9. AIR BREATHING IN FISHES
391
The higher level of CO, tension in the jugular vein than in the arterial
blood suggests that systemic venous blood returning via the posterior
cardinal veins, which receive the major cutaneous drainage, must have
the lowest CO, tension; otherwise CO, must have been largely eliminated
during passage through the vestigial gills.
The nomogram (Fig. 13) summarizes results obtained in Electrophorus and substantiates the efficiency loss in gas transport caused by
the vascular shunt from the mouth organ (Johansen et al., 1968).
D. Control of Breathing in Air-Breathing Fishes
Breathing in vertebrates is, in general, geared to satisfy changing
metabolic requirements by maintaining stable internal conditions for
gas exchange and gas transport. The factors affecting the control of
breathing are markedly different for water breathers and air breathers.
The aquatic animal is surrounded by an environment of changing gas
composition in which O2 and CO, tensions may change in the same or in
opposite directions. Also, in aquatic animals the external and internal
environments are in intimate diffusion contact at the gills, and external
changes will quickly change the internal environment. Air-breathing terrestrial animals, on the other hand, normally experience an external environment stable in its respiratory qualities; in addition, the presence
of an internal gas compartment in the lungs will protect the internal
environment against external changes. Control of breathing in the air
breather can hence be based on negative feedback from receptors sensing
internal parameters important to gas transport such as 0, and CO, tension and pH of arterial blood. The aquatic animal has a more difIicult
problem in regulation because an internal factor like the arterial O2
tension may change as a result of external, as well as, internal changes.
If, for instance, internal hypoxia occurs when a fish enters severely 02deficient water, an increased breathing effort may further aggravate the
internal hypoxia. It would be advantageous for the fish to move to water
of higher 0, content. Control of breathing should hence allow the fish
to distinguish between changes occurring internally or externally. This
complex scheme calls for chemosensitive mechanisms that can screen
external as well as internal changes.
It has long been contended that fishes show preference reactions to
external gradients in gas composition and that this is linked with an
ability to detect the gas tensions in the water by external sensors
(Hoglund, 1961). Many authors view the orientation of fish to the
higher 0, availability in a gradient as a nondirective escape reaction
391
The higher level of CO, tension in the jugular vein than in the arterial
blood suggests that systemic venous blood returning via the posterior
cardinal veins, which receive the major cutaneous drainage, must have
the lowest CO, tension; otherwise CO, must have been largely eliminated
during passage through the vestigial gills.
The nomogram (Fig. 13) summarizes results obtained in Electrophorus and substantiates the efficiency loss in gas transport caused by
the vascular shunt from the mouth organ (Johansen et al., 1968).
D. Control of Breathing in Air-Breathing Fishes
Breathing in vertebrates is, in general, geared to satisfy changing
metabolic requirements by maintaining stable internal conditions for
gas exchange and gas transport. The factors affecting the control of
breathing are markedly different for water breathers and air breathers.
The aquatic animal is surrounded by an environment of changing gas
composition in which O2 and CO, tensions may change in the same or in
opposite directions. Also, in aquatic animals the external and internal
environments are in intimate diffusion contact at the gills, and external
changes will quickly change the internal environment. Air-breathing terrestrial animals, on the other hand, normally experience an external environment stable in its respiratory qualities; in addition, the presence
of an internal gas compartment in the lungs will protect the internal
environment against external changes. Control of breathing in the air
breather can hence be based on negative feedback from receptors sensing
internal parameters important to gas transport such as 0, and CO, tension and pH of arterial blood. The aquatic animal has a more difIicult
problem in regulation because an internal factor like the arterial O2
tension may change as a result of external, as well as, internal changes.
If, for instance, internal hypoxia occurs when a fish enters severely 02deficient water, an increased breathing effort may further aggravate the
internal hypoxia. It would be advantageous for the fish to move to water
of higher 0, content. Control of breathing should hence allow the fish
to distinguish between changes occurring internally or externally. This
complex scheme calls for chemosensitive mechanisms that can screen
external as well as internal changes.
It has long been contended that fishes show preference reactions to
external gradients in gas composition and that this is linked with an
ability to detect the gas tensions in the water by external sensors
(Hoglund, 1961). Many authors view the orientation of fish to the
higher 0, availability in a gradient as a nondirective escape reaction