338
C. SHELTON
The data at present available do not allow firm conclusions to be
reached about the effective upper limit of ventilation which would restrict
the availability of oxygen to the body. If the needs of the whole body, except for the respiratory musculature, are considered then falling utilization, increased work of breathing, and limitations in the stroke volume
and rate of operation of the branchial pump all operate to determine
that upper limit. Little is known of the work factor which in any case
will differ in moving and nonmoving fish. Even if a simpler view is
taken and the changes in the work of breathing are ignored the situation
is still far from simple. Since ventilation-utilization relationships are
variable, the balance between factors will change with conditions. In
none of the experiments (van Dam, 1938; Saunders, 1962; Holeton and
Randall, 196%) in which hyperventilation was the result of low environmental oxygen did utilization decrease more rapidly than ventilation increased (Fig. IS), as would have been the case if utilization alone had
been limiting. In order to maintain oxygen consumption at a steady
level it is necessary for the ventilation volume to go up at the same
rate as the product of oxygen concentration and utilization declines. This
just failed to be the case in the trout examined by Holeton and Randall
(196%) which showed signs of dependence below oxygen tensions of
80 mm Hg. It is significant that ventilation was not maximal but increased
almost by a factor of two as the oxygen concentration was decreased
further. The inference from this is that the control systems may also
participate in the failure to maintain oxygen availability before the
pump has reached its maximum capacity.
2. COORDINATION OF THE RESPONSES TO CHANGES
IN OXYGEN CONCENTRATION
The evidence cited above shows that in the majority of teleosts
examined depletion of oxygen in the medium results in an increased
ventilation volume over a wide range. Other reactions, for example,
avoidance behavior, have also been described in a number of telcosts
(Erichsen Jones, 1952; Whitmore et al., 1960); but it seems likely that
the respiratory response is the primary one. The situation is not so
clearly resolved in the case of elasmobranchs. According to Ogden
(1945) a decrease of environmental oxygen produced no change in the
ventilation volume of the dogfish, Mustelus, although the animal struggled violently under these conditions. Satchel1 (1961) showed that perfusion of dogfish, Squalus acanthins, gills with completely deoxygenated
water caused only a very slight increase in amplitude, together with a
decrease in the rate of breathing. Because of the extreme nature of tho
C. SHELTON
The data at present available do not allow firm conclusions to be
reached about the effective upper limit of ventilation which would restrict
the availability of oxygen to the body. If the needs of the whole body, except for the respiratory musculature, are considered then falling utilization, increased work of breathing, and limitations in the stroke volume
and rate of operation of the branchial pump all operate to determine
that upper limit. Little is known of the work factor which in any case
will differ in moving and nonmoving fish. Even if a simpler view is
taken and the changes in the work of breathing are ignored the situation
is still far from simple. Since ventilation-utilization relationships are
variable, the balance between factors will change with conditions. In
none of the experiments (van Dam, 1938; Saunders, 1962; Holeton and
Randall, 196%) in which hyperventilation was the result of low environmental oxygen did utilization decrease more rapidly than ventilation increased (Fig. IS), as would have been the case if utilization alone had
been limiting. In order to maintain oxygen consumption at a steady
level it is necessary for the ventilation volume to go up at the same
rate as the product of oxygen concentration and utilization declines. This
just failed to be the case in the trout examined by Holeton and Randall
(196%) which showed signs of dependence below oxygen tensions of
80 mm Hg. It is significant that ventilation was not maximal but increased
almost by a factor of two as the oxygen concentration was decreased
further. The inference from this is that the control systems may also
participate in the failure to maintain oxygen availability before the
pump has reached its maximum capacity.
2. COORDINATION OF THE RESPONSES TO CHANGES
IN OXYGEN CONCENTRATION
The evidence cited above shows that in the majority of teleosts
examined depletion of oxygen in the medium results in an increased
ventilation volume over a wide range. Other reactions, for example,
avoidance behavior, have also been described in a number of telcosts
(Erichsen Jones, 1952; Whitmore et al., 1960); but it seems likely that
the respiratory response is the primary one. The situation is not so
clearly resolved in the case of elasmobranchs. According to Ogden
(1945) a decrease of environmental oxygen produced no change in the
ventilation volume of the dogfish, Mustelus, although the animal struggled violently under these conditions. Satchel1 (1961) showed that perfusion of dogfish, Squalus acanthins, gills with completely deoxygenated
water caused only a very slight increase in amplitude, together with a
decrease in the rate of breathing. Because of the extreme nature of tho