350
C. SHELTON
Raynaud, 1957; Hughes, 1961b; Hughes and Umezawa, 1968b), although
there is often a greater tendency for the heart to beat during mouth
closing than in other phases of breathing in the tench (Shelton and
Randall, 1962). Although the relationship is not so clear cut, there is
still a great deal of evidence for control in teleosts. Shelton and Randall
(1962), working on eels and tench, and Peyraud and Serfaty (1964),
working on carp, showed that periodic bursts of breathing movements
were accompanied by an increase in heart rate, whereas the rate fell in
periods of apnoea between bursts. A very clear synchrony developed
in trout if the oxygen content of the environment was reduced, and, as
in the dogfish, it was shown that the receptors were in the branchial
region and the efferent fibers were cardio-inhibitory ones carried in the
vagus (Randall, 1966; Randall and Smith, 1967). As in the case of
elasmobranchs, it was suggested that phasic cardiac inhibition allowed
the heart to beat during the period of maximum water flow over the
gills.
Different environmental conditions and variations in activity place
different demands on heart output just as they do on the gill ventilating
mechanism, although the responses of the two systems need not necessarily be similar in nature. Bradycardia occurs as a response to hypoxia
in both elasmobranchs (Satchell, 1961) and teleosts (Randall and
Shelton, 1963; Serfaty et al., 1965; Labat, 1966) and is attributable to
an increase in vagal tone in both cases. Randall and Shelton (1963)
suggested that the bradycardia represented a fall in cardiac output, but
Holeton and Randall (1967b) have recently shown that this is not so.
In the trout, hypoxia produced no substantial change in cardiac output
although there was a shift from high rate-low stroke volume to low
rate-high stroke volume (Fig, 23). While the gills can maintain complete saturation of the hemoglobin during hypoxia, no change in cardiac
output is necessary to maintain a uniform oxygen consumption. The
significance of the bradycardia is not clear. If the arterial oxygen tensions fall below the level for complete saturation, then the heart output
must go up, or there must be greater extraction in the tissues giving
lowered venous oxygen tensions. The latter seems to be the case in
trout.
Exercise causes substantial increase in oxygen consumption and will
therefore require greater perfusion rates to transport the oxygen from
gills to tissues. Increased cardiac output and ventilation volume should
proceed together in the absence of any very great tension changes in
the different parts of the system. This was in fact the case in the trout
(Stevens and Randall, 1967b), heart rate and stroke volume both going
up to produce the increased cardiac output. The control of this response
C. SHELTON
Raynaud, 1957; Hughes, 1961b; Hughes and Umezawa, 1968b), although
there is often a greater tendency for the heart to beat during mouth
closing than in other phases of breathing in the tench (Shelton and
Randall, 1962). Although the relationship is not so clear cut, there is
still a great deal of evidence for control in teleosts. Shelton and Randall
(1962), working on eels and tench, and Peyraud and Serfaty (1964),
working on carp, showed that periodic bursts of breathing movements
were accompanied by an increase in heart rate, whereas the rate fell in
periods of apnoea between bursts. A very clear synchrony developed
in trout if the oxygen content of the environment was reduced, and, as
in the dogfish, it was shown that the receptors were in the branchial
region and the efferent fibers were cardio-inhibitory ones carried in the
vagus (Randall, 1966; Randall and Smith, 1967). As in the case of
elasmobranchs, it was suggested that phasic cardiac inhibition allowed
the heart to beat during the period of maximum water flow over the
gills.
Different environmental conditions and variations in activity place
different demands on heart output just as they do on the gill ventilating
mechanism, although the responses of the two systems need not necessarily be similar in nature. Bradycardia occurs as a response to hypoxia
in both elasmobranchs (Satchell, 1961) and teleosts (Randall and
Shelton, 1963; Serfaty et al., 1965; Labat, 1966) and is attributable to
an increase in vagal tone in both cases. Randall and Shelton (1963)
suggested that the bradycardia represented a fall in cardiac output, but
Holeton and Randall (1967b) have recently shown that this is not so.
In the trout, hypoxia produced no substantial change in cardiac output
although there was a shift from high rate-low stroke volume to low
rate-high stroke volume (Fig, 23). While the gills can maintain complete saturation of the hemoglobin during hypoxia, no change in cardiac
output is necessary to maintain a uniform oxygen consumption. The
significance of the bradycardia is not clear. If the arterial oxygen tensions fall below the level for complete saturation, then the heart output
must go up, or there must be greater extraction in the tissues giving
lowered venous oxygen tensions. The latter seems to be the case in
trout.
Exercise causes substantial increase in oxygen consumption and will
therefore require greater perfusion rates to transport the oxygen from
gills to tissues. Increased cardiac output and ventilation volume should
proceed together in the absence of any very great tension changes in
the different parts of the system. This was in fact the case in the trout
(Stevens and Randall, 1967b), heart rate and stroke volume both going
up to produce the increased cardiac output. The control of this response