7. GAS EXCHANGE IN FISH
279
High oxygen levels in the vitreous humor are associated with the presence of a rete, and in some cases the oxygen tensions are in excess of
one atmosphere (Wittenberg and Wittenberg, 1962). Carbonic anhydrase
and bicarbonate levels in fish eyes are in excess of those in the plasma
(Hoffert, 1966; Hoffert and Fromm, 1966; Maren, 1967). The concentration of bicarbonate is possibly associated with the formation of the
aqueous humor and the maintainence of intraocular pressure in fish as
well as mammals (Maren, 1967; Hoffert, 1966). Bicarbonate accumulation in the aqueous humor of the rabbit is owing to formation from
CO, and not the transport of the ion ( Maren, 1967). In fish the rete
could maintain CO, levels in the eye that are higher than those in the
plssma, which are only of the order of 2-5 mm Hg, and therefore assist
in bicarbonate concentration and the maintainence of intraocular pressure. Therefore, high oxygen levels in the eye may not be functionally
significant but only a byproduct of the presence of a rete.
Some fish are viviparous, and in a few instances a complex system
exists for exchanging material between adult and young. The anatomy
of circulatory structures associated with development of viviparity in fish
have been described (see chapter by Hoar, Volume 111), but nothing
is known of the exchange processes in these maternal fetal connections.
IV. METHODS OF ANALYSIS OF THE
TRANSFER OF GASES
A. Introduction
The O,:CO, diagram has been used extensively to analyze gas exchange in mammals (Rahn and Fenn, 1956; West, 1965). In aquatic
respiration the relationship between P,,, and Po, in expired water is not
linear, as in air, for a given respiratory quotient and a series of ventilation volumes. This nonlinearity results from the CO, buffering capacity
of the water (Dejours et al., 1968) and the slow rate of formation of
CO, from bicarbonate, both of which tend to reduce the Pm, in water
in contact with the respiratory surface. In fish, oxygen tensions on either
side of the gills do not approach an equilibrium point, and some of the
factors that determine the rate of CO, removal appear to be different
from those that govern 0, uptake. This, plus the general paucity of
relevant data, makes an extensive analysis of gas exchange in fish using
the O,:CO, diagram difficult. No system for the graphical analysis
of gas transfer in fish, similar to the O,:CO, diagrams, has been de-
279
High oxygen levels in the vitreous humor are associated with the presence of a rete, and in some cases the oxygen tensions are in excess of
one atmosphere (Wittenberg and Wittenberg, 1962). Carbonic anhydrase
and bicarbonate levels in fish eyes are in excess of those in the plasma
(Hoffert, 1966; Hoffert and Fromm, 1966; Maren, 1967). The concentration of bicarbonate is possibly associated with the formation of the
aqueous humor and the maintainence of intraocular pressure in fish as
well as mammals (Maren, 1967; Hoffert, 1966). Bicarbonate accumulation in the aqueous humor of the rabbit is owing to formation from
CO, and not the transport of the ion ( Maren, 1967). In fish the rete
could maintain CO, levels in the eye that are higher than those in the
plssma, which are only of the order of 2-5 mm Hg, and therefore assist
in bicarbonate concentration and the maintainence of intraocular pressure. Therefore, high oxygen levels in the eye may not be functionally
significant but only a byproduct of the presence of a rete.
Some fish are viviparous, and in a few instances a complex system
exists for exchanging material between adult and young. The anatomy
of circulatory structures associated with development of viviparity in fish
have been described (see chapter by Hoar, Volume 111), but nothing
is known of the exchange processes in these maternal fetal connections.
IV. METHODS OF ANALYSIS OF THE
TRANSFER OF GASES
A. Introduction
The O,:CO, diagram has been used extensively to analyze gas exchange in mammals (Rahn and Fenn, 1956; West, 1965). In aquatic
respiration the relationship between P,,, and Po, in expired water is not
linear, as in air, for a given respiratory quotient and a series of ventilation volumes. This nonlinearity results from the CO, buffering capacity
of the water (Dejours et al., 1968) and the slow rate of formation of
CO, from bicarbonate, both of which tend to reduce the Pm, in water
in contact with the respiratory surface. In fish, oxygen tensions on either
side of the gills do not approach an equilibrium point, and some of the
factors that determine the rate of CO, removal appear to be different
from those that govern 0, uptake. This, plus the general paucity of
relevant data, makes an extensive analysis of gas exchange in fish using
the O,:CO, diagram difficult. No system for the graphical analysis
of gas transfer in fish, similar to the O,:CO, diagrams, has been de-