254
D. J. RANDALL
designed for gas exchange between blood and water. There are many
accessory respiratory surfaces, which in general are associated with the
movement of fish from an hypoxic aquatic to an aerial environment and
are designed for gas exchange between blood and air. The problems
an animal has to face when breathing seawater or freshwater are similar,
although the differences in ionic content, osmolarity, and gas solubility require small changes in the functioning of the gas exchange system. The
problems associated with aquatic gas exchange, however, are different
from those associated with aerial gas exchange. This chapter is confined
to a discussion of gas transfer between water and blood and between
blood and the tissues. Air breathing in fishes is discussed in the chapter
by Johansen, this volume.
Oxygen is brought into close contact with the gills by the bulk flow
of water. Oxygen diffuses across the gills into the blood down a gradient
of between 40 and 100 mm Hg PO2. Oxygen is transported in the blood
from the gills to the capillaries; it then diffuses across the capillary walls
into the tissues. Gas gradients between blood and tissues in fish have not
been measured, but tissue gas tensions are probably in thc range of
1-15 mm Hg Po, and 3-15 mm Hg P,,,. The partial pressure of oxygen
in the environment is dissipated as oxygen molecules move from the
water into the tissues via the blood. The largest pressure drop, and
hence the major resistance to the movement of oxygen, occurs at the
gills between water and blood.
11. GAS EXCHANGE BETWEEN BLOOD AND WATER
Gas exchange between blood and water occurs across the gills and
the general body surface. The surface area of the gills is between 10 and
60 times that of the rest of the body (Parry, 1966) and is the more important site for gas exchange. Much more is known about the movement
of oxygen and carbon dioxide across the gills, and therefore gas exchange
across the gills is discussed separately from and in greater detail than
gas exchange across the skin.
A. Gas Exchange across the Gills
Gas exchange across the gills of fishes has been reviewed by Krogh
(1941), Black (1951), and Fry (1957). More recently, Hughes and
Shelton (1962), Hughes ( 1964, 1967), Lenfant and Johanscn ( 1966),
Rahn (1966a,b, 1967), Garey (1967), Kylstra et al. (1967), Hanson
D. J. RANDALL
designed for gas exchange between blood and water. There are many
accessory respiratory surfaces, which in general are associated with the
movement of fish from an hypoxic aquatic to an aerial environment and
are designed for gas exchange between blood and air. The problems
an animal has to face when breathing seawater or freshwater are similar,
although the differences in ionic content, osmolarity, and gas solubility require small changes in the functioning of the gas exchange system. The
problems associated with aquatic gas exchange, however, are different
from those associated with aerial gas exchange. This chapter is confined
to a discussion of gas transfer between water and blood and between
blood and the tissues. Air breathing in fishes is discussed in the chapter
by Johansen, this volume.
Oxygen is brought into close contact with the gills by the bulk flow
of water. Oxygen diffuses across the gills into the blood down a gradient
of between 40 and 100 mm Hg PO2. Oxygen is transported in the blood
from the gills to the capillaries; it then diffuses across the capillary walls
into the tissues. Gas gradients between blood and tissues in fish have not
been measured, but tissue gas tensions are probably in thc range of
1-15 mm Hg Po, and 3-15 mm Hg P,,,. The partial pressure of oxygen
in the environment is dissipated as oxygen molecules move from the
water into the tissues via the blood. The largest pressure drop, and
hence the major resistance to the movement of oxygen, occurs at the
gills between water and blood.
11. GAS EXCHANGE BETWEEN BLOOD AND WATER
Gas exchange between blood and water occurs across the gills and
the general body surface. The surface area of the gills is between 10 and
60 times that of the rest of the body (Parry, 1966) and is the more important site for gas exchange. Much more is known about the movement
of oxygen and carbon dioxide across the gills, and therefore gas exchange
across the gills is discussed separately from and in greater detail than
gas exchange across the skin.
A. Gas Exchange across the Gills
Gas exchange across the gills of fishes has been reviewed by Krogh
(1941), Black (1951), and Fry (1957). More recently, Hughes and
Shelton (1962), Hughes ( 1964, 1967), Lenfant and Johanscn ( 1966),
Rahn (1966a,b, 1967), Garey (1967), Kylstra et al. (1967), Hanson