7. GAS EXCHANGE IN FISH
273
the slope of the dissociation curve, the smaller the change in partial
pressure per unit volume of gas exchanged.
Fish blood has a high affinity for oxygen and is 95% saturated at
relatively low partial pressures of oxygen compared with pressures required to saturate mammalian blood. Decreases in temperature increase
the oxygen affinity of the blood (Grigg, 1967), except in tuna, where
the oxygen affinity of hemoglobin is largely independent of temperature (Rossi-Fanelli and Antonini, 1960; see Riggs, this volume, for
further details). The P,, (partial pressure for 50% 0, saturation) is
17 mm Hg (lO°C) in the dogfish (Lenfant and Johansen, 1966), 38
mm Hg (SOC) in the trout (Beaumont, 1968), and 4 mm Hg
( 10°C) in the carp (Garey, 1967). At low Pco2 levels ( 2 mm Hg)
carp blood is 95% saturated at Po, tensions as low as 25 mm Hg. This
means that the Pa, of carp blood will not rise above 25-30 mm Hg
until the hemoglobin is fully loaded, provided the rate of reaction of
oxygen with hemoglobin is equal to or more rapid than the rate of
entry of oxygen into the blood. The amount of oxygen that enters the
blood will be increased if the respiratory area is increased, the diffusion
distance decreased, or the oxygen activity gradient across the gills increased. The oxygen gradient can be increased either by lowering the
blood Po, or by increasing the water Po?. A hemoglobin with a high
affinity for oxygen will maintain low Po, levels in the blood until the
hemoglobin is fully saturated. High ventilation volumes with a low percent utilization of oxygen will maintain a Po, in water which is close
to ambient along the whole length of the secondary lamellae. A countercurrent arrangement of the flows of blood and water in teleosts also
enables the fish to maintain a large 0, gradient between blood and
water along the whole length of the secondary lamella. The mean
oxygen pressure gradient across the gills is similar in carp and trout. The
carp, however, has a high utilization of oxygen from the water and a
hemoglobin with a very high affinity for oxygen, whereas the trout has
a low utilization of oxygen from water passing over the gills and a
hemoglobin with a lower affinity for oxygen. The maintenance of large
oxygen gradients enables the fish to utilize a smaller respiratory area
for a given oxygen uptake and so reduce ion and water exchange across
the gills.
Blood oxygen dissociation curves have been described for a number
of fish (Krogh and Leitch, 1919; Root, 1931; Dill et al., 1932; Willmer,
1934; Black and Irving, 1938; Hall and McCutcheon, 1938; Root et al.,
1939; Black, 1940; Irving et al., 1941; Root and Irving, 1941; Ferguson
and Black, 1941; Black and Black, 1946; Fish, 1956; Fry, 1957; Burke,
1965; Black et al., 1966; Swan and Hall, 1966; Lenfant and Johansen,
273
the slope of the dissociation curve, the smaller the change in partial
pressure per unit volume of gas exchanged.
Fish blood has a high affinity for oxygen and is 95% saturated at
relatively low partial pressures of oxygen compared with pressures required to saturate mammalian blood. Decreases in temperature increase
the oxygen affinity of the blood (Grigg, 1967), except in tuna, where
the oxygen affinity of hemoglobin is largely independent of temperature (Rossi-Fanelli and Antonini, 1960; see Riggs, this volume, for
further details). The P,, (partial pressure for 50% 0, saturation) is
17 mm Hg (lO°C) in the dogfish (Lenfant and Johansen, 1966), 38
mm Hg (SOC) in the trout (Beaumont, 1968), and 4 mm Hg
( 10°C) in the carp (Garey, 1967). At low Pco2 levels ( 2 mm Hg)
carp blood is 95% saturated at Po, tensions as low as 25 mm Hg. This
means that the Pa, of carp blood will not rise above 25-30 mm Hg
until the hemoglobin is fully loaded, provided the rate of reaction of
oxygen with hemoglobin is equal to or more rapid than the rate of
entry of oxygen into the blood. The amount of oxygen that enters the
blood will be increased if the respiratory area is increased, the diffusion
distance decreased, or the oxygen activity gradient across the gills increased. The oxygen gradient can be increased either by lowering the
blood Po, or by increasing the water Po?. A hemoglobin with a high
affinity for oxygen will maintain low Po, levels in the blood until the
hemoglobin is fully saturated. High ventilation volumes with a low percent utilization of oxygen will maintain a Po, in water which is close
to ambient along the whole length of the secondary lamellae. A countercurrent arrangement of the flows of blood and water in teleosts also
enables the fish to maintain a large 0, gradient between blood and
water along the whole length of the secondary lamella. The mean
oxygen pressure gradient across the gills is similar in carp and trout. The
carp, however, has a high utilization of oxygen from the water and a
hemoglobin with a very high affinity for oxygen, whereas the trout has
a low utilization of oxygen from water passing over the gills and a
hemoglobin with a lower affinity for oxygen. The maintenance of large
oxygen gradients enables the fish to utilize a smaller respiratory area
for a given oxygen uptake and so reduce ion and water exchange across
the gills.
Blood oxygen dissociation curves have been described for a number
of fish (Krogh and Leitch, 1919; Root, 1931; Dill et al., 1932; Willmer,
1934; Black and Irving, 1938; Hall and McCutcheon, 1938; Root et al.,
1939; Black, 1940; Irving et al., 1941; Root and Irving, 1941; Ferguson
and Black, 1941; Black and Black, 1946; Fish, 1956; Fry, 1957; Burke,
1965; Black et al., 1966; Swan and Hall, 1966; Lenfant and Johansen,