7. GAS EXCHANGE IN FISH
265
of the order of 0.4-2 sec depending on ventilation volume. The diffusion
dead space can be calculated (Fig. 3) from the equation of Kylstra
et al. (1967); D is 1.43 X
cm2/sec at 5°C (from Table I, assuming
a temperature coefficient at 2 % / O C ) . The diffusion distance (a) is taken
to be half the distance between successive secondary lamellae on a gill
filament. The calculated values of diffusion dead space are small in
trout and carp, largely because of the very small distances between
secondary lamellae. Large water shunts past the gills cannot therefore
be explained in terms of a large diffusion dead space. In the trout the
diffusion dead space is only 7% VG (Fig. 3 ) even when VG is 10.3 ml/sec
( t = 0.4 sec) and all the water passes between the secondary lamellae.
A large water shunt can only be explained in terms of a large anatomical
or distribution dead space.
A low percent utilization of oxygen from the water may therefore be
Anatomical and distribution
Residual volume
L
. u
0
0 1 2 3 4
5
6
7 8
9 1 0
Gill ventilation (rnl/sec)
Fig. 3. The effect of ventilation volume on the size of the anatomical ( VI, anatox)
and distribution (Vndisto2) dead space, the residual dead space, and the diffusion
dead space ( V D ~ , W O ~ )
assuming an oxygen uptake ( Vo,) of 0.47 ml/min and a venous
oxygen tension ( P V O ~ )
of 20 mm Hg.