8. THE REGULATION OF BREATHING
347
may or may not involve a decrease in utilization (Fig. 18). By virtue
of the increased transfer factor, active trout maintain a steady utilization (Stevens and Randall, 1967b), whercas less active fish such as carp
and suckers show a marked fall in utilization as ventilation goes up
( Saunders, 1962).
The regulation of ventilation during exercise presents some problems to the physiologist if not to the fish. The changes in both arterial
and venous oxygen tensions are very small, whereas ventilation changes
are marked (Stevens and Randall, 196%). This impressive stability of
tension might be expected in a well-designed feedback control system.
However, in hypoxic conditions there are, necessarily, larger fluctuations
in blood oxygen tensions; but not until these tensions become very low
are they accompanied by ventilation changes as large as in the active fish
(Holeton and Randall, 1967b). The same feedback system could not
operate in both cases since the error in oxygen tension is differently
related to ventilation volume. During hypoxia, the tension changes are
smallest in venous blood, which is why the computer simulation of Taylor
et al. (1968) for both exercising and hypoxic trout works best with the
error detector in the venous circuit. Stevens and Randall (1967a)
suggest that ventilation in swimming fish could be regulated by the
changes in muscular activity in some way. It is well known that there
is a direct effect of exercise on respiratory activity in mammals
(Dejours, 1964). Another possibility might be that the increased levels
of carbon dioxide in the active tissues and venous blood could act in conjunction with the oxygen receptor system, for example, to reset the
level of feedback to work with a smaller error than during hypoxia.
In the absence of further experimental evidence, oxygen, carbon dioxide,
and muscular activity must be assumed to have some part to play in
the regulation of ventilation during exercise (Fig. 19).
VI. THE RELATIONSHIP BETWEEN
VENTILATION AND PERFUSION
In order for gas exchange to occur, water and blood must be brought
into intimate contact. The provision for this at the level of the gill
lamella is discussed elsewhere (see chapter by Randall, “Gas Exchange in
Fish,” this volume), In addition, there must be an adequate flow of both
blood and water to transport the respiratory gases to and from the exchanging surface. The problems involved in the provision and control of
blood flow to the gills, especially in relation to the ventilation volume and
gas exchange have been discussed by Hughes and Shelton (1962), Hughes
347
may or may not involve a decrease in utilization (Fig. 18). By virtue
of the increased transfer factor, active trout maintain a steady utilization (Stevens and Randall, 1967b), whercas less active fish such as carp
and suckers show a marked fall in utilization as ventilation goes up
( Saunders, 1962).
The regulation of ventilation during exercise presents some problems to the physiologist if not to the fish. The changes in both arterial
and venous oxygen tensions are very small, whereas ventilation changes
are marked (Stevens and Randall, 196%). This impressive stability of
tension might be expected in a well-designed feedback control system.
However, in hypoxic conditions there are, necessarily, larger fluctuations
in blood oxygen tensions; but not until these tensions become very low
are they accompanied by ventilation changes as large as in the active fish
(Holeton and Randall, 1967b). The same feedback system could not
operate in both cases since the error in oxygen tension is differently
related to ventilation volume. During hypoxia, the tension changes are
smallest in venous blood, which is why the computer simulation of Taylor
et al. (1968) for both exercising and hypoxic trout works best with the
error detector in the venous circuit. Stevens and Randall (1967a)
suggest that ventilation in swimming fish could be regulated by the
changes in muscular activity in some way. It is well known that there
is a direct effect of exercise on respiratory activity in mammals
(Dejours, 1964). Another possibility might be that the increased levels
of carbon dioxide in the active tissues and venous blood could act in conjunction with the oxygen receptor system, for example, to reset the
level of feedback to work with a smaller error than during hypoxia.
In the absence of further experimental evidence, oxygen, carbon dioxide,
and muscular activity must be assumed to have some part to play in
the regulation of ventilation during exercise (Fig. 19).
VI. THE RELATIONSHIP BETWEEN
VENTILATION AND PERFUSION
In order for gas exchange to occur, water and blood must be brought
into intimate contact. The provision for this at the level of the gill
lamella is discussed elsewhere (see chapter by Randall, “Gas Exchange in
Fish,” this volume), In addition, there must be an adequate flow of both
blood and water to transport the respiratory gases to and from the exchanging surface. The problems involved in the provision and control of
blood flow to the gills, especially in relation to the ventilation volume and
gas exchange have been discussed by Hughes and Shelton (1962), Hughes