266
D. J. RANDALL
associated with either a high venous oxygen tension ( Pv,,), a large anatoniical dead space, or a largc distribution dead spacc. Large differences
between Pv,, and P E , ~ (expired water oxygen tension) may be associated
with either a large anatomical or distribution dead space. The size of
the anatomical dead space can be reduced by maintaining contact between the tips of adjacent filaments and lanicllae, thus forcing water
between the secondary lamallae. The distribution dead spacc could be
reduced by ensuring maximum blood flow to those portions of the gill
receiving maximum water flow. Therc is no evidence at present that
this occurs in fish. h4easured values for P E ~ ~
- Pvo2 are 100 mm Hg in
the trout (Stevens and Randall, 1967b) 40 mm Hg in the carp (Garey,
1967), and 46 mm Hg in the dogfish ( Baumgarten-Schumann and Piiper,
1968), indicating a large water shunt in these fish.
It is important to note that, although there may be large differences
between the mean Po, values for blood and water across the gills and
arterial Po, values may be very different from those in inspired water,
this does not indicate that the gills are not effective in transferring oxygen
between water and blood. In fact, blood leaving the gills is usually between 85 and 95% saturated with oxygen.
3. BLOOD FLOW THROUGH THE GILLS
In fish, unlike mammals, the respiratory and systemic circulations
are in series rather than in parallel. Blood ejected from the heart is conveycd by the vcmtral aorta to the afferent branchial arteries supplying cach
gill arch. Blood flows from the affcrent to the efferent branchial arteries
through the capillary bed of the gills. These efferent branchial vessels
join to form the dorsal aorta through which blood passes to the general
body circulation. Figure 4 illustrates the vascularization of the gills of a
salmonid. Thcrc are a number of alternate pathways of varying distance
from the water interface, for tlie passage of blood through the gills (Steen
and Kryusse, 1964; Hughes and Grimstone, 1965; Newstead, 1967; Datta
Munshi and Singh, 1968). Blood may flow through either a few or all
of the secondary laniaellne on each filament. Some blood may bypass
the lamellae, flowing through capillaries joining afferent and efferent
vessels within the filaments. Alterations in the distribution and volume
of blood flow through thcw channels will change the functional surface area of the gills and the diffusion distance between blood and water,
thus affecting the capacity of the gills to transfer gases. At low ratcs
of oxygen uptake in the eel some blood is probably shunted past the
secondary lamellae, and blood leaving the gills is not fully saturated.
D. J. RANDALL
associated with either a high venous oxygen tension ( Pv,,), a large anatoniical dead space, or a largc distribution dead spacc. Large differences
between Pv,, and P E , ~ (expired water oxygen tension) may be associated
with either a large anatomical or distribution dead space. The size of
the anatomical dead space can be reduced by maintaining contact between the tips of adjacent filaments and lanicllae, thus forcing water
between the secondary lamallae. The distribution dead spacc could be
reduced by ensuring maximum blood flow to those portions of the gill
receiving maximum water flow. Therc is no evidence at present that
this occurs in fish. h4easured values for P E ~ ~
- Pvo2 are 100 mm Hg in
the trout (Stevens and Randall, 1967b) 40 mm Hg in the carp (Garey,
1967), and 46 mm Hg in the dogfish ( Baumgarten-Schumann and Piiper,
1968), indicating a large water shunt in these fish.
It is important to note that, although there may be large differences
between the mean Po, values for blood and water across the gills and
arterial Po, values may be very different from those in inspired water,
this does not indicate that the gills are not effective in transferring oxygen
between water and blood. In fact, blood leaving the gills is usually between 85 and 95% saturated with oxygen.
3. BLOOD FLOW THROUGH THE GILLS
In fish, unlike mammals, the respiratory and systemic circulations
are in series rather than in parallel. Blood ejected from the heart is conveycd by the vcmtral aorta to the afferent branchial arteries supplying cach
gill arch. Blood flows from the affcrent to the efferent branchial arteries
through the capillary bed of the gills. These efferent branchial vessels
join to form the dorsal aorta through which blood passes to the general
body circulation. Figure 4 illustrates the vascularization of the gills of a
salmonid. Thcrc are a number of alternate pathways of varying distance
from the water interface, for tlie passage of blood through the gills (Steen
and Kryusse, 1964; Hughes and Grimstone, 1965; Newstead, 1967; Datta
Munshi and Singh, 1968). Blood may flow through either a few or all
of the secondary laniaellne on each filament. Some blood may bypass
the lamellae, flowing through capillaries joining afferent and efferent
vessels within the filaments. Alterations in the distribution and volume
of blood flow through thcw channels will change the functional surface area of the gills and the diffusion distance between blood and water,
thus affecting the capacity of the gills to transfer gases. At low ratcs
of oxygen uptake in the eel some blood is probably shunted past the
secondary lamellae, and blood leaving the gills is not fully saturated.