9. AIR BREATHING IN FISHES
405
I. Coupling of Respiratory and Circulatory Events
Cardiovascular and respiratory events are strongly interrelated in
fishes. Cardiac output determination by the Fick principle in Neoceratodus indicated a marked increase during exposure to hypoxic water.
The increase in blood perfusion exceeded the relative increase in the
ventilation of water. Consequently, the ventilation perfusion ratio of the
gills changed from about 12 before hypoxia to about 4, after 45 min
exposure to hypoxic water, then climbed to 17 during recovery. The ratio
of pulmonary blood flow to total blood flow rose with increased rate
of air breathing (Johansen et al., 1967). In Protoptern, direct blood
flow measurements have shown that spontaneous air breathing causes
distinct changes in heart rate, total cardiac output, and regional blood
flow. Cardiac output increased sharply with each air breath and declined slowly during the subsequent breath interval (Fig. 21A) (Johansen et al., 1968). Branchial vascular resistance tended to decrease in conjunction with an air breath. The most consistent change was a marked
increase of pulmonary blood flow which exceeded that in the vena cava
and thus indicated a regional flow shift from the systemic to the
pulmonary circuit. Thus, pulmonary flow could vary from less than 20%
to more than 70% of the cardiac output.
In Electrophorus there is an equally strong interrelationship between
circulatory and respiratory events. When the intervals between air
breaths were relatively long (exceeding 1 min), cardiac output and
heart rate decreased progressively in the late phase of the interval. At
the next breath, both heart rate and blood velocity promptly increased
and were reestablished at the levels prevailing just after the preceding
breath (Fig. 21B). This cyclic phenomenon occurred as a normal event
when the intervals between breaths were long, whereas no time-dependent changes were recorded at short intervals. The sudden changes
in blood perfusion elicited by air breaths are at least, in part, controlled
by the degree of mechanical distension of the air-breathing organ inside
the mouth. Artificial inflation of the mouth via a catheter elicited an increase in heart rate and cardiac output whether nitrogen, air, or
oxygen were injected; thus, the chemical composition of the gas in the
mouth does not constitute the actual stimulus. A similar interrelation
between cardiovascular events and distention of the air-breathing
organ has been reported for Symbranclzus (Johansen, 1966). Based on
blood gas values the fractional distribution of cardiac output has been
calculated in Electrophorus. Just after a breath the blood flow fraction
405
I. Coupling of Respiratory and Circulatory Events
Cardiovascular and respiratory events are strongly interrelated in
fishes. Cardiac output determination by the Fick principle in Neoceratodus indicated a marked increase during exposure to hypoxic water.
The increase in blood perfusion exceeded the relative increase in the
ventilation of water. Consequently, the ventilation perfusion ratio of the
gills changed from about 12 before hypoxia to about 4, after 45 min
exposure to hypoxic water, then climbed to 17 during recovery. The ratio
of pulmonary blood flow to total blood flow rose with increased rate
of air breathing (Johansen et al., 1967). In Protoptern, direct blood
flow measurements have shown that spontaneous air breathing causes
distinct changes in heart rate, total cardiac output, and regional blood
flow. Cardiac output increased sharply with each air breath and declined slowly during the subsequent breath interval (Fig. 21A) (Johansen et al., 1968). Branchial vascular resistance tended to decrease in conjunction with an air breath. The most consistent change was a marked
increase of pulmonary blood flow which exceeded that in the vena cava
and thus indicated a regional flow shift from the systemic to the
pulmonary circuit. Thus, pulmonary flow could vary from less than 20%
to more than 70% of the cardiac output.
In Electrophorus there is an equally strong interrelationship between
circulatory and respiratory events. When the intervals between air
breaths were relatively long (exceeding 1 min), cardiac output and
heart rate decreased progressively in the late phase of the interval. At
the next breath, both heart rate and blood velocity promptly increased
and were reestablished at the levels prevailing just after the preceding
breath (Fig. 21B). This cyclic phenomenon occurred as a normal event
when the intervals between breaths were long, whereas no time-dependent changes were recorded at short intervals. The sudden changes
in blood perfusion elicited by air breaths are at least, in part, controlled
by the degree of mechanical distension of the air-breathing organ inside
the mouth. Artificial inflation of the mouth via a catheter elicited an increase in heart rate and cardiac output whether nitrogen, air, or
oxygen were injected; thus, the chemical composition of the gas in the
mouth does not constitute the actual stimulus. A similar interrelation
between cardiovascular events and distention of the air-breathing
organ has been reported for Symbranclzus (Johansen, 1966). Based on
blood gas values the fractional distribution of cardiac output has been
calculated in Electrophorus. Just after a breath the blood flow fraction