268
D. J. RANDALL
dorsal aortic blood pressure is not seen after a-adrenergic receptor blockade with phenoxybenzamine. There is a decrease in the resistance to
blood flow 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 flow 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 flow through the gills
and decreases lamellar blood flow (Steen and Kruysse, 1964). Hypoxia
in the water flowing over the gills of trout increases the resistance to
blood flow through the gills (Holeton and Randall, 1967a) but does not
impair the capacity of the gills to transfer gases.
The pattern of capillaries in the secondary lamellae of the tuna
appear to be different from the rest of teleosts. The capillaries are not
divided into respiratory and nonrespiratory vessels, but all blood passes
through the secondary lamellae (Muir, 1970). Tuna have typical afferent and efferent vessels, but each filament afferent gives off about 20
lamellar afferents to each secondary lamellae. Each of the lamellar
afferents subdivides forming a large number of blood channels in the
secondary lamellae, all of which are respiratory. The respiratory surface
area of the gills of the tuna may be regulated by altering the number
of lamellar derents open to blood flow. In some tuna species there are
valvelike flaps in the filament afferent that may play a role in regulating
blood flow to the lamellae.
The capillaries in teleost gills are wide enough to allow the passage
of nucleated red blood cells. The erythrocytes are about 11 p long and
6.5 p wide in tuna ( Muir, 1967), similar in size to those of Scomberomorus
(Bastos, 1966). The diameter of the capillaries appears to be less than that
of the erythrocyte, which becomes sausage-shaped as it is forced through
the gill capillaries. Red blood cells in fish swell markedly if blood CO,
levels increase (Ferguson and Black, 1941; Holeton and Randall, 1967b).
The importance of countercurrent exchange in fishes has been stressed
by a number of investigators (van Dam, 1938; Hazelhoff and Evenhuis,
1952; Hughes and Shelton, 1962). Higher oxygen levels in arterial blood
than efferent water have been recorded in both teleosts (Holeton and
Randall, 1967b) and elasmobranchs ( Piiper and Baumgarten-Schumann, 1968a) indicating the presence of a functional countercurrent
arrangement of the flows of blood and water. However, in a number of
instances (Saunders, 1962; Lenfant and Johansen, 1966), the recorded
oxygen level in arterial blood was below that of the efferent water. These
D. J. RANDALL
dorsal aortic blood pressure is not seen after a-adrenergic receptor blockade with phenoxybenzamine. There is a decrease in the resistance to
blood flow 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 flow 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 flow through the gills
and decreases lamellar blood flow (Steen and Kruysse, 1964). Hypoxia
in the water flowing over the gills of trout increases the resistance to
blood flow through the gills (Holeton and Randall, 1967a) but does not
impair the capacity of the gills to transfer gases.
The pattern of capillaries in the secondary lamellae of the tuna
appear to be different from the rest of teleosts. The capillaries are not
divided into respiratory and nonrespiratory vessels, but all blood passes
through the secondary lamellae (Muir, 1970). Tuna have typical afferent and efferent vessels, but each filament afferent gives off about 20
lamellar afferents to each secondary lamellae. Each of the lamellar
afferents subdivides forming a large number of blood channels in the
secondary lamellae, all of which are respiratory. The respiratory surface
area of the gills of the tuna may be regulated by altering the number
of lamellar derents open to blood flow. In some tuna species there are
valvelike flaps in the filament afferent that may play a role in regulating
blood flow to the lamellae.
The capillaries in teleost gills are wide enough to allow the passage
of nucleated red blood cells. The erythrocytes are about 11 p long and
6.5 p wide in tuna ( Muir, 1967), similar in size to those of Scomberomorus
(Bastos, 1966). The diameter of the capillaries appears to be less than that
of the erythrocyte, which becomes sausage-shaped as it is forced through
the gill capillaries. Red blood cells in fish swell markedly if blood CO,
levels increase (Ferguson and Black, 1941; Holeton and Randall, 1967b).
The importance of countercurrent exchange in fishes has been stressed
by a number of investigators (van Dam, 1938; Hazelhoff and Evenhuis,
1952; Hughes and Shelton, 1962). Higher oxygen levels in arterial blood
than efferent water have been recorded in both teleosts (Holeton and
Randall, 1967b) and elasmobranchs ( Piiper and Baumgarten-Schumann, 1968a) indicating the presence of a functional countercurrent
arrangement of the flows of blood and water. However, in a number of
instances (Saunders, 1962; Lenfant and Johansen, 1966), the recorded
oxygen level in arterial blood was below that of the efferent water. These