8. THE REGULATION OF BREATHING
303
flow range, but invariably discontinuities were found in the relationship.
The most usual pattern was one in which the gill resistance remained
constant down to a certain flow rate, below which the resistance increased gradually as the flow decreased. The values for resistance obtained in these experiments agreed reasonably well with those from the
actively ventilating tench. Increase in resistance with decreasing ventilation could be explained in terms of change in gill geometry, for
example, in the position of the filament tips. The discontinuity in the
relationship could result from some new factor such as size of mouth
and opercular openings, assuming greater importance at high flow rates.
Experiments on the relationship between ventilation and differential
pressure across the gills have also been done on CaZZionymus (Hughes
and Umezawa, 1968b). Because of the convenient anatomy of this
animal it is possible to glue rubber connections around the opercular
openings and impose external changes of pressure across the gills of
otherwise normally breathing fish. Hughes and Umezawa found a
change in slope of the pressure-volume relationship at the point where
the imposed pressure gradient was zero (Fig. 4b). Larger changes in
ventilation were produced by increasing the gradient from buccal to
opercular cavities than were seen when similar gradients were imposed
in the reverse direction. The change in slope is not necessarily to be interpreted in terms of a change in gill resistance because the fish is
actively pumping water and may respond to the imposed gradient. A
change in activity may be more important than a change in gill resistance
in this case.
d. The Work of Breathing. Breathing muscles do work against three
main categories of force, the magnitude of which may vary greatly with
the environment in which an animal lives. The flow resistive forces offered
by the gill curtain of fish or by the airways in a mammalian lung will
depend upon the viscosity of water and air, respectively (these are
related in a ratio of approximately 55: 1); similarly inertial forces will
depend on the mass of water or air (densities related approximately
840:l) and of the tissues being accelerated. Only the third category,
the elastic forces developed in the tissues of the ventilating system, may
mouth was held open by means of a glass tube. Slopes different from 1 on the log-log
plot indicate changing gill resistance. From Hughes and Shelton (1962). ( b ) In
Callionymus ( 4 animals each of approximately 100 g ) which were breathing normally.
The applied pressure changes were therefore not the only pressures produced in the
system, and a reversed gradient could be applied without reversing the flow. The
change in slope at zero applied pressure need not reflect a change in gill resistance
since the animal may change its breathing pattern. From Hughes and Umezawa
(1968b).
303
flow range, but invariably discontinuities were found in the relationship.
The most usual pattern was one in which the gill resistance remained
constant down to a certain flow rate, below which the resistance increased gradually as the flow decreased. The values for resistance obtained in these experiments agreed reasonably well with those from the
actively ventilating tench. Increase in resistance with decreasing ventilation could be explained in terms of change in gill geometry, for
example, in the position of the filament tips. The discontinuity in the
relationship could result from some new factor such as size of mouth
and opercular openings, assuming greater importance at high flow rates.
Experiments on the relationship between ventilation and differential
pressure across the gills have also been done on CaZZionymus (Hughes
and Umezawa, 1968b). Because of the convenient anatomy of this
animal it is possible to glue rubber connections around the opercular
openings and impose external changes of pressure across the gills of
otherwise normally breathing fish. Hughes and Umezawa found a
change in slope of the pressure-volume relationship at the point where
the imposed pressure gradient was zero (Fig. 4b). Larger changes in
ventilation were produced by increasing the gradient from buccal to
opercular cavities than were seen when similar gradients were imposed
in the reverse direction. The change in slope is not necessarily to be interpreted in terms of a change in gill resistance because the fish is
actively pumping water and may respond to the imposed gradient. A
change in activity may be more important than a change in gill resistance
in this case.
d. The Work of Breathing. Breathing muscles do work against three
main categories of force, the magnitude of which may vary greatly with
the environment in which an animal lives. The flow resistive forces offered
by the gill curtain of fish or by the airways in a mammalian lung will
depend upon the viscosity of water and air, respectively (these are
related in a ratio of approximately 55: 1); similarly inertial forces will
depend on the mass of water or air (densities related approximately
840:l) and of the tissues being accelerated. Only the third category,
the elastic forces developed in the tissues of the ventilating system, may
mouth was held open by means of a glass tube. Slopes different from 1 on the log-log
plot indicate changing gill resistance. From Hughes and Shelton (1962). ( b ) In
Callionymus ( 4 animals each of approximately 100 g ) which were breathing normally.
The applied pressure changes were therefore not the only pressures produced in the
system, and a reversed gradient could be applied without reversing the flow. The
change in slope at zero applied pressure need not reflect a change in gill resistance
since the animal may change its breathing pattern. From Hughes and Umezawa
(1968b).