7. GAS EXCHANGE IN FISH
281
thickness of the boundary layer of water at the gill surface may also
be altered and affect the transfer factor, but this effect is probably not
important (see Section 11, A, 2, c). The velocity of the various hemoglobin reactions and the rate of bicarbonate formation will also affect
the respective transfer factor of the gills for oxygen and carbon dioxide.
The relative importance of these parameters in determining the size
of the transfer factor has yet to be investigated.
Hughes and Shelton (1962) and Hughes (1964) introduced a term
analogous to the transfer factor. This term “the number of transfer units”
is determined by the ratio between the capacity of the gills for transfer of gases and the load imposed on the system by the blood flow
(Hughes, 1964). In practical terms it is difficult to assign a numerical
value to this term, and the transfer factor appears to be a more suitable
estimate of the transfer capacity of the gills.
C. The Effectiveness of Gas Transfer
This term introduced by Hughes and Shelton (1962) is a measure
of the relative ability of the system to transfer a particular gas. The
effectiveness of gas transfer is the ratio of the actual rate of gas exchange
to the maximum rate of gas exchange possible. The maximum rate of
oxygen removal from water occurs (assuming that no active processes are
involved) when water leaving the gills is in equilibrium with venous
blood entering the gills.
The effectiveness of gas exchange can be applied to the removal of
oxygen from water, the uptake of oxygen by the blood, the removal of
CO, from the blood and the uptake of CO, by water, this has been done
for trout (Randall et al., 1967) and dogfish (Piiper and BaumgartenSchumann, 1968a). In both trout and dogfish the effectiveness of oxygenating the blood is high, ranging from 79% in the unanesthetized
dogfish to 9%100% in the trout. The effectiveness of CO, removal from
the blood is not so high (3540%) in trout or dogfish, but in both
cases it is considerably more effective than CO, removal from human
blood. The effectiveness of CO, uptake by the water is between 38 and
43% in dogfish and 4 4 % in trout. The low effectiveness of oxygen removal
from water (1130%) passing over the trout gills is related to the very
high ventilation volumes and the low utilization of oxygen recorded
in this animal (Randall et al., 1967). The difference between the effectiveness of oxygen removal from the water and the percent utilization of oxygen is that effectiveness takes into consideration the venous
Po2. Effectiveness, therefore, is a more accurate estimate of the efficiency
281
thickness of the boundary layer of water at the gill surface may also
be altered and affect the transfer factor, but this effect is probably not
important (see Section 11, A, 2, c). The velocity of the various hemoglobin reactions and the rate of bicarbonate formation will also affect
the respective transfer factor of the gills for oxygen and carbon dioxide.
The relative importance of these parameters in determining the size
of the transfer factor has yet to be investigated.
Hughes and Shelton (1962) and Hughes (1964) introduced a term
analogous to the transfer factor. This term “the number of transfer units”
is determined by the ratio between the capacity of the gills for transfer of gases and the load imposed on the system by the blood flow
(Hughes, 1964). In practical terms it is difficult to assign a numerical
value to this term, and the transfer factor appears to be a more suitable
estimate of the transfer capacity of the gills.
C. The Effectiveness of Gas Transfer
This term introduced by Hughes and Shelton (1962) is a measure
of the relative ability of the system to transfer a particular gas. The
effectiveness of gas transfer is the ratio of the actual rate of gas exchange
to the maximum rate of gas exchange possible. The maximum rate of
oxygen removal from water occurs (assuming that no active processes are
involved) when water leaving the gills is in equilibrium with venous
blood entering the gills.
The effectiveness of gas exchange can be applied to the removal of
oxygen from water, the uptake of oxygen by the blood, the removal of
CO, from the blood and the uptake of CO, by water, this has been done
for trout (Randall et al., 1967) and dogfish (Piiper and BaumgartenSchumann, 1968a). In both trout and dogfish the effectiveness of oxygenating the blood is high, ranging from 79% in the unanesthetized
dogfish to 9%100% in the trout. The effectiveness of CO, removal from
the blood is not so high (3540%) in trout or dogfish, but in both
cases it is considerably more effective than CO, removal from human
blood. The effectiveness of CO, uptake by the water is between 38 and
43% in dogfish and 4 4 % in trout. The low effectiveness of oxygen removal
from water (1130%) passing over the trout gills is related to the very
high ventilation volumes and the low utilization of oxygen recorded
in this animal (Randall et al., 1967). The difference between the effectiveness of oxygen removal from the water and the percent utilization of oxygen is that effectiveness takes into consideration the venous
Po2. Effectiveness, therefore, is a more accurate estimate of the efficiency