7. GAS EXCHANGE IN FISH
275
effect (Lenfant et al., 1966-1967). The reader is referred to the chapter
by Riggs, this volume, for further information on this subject.
The much higher CO, than 0, solubility in water means that if the
respiratory quotient is around unity, the changes in CO, tension in the
water as it passes over the gills are small and of the order of a few mm
Hg whereas the changes in Po. are large. Under these circumstances
the ratio of the Po? to Pco, changes will be equal to the ratio of the
solubilities of 0, and CO, irr water, that is, between 20 and 30:l. This
effect of solubility on the changes in partial pressure is described by the
0,: CO, diagram of Rahn (1966a).
The relationship between the amount of CO, in solution and the
partial pressure of the gas is not linear in either blood or water. The
CO, dissociation curves are curvilinear in water below 2 mm Hg Pco,,
particularly in seawater or highly carbonated freshwater (Fig. 6). Carbon dioxide entering water is buffered by a carbonate-bicarbonate
system reducing the magnitude of the P,,, increase (Dejours et al.,
1968). As water passes over the gills of teleosts the excretion of ammonium ions increase the CO, buffering capacity of water, further reducing
the magnitude of the rise in P,,, in efferent water. This latter effect is
probably not seen in elasmobranchs because they are ureotelic and not
ammonotelic ( Piiper and Baumgarten-Schumann, 1968b).
Carbonic anhydrase has been located in fish red blood cells and the
gill epithelium (Maren, 1967) and bicarbonate is exchanged for chloride
by an exchange diffusion mechanism across the gill epithelium of freshwater teleosts (Maetz and Garcia Romeu, 1964). Thus because of the
presence of carbonic anhydrase and an exchange diffusion mechanism, bicarbonate as well as CO, enters the water passing over the gills (Fig. 8).
If this is so then the slow formation of CO, from bicarbonate in water will
delay the rise in P,,, until the water has left the respiratory surface. A body
of water is in contact with the gill epithelium for about a second, the
time required for the formation of CO, from bicarbonate in water
is of the order of several seconds; hence, the major portion of the rise
in P,.,, will occur after the water has left the respiratory surface. The
relative importance of the buffering system, the excretion of ammonium
ions, and the slow formation of CO, from bicarbonate, in water, in maintaining the CO, gradient across the gills will depend on the water flow
rate and the ambient temperature, but all tend to reduce or retard the
rise in water Pro, and maintain the Pco, gradient between blood and
water.
The rate of formation of CO, in the red blood cell, and hence the
rate of excretion of CO?, will depend upon the presence of carbonic
anhydrase (Fig. 8). The activity of carbonic anhydrase in fish blood
275
effect (Lenfant et al., 1966-1967). The reader is referred to the chapter
by Riggs, this volume, for further information on this subject.
The much higher CO, than 0, solubility in water means that if the
respiratory quotient is around unity, the changes in CO, tension in the
water as it passes over the gills are small and of the order of a few mm
Hg whereas the changes in Po. are large. Under these circumstances
the ratio of the Po? to Pco, changes will be equal to the ratio of the
solubilities of 0, and CO, irr water, that is, between 20 and 30:l. This
effect of solubility on the changes in partial pressure is described by the
0,: CO, diagram of Rahn (1966a).
The relationship between the amount of CO, in solution and the
partial pressure of the gas is not linear in either blood or water. The
CO, dissociation curves are curvilinear in water below 2 mm Hg Pco,,
particularly in seawater or highly carbonated freshwater (Fig. 6). Carbon dioxide entering water is buffered by a carbonate-bicarbonate
system reducing the magnitude of the P,,, increase (Dejours et al.,
1968). As water passes over the gills of teleosts the excretion of ammonium ions increase the CO, buffering capacity of water, further reducing
the magnitude of the rise in P,,, in efferent water. This latter effect is
probably not seen in elasmobranchs because they are ureotelic and not
ammonotelic ( Piiper and Baumgarten-Schumann, 1968b).
Carbonic anhydrase has been located in fish red blood cells and the
gill epithelium (Maren, 1967) and bicarbonate is exchanged for chloride
by an exchange diffusion mechanism across the gill epithelium of freshwater teleosts (Maetz and Garcia Romeu, 1964). Thus because of the
presence of carbonic anhydrase and an exchange diffusion mechanism, bicarbonate as well as CO, enters the water passing over the gills (Fig. 8).
If this is so then the slow formation of CO, from bicarbonate in water will
delay the rise in P,,, until the water has left the respiratory surface. A body
of water is in contact with the gill epithelium for about a second, the
time required for the formation of CO, from bicarbonate in water
is of the order of several seconds; hence, the major portion of the rise
in P,.,, will occur after the water has left the respiratory surface. The
relative importance of the buffering system, the excretion of ammonium
ions, and the slow formation of CO, from bicarbonate, in water, in maintaining the CO, gradient across the gills will depend on the water flow
rate and the ambient temperature, but all tend to reduce or retard the
rise in water Pro, and maintain the Pco, gradient between blood and
water.
The rate of formation of CO, in the red blood cell, and hence the
rate of excretion of CO?, will depend upon the presence of carbonic
anhydrase (Fig. 8). The activity of carbonic anhydrase in fish blood