270
D. J. RANDALL
of the dogfish that were higher than in expired water. Thus, both teleosts
and elasmobranchs can have arterial oxygen tensions higher than that
in expired water; however, in teleosts, these are the result of a countercurrent arrangement of flows, whereas in elasmobranchs a serial multicapillary system may predominate. These high arterial oxygen tensions are
also explicable in terms of the active transport or exchange diffusion
of oxygen across the gill epithelium (see the discussion on pages 104105
and 388-391 in the “Development of the Lung,” de Reuck and Porter,
eds., 1967). Although active transport or exchange diffusion of oxygen
cannot be excluded, there is no evidence for the involvement of these
processes in oxygen transport across the gills.
4. GAS CONTENT IN BLOOD AND WATER
The concentration of CO, and 0, in both scawater and freshwater
is extremely variable (Table I ) . The amount of oxygen dissolved in water
per mm Hg varies with the temperature and ionic content of the water.
All the oxygen in water is in physical solution; CO, in blood and water
and 0, in blood is not simply in physical solution, and the gas content
per mm Hg varies with the partial pressure of the gas. The relationship
between gas content and the partial pressure of gas in solution is described by a series of CO, and 0, dissociation curves of water and blood
(Figs. 6 and 7). Carbon dioxide is between 20 and 30 times more soluble
in water than oxygen. Fully saturated blood contains between 10 and 20
times more oxygen, than water at the same partial pressure. Arterial
blood contains twice as much CO, as 0, even though the partial pressure
of 0, may be 40 times that of CO,.
The rate of gas exchange across a respiratory epithelium depends
on the dimensions of the epithelium, the concentration gradient, and
the diffusion coefficient of the gas. The diffusion coefficient of carbon
dioxide is only slightly less than that for oxygen, and the concentration
gradient for free carbon dioxide is similar to that for physically dissolved
oxygen in the reverse direction. Consequently, the exchange of oxygen
and carbon dioxide across the gills occurs at more or less the same rate.
Although the concentration gradients for carbon dioxide and oxygen
across the gills are similar, the tension gradient for carbon dioxide across
the gills is much less than that for oxygen in the reverse direction, because the solubilities of oxygen and carbon dioxide in blood and water
are different.
The oxygen capacity of fish blood is variable between species.
The antarctic icefish have no erythrocytcs or hcmoglobin in their blood
(Ruud, 1954) and an instance of a carp without hemoglobin has
D. J. RANDALL
of the dogfish that were higher than in expired water. Thus, both teleosts
and elasmobranchs can have arterial oxygen tensions higher than that
in expired water; however, in teleosts, these are the result of a countercurrent arrangement of flows, whereas in elasmobranchs a serial multicapillary system may predominate. These high arterial oxygen tensions are
also explicable in terms of the active transport or exchange diffusion
of oxygen across the gill epithelium (see the discussion on pages 104105
and 388-391 in the “Development of the Lung,” de Reuck and Porter,
eds., 1967). Although active transport or exchange diffusion of oxygen
cannot be excluded, there is no evidence for the involvement of these
processes in oxygen transport across the gills.
4. GAS CONTENT IN BLOOD AND WATER
The concentration of CO, and 0, in both scawater and freshwater
is extremely variable (Table I ) . The amount of oxygen dissolved in water
per mm Hg varies with the temperature and ionic content of the water.
All the oxygen in water is in physical solution; CO, in blood and water
and 0, in blood is not simply in physical solution, and the gas content
per mm Hg varies with the partial pressure of the gas. The relationship
between gas content and the partial pressure of gas in solution is described by a series of CO, and 0, dissociation curves of water and blood
(Figs. 6 and 7). Carbon dioxide is between 20 and 30 times more soluble
in water than oxygen. Fully saturated blood contains between 10 and 20
times more oxygen, than water at the same partial pressure. Arterial
blood contains twice as much CO, as 0, even though the partial pressure
of 0, may be 40 times that of CO,.
The rate of gas exchange across a respiratory epithelium depends
on the dimensions of the epithelium, the concentration gradient, and
the diffusion coefficient of the gas. The diffusion coefficient of carbon
dioxide is only slightly less than that for oxygen, and the concentration
gradient for free carbon dioxide is similar to that for physically dissolved
oxygen in the reverse direction. Consequently, the exchange of oxygen
and carbon dioxide across the gills occurs at more or less the same rate.
Although the concentration gradients for carbon dioxide and oxygen
across the gills are similar, the tension gradient for carbon dioxide across
the gills is much less than that for oxygen in the reverse direction, because the solubilities of oxygen and carbon dioxide in blood and water
are different.
The oxygen capacity of fish blood is variable between species.
The antarctic icefish have no erythrocytcs or hcmoglobin in their blood
(Ruud, 1954) and an instance of a carp without hemoglobin has