346
G. SHELTON
relatively small, and the evidence on responses to low level effects is
conflicting. Moreover, a control system of this type could not be universally effective and would, for example, be unsatisfactory during
hypoxia. There may be some role for it, however, in conjunction with
an oxygen control system such as is known to exist.
2. COORDINATION OF THE RESPONSES TO CHANGES IN
CARBON DIOXIDE CONCENTRATION
Very little is known about the sensory mechanisms involved in the
response to carbon dioxide. The relative importance of carbon dioxide
tension or p H change as the effective stimulus has not been established.
Receptors could be in the three general areas suggested earlier for oxygen
receptors, but the arguments above suggest that environmental and
arterial blood receptors would have very limited use in respiratory
controI. Carbon dioxide sensitivity has been shown in the pseudobranch
(Laurent, 1967). However, Shelton (in Hughes and Shelton, 1962)
demonstrated that substantial ventilatory changes to carbon dioxide
persisted after total denervation of the gill area by section of the IXth
and Xth cranial nerves. Injection of small quantities of saline containing carbon dioxide into some regions of the teleost medulla caused
changes in the breathing movements, but the precise nature and location
of the stimulated area was not decided.
C. Respiratory Homeostasis in Resting and Active Fish
The validity of experiments on the effects of changes in environmental gas concentrations on some species of fish is occasionally questioned if that species is only found in well-aerated environments. One
justification for such work is that the changes may elicit responses from
the same overall system as would be activated by minor internal fluctuations in the resting animaI, or major alterations in demand as the
animal became active. The results at present available would suggest
that homeostasis is achieved in the resting animal by monitoring oxygen
tensions in the blood with carbon dioxide not being involved in any
significant role. The precise location of the oxygen receptors is not
settled.
Differences in the level of activity are the main cause of short-term
variation in oxygen consumption; the relationship has been well documented by Brett (1964). Adjustments in ventilation which meet increased
oxygen demand during exercise are also accompanied by increasing
transfer factor as Randall et al. (1967) have shown in the trout. They
G. SHELTON
relatively small, and the evidence on responses to low level effects is
conflicting. Moreover, a control system of this type could not be universally effective and would, for example, be unsatisfactory during
hypoxia. There may be some role for it, however, in conjunction with
an oxygen control system such as is known to exist.
2. COORDINATION OF THE RESPONSES TO CHANGES IN
CARBON DIOXIDE CONCENTRATION
Very little is known about the sensory mechanisms involved in the
response to carbon dioxide. The relative importance of carbon dioxide
tension or p H change as the effective stimulus has not been established.
Receptors could be in the three general areas suggested earlier for oxygen
receptors, but the arguments above suggest that environmental and
arterial blood receptors would have very limited use in respiratory
controI. Carbon dioxide sensitivity has been shown in the pseudobranch
(Laurent, 1967). However, Shelton (in Hughes and Shelton, 1962)
demonstrated that substantial ventilatory changes to carbon dioxide
persisted after total denervation of the gill area by section of the IXth
and Xth cranial nerves. Injection of small quantities of saline containing carbon dioxide into some regions of the teleost medulla caused
changes in the breathing movements, but the precise nature and location
of the stimulated area was not decided.
C. Respiratory Homeostasis in Resting and Active Fish
The validity of experiments on the effects of changes in environmental gas concentrations on some species of fish is occasionally questioned if that species is only found in well-aerated environments. One
justification for such work is that the changes may elicit responses from
the same overall system as would be activated by minor internal fluctuations in the resting animaI, or major alterations in demand as the
animal became active. The results at present available would suggest
that homeostasis is achieved in the resting animal by monitoring oxygen
tensions in the blood with carbon dioxide not being involved in any
significant role. The precise location of the oxygen receptors is not
settled.
Differences in the level of activity are the main cause of short-term
variation in oxygen consumption; the relationship has been well documented by Brett (1964). Adjustments in ventilation which meet increased
oxygen demand during exercise are also accompanied by increasing
transfer factor as Randall et al. (1967) have shown in the trout. They