342
C. SHELTON
or venous carbon dioxide tensions were used as reference factors in the
feedback equations. Equally, the use of arterial oxygen tensions produced responses in the model which did not correspond with those of
the fish. However, if venous oxygen tension was used as a reference
to control ventilation, blood flow, and gill area, the model responded in
a way which resembled the resting and exercising trout and had a
qualitative similarity to the hypoxic animal. The model has some shortcomings, for example, in relating cardiac output directly to ventilation,
but in spite of these it is reasonably consistent with experimental
evidence. It is not yet clear that the venous location for the receptor
system is justified. The dissociation curve would normally be steepest
at venous oxygen tensions, and there may be disadvantages in locating
the error detector where the error is smallest.
There are therefore reasons to believe that oxygen receptors may be
found in the ventilation stream and in arterial and venous blood. Much
of the evidence needs more detailed experimental examination but, if
confirmed, points to a control system of greater complexity than hitherto
supposed.
B. Carbon Dioxide
1. EFFECTS OF CARBON DIOXIDE CONCENTRATIONS ON OXYGEN
CONSUMPTION, GILL VENTILATION, AND UTILIZATION
The precise role played by changes in carbon dioxide concentration,
either in the environment or within the body of a fish, in regulating
breathing is not yet understood. Various effects of increased environmental carbon dioxide have been described. At very high levels the
gas is toxic and completely inhibits oxygen consumption. Below these
levels fish survive for varying periods of time, although the ability of an
animal to extract oxygen from the environment is reduced to an extent
which depends on the amount of carbon dioxide present (Black et nl.,
1954) or on hydrogen ion concentration (Anthony, 1961). Basu (1959)
found that the rate of oxygen consumption by active suckers, bullhead,
and carp fell as carbon dioxide levels increased, the logarithm of the
consumption rate being linearly related to carbon dioxide tcnsions up
to 50 mm Hg. The same linear relationship was found for trout, using
carbon dioxide tensions up to 25 mm Hg (Fig. 20). The rcsiilts on
carp were confirmed by Bcamish (1964d), who also examined the
effects of similar ranges of carbon dioxide on the standard rates of
oxygen consumption. He found that carbon dioxide usually produccd
no significant change in the standard metabolic rate (Fig. 20).
C. SHELTON
or venous carbon dioxide tensions were used as reference factors in the
feedback equations. Equally, the use of arterial oxygen tensions produced responses in the model which did not correspond with those of
the fish. However, if venous oxygen tension was used as a reference
to control ventilation, blood flow, and gill area, the model responded in
a way which resembled the resting and exercising trout and had a
qualitative similarity to the hypoxic animal. The model has some shortcomings, for example, in relating cardiac output directly to ventilation,
but in spite of these it is reasonably consistent with experimental
evidence. It is not yet clear that the venous location for the receptor
system is justified. The dissociation curve would normally be steepest
at venous oxygen tensions, and there may be disadvantages in locating
the error detector where the error is smallest.
There are therefore reasons to believe that oxygen receptors may be
found in the ventilation stream and in arterial and venous blood. Much
of the evidence needs more detailed experimental examination but, if
confirmed, points to a control system of greater complexity than hitherto
supposed.
B. Carbon Dioxide
1. EFFECTS OF CARBON DIOXIDE CONCENTRATIONS ON OXYGEN
CONSUMPTION, GILL VENTILATION, AND UTILIZATION
The precise role played by changes in carbon dioxide concentration,
either in the environment or within the body of a fish, in regulating
breathing is not yet understood. Various effects of increased environmental carbon dioxide have been described. At very high levels the
gas is toxic and completely inhibits oxygen consumption. Below these
levels fish survive for varying periods of time, although the ability of an
animal to extract oxygen from the environment is reduced to an extent
which depends on the amount of carbon dioxide present (Black et nl.,
1954) or on hydrogen ion concentration (Anthony, 1961). Basu (1959)
found that the rate of oxygen consumption by active suckers, bullhead,
and carp fell as carbon dioxide levels increased, the logarithm of the
consumption rate being linearly related to carbon dioxide tcnsions up
to 50 mm Hg. The same linear relationship was found for trout, using
carbon dioxide tensions up to 25 mm Hg (Fig. 20). The rcsiilts on
carp were confirmed by Bcamish (1964d), who also examined the
effects of similar ranges of carbon dioxide on the standard rates of
oxygen consumption. He found that carbon dioxide usually produccd
no significant change in the standard metabolic rate (Fig. 20).