284
D. J. RANDALL
B. Hypoxia
The effects of hypoxia on 0, uptake and C 0 2 removal have been
studied in the trout ( Holeton and Randall, 1967a)b) and the carp (Garey,
1967). A reduction of the oxygen level in the water causes an increase
in breathing rate and amplitude, a decrease in heart rate, but an increase
in stroke volume of the heart in the trout. Cardiac output varies little
but the ventilation-perfusion ratio increases because of a large increase
in water flow over the gills, The transfer factor of the gills for oxygen
is increased, resulting in a decrease in the mean pressure gradient for
oxygen between blood and water across the gills. The partial pressure
of oxygen in venous blood falls as the animal utilizes the venous oxygen
reserve. The end result of the increase in transfer factor and ventilationperfusion ratio and the decrease in venous oxygen content is that oxygen
uptake is maintained in the face of a decreased amount of oxygen in the
water (Holeton and Randall, 1967b). Oxygen uptake falls at low oxygen
levels in the water, and there is an increased production of lactate by the
fish. The exact level at which oxygen uptake falls varies between fish.
Teleosts appear to be able to regulate oxygen uptake over a wide range
of oxygen levels in the water; elasmobranchs appear to be less able in
this respect. Both groups become more active in hypoxic conditions and
attempt to leave the oxygen-depleted environment.
An increase in ventilation volume associated with a decreased heart
rate appears to be a general response of fish to hypoxia. The increase
in ventilation volume maintains the delivery of oxygen to the respiratory
surface as the oxygen content falls. The decreased heart rate is offset
by an increase in stroke volume in the trout such that there are only
small changes in cardiac output. During hypoxia, therefore, the pattern
of blood flow, rather than the cardiac output, changes. The changing
pattern of blood flow through the gills may, as suggested by Satchel1
( 1960), augment gas exchange.
C. Exercise
There have been a large number of studies on the changes in oxygen
uptake with exercise in fish (Brett, 1964; Fry, 1957). Stevens and Randall
(1967a,b) and Randall et al. (1967) have investigated gas exchange
across the gills of trout during swimming. Moderate exercise in the
trout is associated with an increase in cardiac output, ventilation volume,
and the gill transfer factor for oxygen. The increase in cardiac output
is largely the result of an increase in stroke volume.
D. J. RANDALL
B. Hypoxia
The effects of hypoxia on 0, uptake and C 0 2 removal have been
studied in the trout ( Holeton and Randall, 1967a)b) and the carp (Garey,
1967). A reduction of the oxygen level in the water causes an increase
in breathing rate and amplitude, a decrease in heart rate, but an increase
in stroke volume of the heart in the trout. Cardiac output varies little
but the ventilation-perfusion ratio increases because of a large increase
in water flow over the gills, The transfer factor of the gills for oxygen
is increased, resulting in a decrease in the mean pressure gradient for
oxygen between blood and water across the gills. The partial pressure
of oxygen in venous blood falls as the animal utilizes the venous oxygen
reserve. The end result of the increase in transfer factor and ventilationperfusion ratio and the decrease in venous oxygen content is that oxygen
uptake is maintained in the face of a decreased amount of oxygen in the
water (Holeton and Randall, 1967b). Oxygen uptake falls at low oxygen
levels in the water, and there is an increased production of lactate by the
fish. The exact level at which oxygen uptake falls varies between fish.
Teleosts appear to be able to regulate oxygen uptake over a wide range
of oxygen levels in the water; elasmobranchs appear to be less able in
this respect. Both groups become more active in hypoxic conditions and
attempt to leave the oxygen-depleted environment.
An increase in ventilation volume associated with a decreased heart
rate appears to be a general response of fish to hypoxia. The increase
in ventilation volume maintains the delivery of oxygen to the respiratory
surface as the oxygen content falls. The decreased heart rate is offset
by an increase in stroke volume in the trout such that there are only
small changes in cardiac output. During hypoxia, therefore, the pattern
of blood flow, rather than the cardiac output, changes. The changing
pattern of blood flow through the gills may, as suggested by Satchel1
( 1960), augment gas exchange.
C. Exercise
There have been a large number of studies on the changes in oxygen
uptake with exercise in fish (Brett, 1964; Fry, 1957). Stevens and Randall
(1967a,b) and Randall et al. (1967) have investigated gas exchange
across the gills of trout during swimming. Moderate exercise in the
trout is associated with an increase in cardiac output, ventilation volume,
and the gill transfer factor for oxygen. The increase in cardiac output
is largely the result of an increase in stroke volume.