in salmonids is associated with an increase in the level of cir
culating catecholamines (Nakano and Tomlinson, 1967 ) and a rise in
dorsal aortic blood pressure (Randall and Stevens, 1967 ). This rise in
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